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Electromagnetic compatibility (EMC) is the branch of electronics that concerns the unintentional generation, propagation, and reception of electromagnetic energy.  The formal process of compliance testing ensures that unwanted effects like electromagnetic interference (EMI) or physical damage in operational electronic equipment is not present, making sure that devices will safely work in a reliable manner.  The goal of EMC is to correct operation of different equipment in a common electromagnetic environment. 

 

In the last blog, we talked about the difference between Pre-Compliance and Compliance testing – where Pre-Compliance is an informal, cost-effective, and low risk method to ensure your Compliance testing will pass. 

Today we will talk about the formal Compliance regulatory standards and the general process for final compliance testing.

 

 

EMC Compliance Testing deals with 4 main tests:

  • Emission
  • Susceptibility
  • Immunity
  • Coupling

 

Emission issues involve the deliberate or accidental generation of electromagnetic energy. 

Susceptibility involves the tendency of electrical equipment to malfunction or break down in the presence of unwanted emissions or radio frequency inference (RFI). 

 

Immunity is the opposite of susceptibility; it is the ability of the equipment to function correctly in the presence of RFI.

Coupling is the mechanism by which emitted interference reaches the DUT.  There are several types of coupling:

 

  • Conductive:  When coupling path between source and the receptor is formed by direct electrical contact with a conductive surface (i.e: transmission line, wire, cable, etc.)
  • Inductive:  When a source and receiver are separated by a short distance
  • Capacitive:  When a varying electrical field exists between two adjacent conductors, inducing a change in voltage on the receiving conductor
  • Magnetic:  Type of inductive coupling, when a varying magnetic field exists between two parallel conductors, including a change in voltage along the receiving conductor
  • Radiative:  Occurs when a source and DUT are separated by a large distance.  The source and DUT act as radio antennas, the source radiates an electromagnetic wave

Figure 1:  The use of an anechoic chamber is a crucial component for final compliance testing.

 

The EMC testing process involves open-air test sites that are the reference point in most CISPR standards.  This Is especially useful for emissions testing of large equipment systems.  RF testing of a physical prototype is most often carried out indoors in an EMC test chamber, like an anechoic chamber, which is a room designed to completely absorb reflections of either sound or electromagnetic waves.  This room is isolated from external waves from entering its surroundings.

 

EMC tests are regulated for standard compliance.  These standards help regulate and make uniform product EMC performance.  An example of one of the standards is CISPR, as mentioned in the previous blog.  CISPR’s work involves the equipment and methods for measuring interference, and establishes limits and immunity requirements for electronic devices.  Different countries have different organizations that enforce these requirements.  In America, the Federal Communications Commission (FCC) is the group that enforces these compliance testing and certifications.  This means, in America, the FCC enforces specific CISPR requirements for electronics that are sold, while another country may enforce another group of CISPR requirements.  The FCC requirements only relate to radiated and conducted emissions.  The difference between America and Europe is that there are no immunity limits, this is associated with European EMC certification.  Generally compliance with national or international standards are usually laid down by laws passed by individual nations, so it will vary from place to place.

 

CISPR is divided into various subcommittees depending on the specific type of electronics.  The different subcommittees are:

  • CIS/A - covers radio interference measurements and statistical methods
  • CIS/B - covers interference pertaining to industrial, medical, and scientific RF equipment
  • CIS/D - deals with electromagnetic disturbances that are related to electronic equipment on vehicles, and other devices that are power ed by internal-combustion engines
  • CIS/F - deals with interference relating to household appliances and lighting
  • CIS/H - sets the limits for the protection of radio services
  • CIS/I/ - deals with EMC of information technology, multimedia equipment, and receivers

Figure:  The different CISPR standards provide a guide to which standards your device needs to pass.

Figure 2:  CISPR regulations guide you to what standards your device needs to pass.

 

Compliance testing is a very formal process that is heavily regulated from place to place, so it is best to ensure your devices are likely to pass this compliance testing with the use of pre-compliance testing, which is a low risk, cost effective method to ensure you meet the final compliance requirements, depending on what country your product will be sold.

 

For more information on pre-compliance testing, check out the Making Conducted and Radiated Emissions Measurements application note for more information.  Please like, comment, or share! 

A big question that you might have is, what is the difference between Pre-Compliance and Compliance testing?  What makes Pre-Compliance important?  Pre-Compliance is a low risk, cost effective method to ensure your DUT will pass final Compliance testing.  Waiting until the end of a product development cycle for compliance testing is risky due to its high cost.  The cost includes reserving time in a compliance test lab, and the cost of redesigning your DUT if compliance testing does not pass.  Reserving time in the lab can be difficult, as these labs are in high demand.  It may be a long period of time before the lab is available, which could mean launch delays.  These are all unexpected expenses, in addition to expensive test time.

     

What is EMC Compliance Testing?

Electromagnetic Compatibility (EMC) Testing, is the interaction of electrical equipment with its electromagnetic environment, and other equipment.  All electronic devices have the potential to emit electromagnetic fields, and compliance testing is the final stage of testing  that ensures the electronic devices operate safely.

An example of poor compliance is when your TV picture quality is wrecked by wavy interference lines each time you turn on the kitchen blender.  This is something that shouldn’t happen – you should be able to have your TV operate normally, regardless of a kitchen blender running.  The blender produces electromagnetic waves that interfere with the TV signal.  Electronics need proper shielding to avoid interference with other devices.  This example is quite harmless, but if you think about unintentional electromagnetic interference on a larger scale, it could be a safety hazard, for example, it could corrupt data. 

Figure 1:  Final compliance testing is performed in an anechoic chamber

Figure 1:  An anechoic chamber is required for compliance testing

 

Common Measurements for Compliance

Developing your own EMC test lab will help you check your designs for compliance while they are in development and undergoing revisions.  Verifying your designs on your own is called pre-compliance testing.  Pre-compliance testing closely simulates the way compliance test s are run – putting your designs to test against actual test limits.  Once you are confident in your design, you can take it to a third party lab for final compliance testing.  This will make your testing more efficient and cost effective than relying on limited external laboratory tests.

The top most common compliance test failures are:

  1. Radiated Emissions
  2. Radiated Immunity
  3. Conducted Emissions
  4. Electrostatic Discharge (ESD)

Let’s go over what each of these are:

 

Radiated emissions testing

Radiated emission testing measures the radiated E-fields emanating from the DUT, and is usually the most common test failure. All devices will have some amount of emissions, but as long as they meet the requirements of your standards body you will be compliant.  A radiated emissions test involves measuring a DUT’s radiated emissions using a signal analyzer and an antenna.

 

Radiated immunity testing

The next common point of failure is radiated immunity.  Radiated immunity is a measure of how much external electric fields from external sources the CUT can tolerate before its performance starts to degrade.  This test set up requires 3 signal generators to cover the entire frequency range, RF broadband power amplifiers, and 2 – 3 antennas.

Figure 3:  Shows the relationship between radiated emissions vs. radiated immunity 

Figure 3:  Demonstration of the difference of Emissions and Immunity / Susceptibility

 

Conducted emissions testing

Conducted emissions testing focuses on the unwanted signals the DUT generates on the AC mains.  Both radiated and conducted testing are very important as you will not pass compliance testing if either of these fails.  For conducted emissions testing, your will need a spectrum analyzer equipped with EMC pre-compliance measurement software, line impedance stabilization network (LISN), and a limiter.  For more info refer back to the blog, "Complete your EMC conducted emissions testing in just 7 steps".

Figure 2: Radiated emissions set-up

Figure 2:  Sample set-up of a radiated emissions measurement

 

Electrostatic Discharge testing

ESD shield testing checks how immune the DUT is to static discharges, usually from operators touching key pads or touchscreens. The set up for ESD testing requires a ground plane that the DUT is connected to and several sheets of metal of various thickness to observe how the DUT interacts with the various planes, thickness and materials.    

 

Proper pre-compliance testing is crucial if you want to avoid surprises during compliance testing.  By checking your designs for electromagnetic compatibility during your design and verification work, you can ensure that you will pass compliance testing on the first try.  If you fail compliance testing, there’s a best-case scenario.  A simple design tweak fixes the issue and your only added cost is more time with the compliance lab.  As you probably know, the best-case scenario rarely happens.  Many times a failed compliance test means you have to do a significant design rework, which can mean a delayed product ship date.

 

For more information on pre-compliance testing, check out the Making Conducted and Radiated Emissions Measurements application note for more information.  Please like, comment, or share!  Stay tuned for the next one!

There are two kinds of EMC Pre-Compliance tests you can perform – radiated and conducted.  Today, we will review conducted emissions testing – what it is and why it’s important.  For a similar discussion on radiated testing, check out EMC Basics:  What is Radiated Emissions & Immunity Testing?.

 

Conducted emissions tests focus on the unwanted signals that are on the AC mains generated by the device under test (DUT).  Conducted RF emissions are electromagnetic disturbances (noise voltages and currents) that are caused by electrical activity in a DUT and is conducted out of the DUT along its interconnecting cables – for instance, power, signal, or data cables.  Conducted disturbances, in particular, a conductor, can couple directly into another electronic device or component within the same device.  This will provide unwanted signals that could lead to issues, like inaccurate performance.  This type of testing is one of the first group of tests performed in the process for EMC Pre-Compliance, followed by Radiated Emissions testing, Radiated Immunity, and conducted immunity testing.  The general procedure is to connect the appropriate equipment, load the limit, and load the correction factors.

Before we go through the steps to complete the conducted emissions testing process, let’s gather the equipment required.  These are common items that a test bench should have – these include:

 

  • Spectrum Analyzer equipped with EMC pre-compliance measurement software
  • Line impedance stabilization network (LISN) - The LISN is important because it isolates power mains from the DUT, which must have as clean of a signal as possible
  • Limiter
  • DUT

 

Now let’s go through the conducted emissions testing process in seven steps:

 

1.  Set up your test

Connect the signal analyzer to the limiter, LISN, and DUT.  Make sure the cord between the DUT and LISN is as short as possible to avoid the power cord from becoming an antenna.  Measure the signals on the power line with the DUT off.  If you see a signal approaching the established limit lines, you’ll want to set up some additional shielding so that these signals do not interfere with your possible conducted emissions from your DUT.  Shielding isolates components from each other to avoid coupling and interference that unwanted.

 

2.  Select your frequency range

Be sure you are measuring within 150 kHz and 30 MHz, which is the correct bandwidth for this measurement.   This is the corresponding frequency span that meets the CISPR requirement, which is a standard that is used for compliance testing. We will talk more about CISPR in another blog.

 

3.  Load the limit lines and correction factors

 

The two limit lines used for conducted emissions are EN5502 Class A quasi-peak and EN55022 Class A EMI average.  To compensate for measurement errors, add a margin to each limit line.

 

Figure 1:  Scan table where you can select the frequency span needed for the corresponding measurement.

 

Figure 2:  Conducted emissions display with limit lines and margin set

 

4.  Correct  for the LISN and the transient limiter

 

The transient limiter is used to protect the input mixer, basically acting as a filter or attenuator and is used with the LISN.  The correction factors for the LISN and the transient limiter are stored within the signal analyzer and can be easily recalled.  Correction factors adjust the reference plane for the DUT compensate for any loss through cables, space, etc.  Now you are able to view ambient emissions.  During this step, the DUT must be turned off.  If your emissions are above the limit, the cord between the LISN and DUT may need to be shortened.

Most radiated and conducted limits in EMC testing are based on quasi-peak detection mode.  Quasi-peak detectors weigh signals according to their repetition rate, which is done by having a charge rate faster than the discharge rate.  As the repetition rate increases, the quasi-peak detector does not have enough time to discharge completely, resulting in a higher voltage output.

 

The quasi-peak and average of the signals need to be measured and compared to their respective limits.  There are three detectors – Detector 1 will be set to peak, Detector 2 to Quasi-peak, and Detector 3 to EMI average.

 

Figure 3:  Loading correction factor files

 

5.  Locate signals above the limit lines

 

Switch on the DUT to find signals above the limit lines.  This is a good time to check to make sure the input of the signal analyzer is not overloaded by stepping the input attenuator up in value and seeing if they display levels do not change.

 

Figure 4:  Scan and search for signals above the limit lines

 

6.  Measure the Quasi-peak and average of the signals 

 

Most radiated and conducted limits in EMC testing are based on quasi-peak detection mode, which is available in the EMC X application.  Quasi-peak detectors weigh signals according to their repetition rate, which is done by having a charge rate faster than the discharge rate.  As the repetition rate increases, the quasi-peak detector does not have enough time to discharge completely, resulting in a higher voltage output. 

 

The quasi-peak and average of the signals need to be measured and compared to their respective limits.  There are three detectors – Detector 1 will be set to peak, Detector 2 to Quasi-peak, and Detector 3 to EMI average. 

 

Almost there!  We’ve got one more step to go!

 

7.  Review the measurement results 

 

The quasi-peak detector delta to Limit Line 1 & average detector delta to Limit Line 2 should all have negative values.  If there are some measurements that are positive, then there is a problem with conducted emissions from the DUT.  Before redesigning / troubleshooting the DUT with these results, check to ensure there is proper grounding if there are conducted emissions problems.

 

Figure 5: Quasi-peak and average delta to limit - the measurement results

 

Check these tips out for any troubleshooting issues: 

  • If the signals you are looking at are in the lower frequency range of the conducted band (2MHz or lower), you can reduce the stop frequency to get a closer look
  • You can add more data points by changing the scan table
    • The default scan table is two data points per bandwidth, or 4.5 kHz per point

 

To get more data points, change the points per bandwidth to 2.25 or 1.125 to give four or eight points per bandwidth. 

 

For more details on conducted emissions testings, check out the Making Conducted and Radiated Emissions Measurements application note for more information.  Please like, comment, or share!  Stay tuned for the next one!

After reading my last blog you’ve been given a brief intro on What are EMC & EMI Measurements and why it is important to measure for to ensure your device is pre-compliant.

 

In this blog we’ll discuss 2 of the 4 EMC pre-compliance tests – radiated tests, emissions and immunity – in more detail.

 

Radiated Testing

Radiated tests, as mentioned in the previous EMC basics blog, entail characterizing unintentional electromagnetic energy release from an electronic device. Radiated tests are the most common EMC test done around the world.

 

Many regulatory bodies across the globe set emission limit standards that all electronic products must meet. Looking at Figure 1 below we see a setup that is commonly seen at a test house, where your product will eventually have to acquire certification from. Your product is placed in a semi-anechoic chamber with an antenna directed at it to capture any phenomena radiating from the device.

 

However, before even sending your device to a test house, it’s important to first do pre-compliance tests of your own. This will not only save you money, but also avoid throwing off your schedule if your device fails at a test house and redesign work is needed. Before we get into how the tests are made, let’s get a better idea of the different kind of radiated tests.

 

Figure 1: Semi-Anechoic chamber where an EMC/EMI test is being conducted with dual antennas. The material of the chamber does not allow signals to leave or enter the room to ensure accurate measurements are made.

 

Radiated Testing - Emissions

Radiated emissions testing can be a bit more complicated than say, conducted emissions testing. This is because radiated tests entail through the air testing, which adds some complexity in how we can accurately measure emissions from a device (Figure 2). The complexity is attributed to the ambient environment, which can interfere with your device under test’s (DUT’s) emissions measurement. So, as part of emissions test, it is important to be able to identify what signals are coming from your device versus the ones coming from the environment. The topic of conflicting ambient signals will be discussed in more detail in a later blog.

 

When testing for radiated emissions, your intention is to determine the electromagnetic energy strength of the emissions that are being output by your device. Most devices usually have some sort of emission, but it is a question of whether those emissions from your device are compliant with the standards set by the regulatory body of your respective region. Pre-compliance testing is done to answer this very question.

 

Figure 2: An example of what emissions and immunity testing entail.

 

Radiated Testing – Immunity

Radiated tests for immunity entail testing the susceptibility of your device to emissions from other surrounding devices (Figure 2). Let’s say you work in a company that designs and manufactures phones. At some point you must determine what these phones’ susceptibility are to the emissions that, let’s say, a nearby laptop may have. Immunity is important to test for because you do not want your device to be accidentally influenced by a neighboring device.

 

Conclusion

Now that you have a better understanding of the types of radiated tests that exist and what they are, knowing how to test for them is the next step. Furthermore, let’s not forget that radiated tests only give you half of the story of EMC pre-compliance testing, as you should also have a good grasp on what conducted tests are – radiated and immunity. This will be discussed in a later blog here on the RF Test blog.

 

For an even more expansive and detailed look into EMC tests, check out the Making Conducted and Radiated Emissions Measurements for more information. Finally, if you enjoyed the blog make sure to give it a like, comment or share! Thanks for reading, and I look forward to seeing you in the next one.

Introduction:

Conceptualizing your next product, designing it, and then releasing it to market are more or less the main phases encountered during a product development cycle. But what if you had gotten as far as finishing your product design only to discover late that you cannot push the product to market, because it didn’t meet some standards set by local regulators. Therefore, testing your product early in the cycle to make sure it works appropriately is just as important as designing it. In this blog we’ll discuss the basics of EMC and EMI pre-compliance – something all electronic products will eventually have to get certified compliance for in a test house. However, compliance testing certification occurs very late in the development process and if you instead did pre-compliance testing earlier on, fixing these problems are easier and less costly (Figure 1).

 

Product Development Cycle including EMI Testing

Figure 1: Pre-compliance testing can uncover problems during earlier stages of development, where solutions are easier and less costly. It can also reduce the risk of design rework and associated schedule delays.

 

What is It

Let’s first figure out the difference between EMC and EMI. EMC and EMI stand for electromagnetic compatibility and electromagnetic interference respectively. EMC, you can say, is the umbrella term whereas EMI is the actual phenomena you will be testing for.

 

All electronic products, whether it be your smartphone or your smart refrigerator, must eventually pass compliance tests in a recognized test house and be certified before they can be brought to market. This certification is required as it demonstrates that your product won’t electromagnetically interfere or be interfered with by any other electronic products in close proximity. To get your device certified, test houses are mainly concerned with making sure your device can pass 4 EMC tests in particular – radiated emissions and immunity tests and conducted emissions and immunity tests.

 

In short, the difference between radiated tests (emissions and immunity) and conducted tests (emissions and immunity) is that the first refers to unintentional release of electromagnetic energy from an electronic device. The latter refers to internal electromagnetic emissions propagated along a power or signal cable, creating noise. Looking at Figure 2 below we see a snapshot of the 4 different tests.

 

EMC Testing, Conducted & Radiated Emissions

Figure 2: Four types of EMC Measurements.

 

The difference between emissions and immunity tests are that the emissions is concerned with the amount of electromagnetic energy emitted from your device while immunity is concerned with how susceptible your device is to electromagnetic energy being emitted from surrounding devices.

 

Why Testing For It Matters

Regulatory bodies, like the FCC in the US, set up standards (CISPR) that devices are tested against. As you would expect, standards vary from one device to another – meaning you would measure the amount of EMI on your smartphone differently than you would military-grade avionic equipment. If you have a good signal analyzer with an EMI application, then you’ll have pre-loaded and configured limits for CISPR and MIL-STD tests that allow you test against these standards quickly. EMI measurements are made to ensure that there is no interference between devices when in operation. When you’ve designed your product, and have it sent off to a test house, the test house will traditionally put your device in an anechoic chamber. An anechoic chamber is used to completely block out signals exiting from within the chamber to outside of it and other signals outside the chamber from entering in it. Within the chamber an antenna is used to test a number of different points on your device. This anechoic chamber setup is similar to the one you see below in Figure 3

 

 EMC Test in an Anechoic Chamber

Figure 3: EMC Test in an Anechoic Chamber. An antenna is directionally pointed at the device under test (DUT).

 

However, simply sending off your product for EMC testing at a certified test house is not the answer. This is because these tests are very expensive to do, and you don’t want to risk having to do design revisions and throwing off your entire schedule. That’s why it’s important to do some due diligence on your end.

 

What’s The Solution

The solution is simple – make sure you conduct your own EMC tests in house (pre-compliance testing) for your device prior to sending it off to a test house for certification (compliance testing). In the off chance that your device doesn’t pass at the test house, it will not only throw off your design cycle and time to market, but also cost you a lot of money.

 

This is where signal analyzers come in handy. Using a signal analyzer, EMI close-field probes and an EMI application, you can conduct your own EMI measurements in house to ensure your device is on track to fulfilling the EMC requirements of the standard that it will be tested against at a test house. You can take a look at Figure 4 below to see some example tools you can equip yourself with to conduct EMC pre-compliance measurements.

 

EMC Testing Tools

Figure 4: A set of tools you can use to conduct your own EMC testing.

 

Conclusion

So, in summary, if you are working towards getting your device to market you definitely need to make sure your device is EMC pre-compliant prior to sending it off to a test house for certification. A signal analyzer with an EMI application and EMI close-field probes are the right tools you need for making sure you can test for EMI accurately. Your design cycle will not only stay on track, but you will also make your manager one happy, and less broke, person.

 

Thanks for taking the time to read this blog, if you enjoyed it feel free to give it a like, comment, and share. Stay tuned for more blogs to come that discusses more about EMC testing. For more in-depth information on EMI conducted and radiated measurements check out the following application note: Making Conducted and Radiated Emissions Measurements.

 

 

 

You have your workspace set up with your oscilloscope, waveform generator, and vector network analyzer. Now all you're missing is some desk space to actually do your work. What if I told you that you could get the same performance from machines half the size of your current setup? Small form factor equipment is a great option for labs where space, cost, or portability are concerns.

 

The P937xA Keysight Streamline Series VNAs are easily controlled via PC.

Figure 1. The P937xA Keysight Streamline Series VNAs are easily controlled via PC.

 

The P937xA is part of Keysight’s Streamline Series, a new family of faceless USB instruments, including vector network analyzers (VNAs), oscilloscopes, and an arbitrary waveform generator (AWG). By moving the user interface (UI) to a computer, we were able to pack our proven hardware into small devices with incredible performance for the price.

 

When you buy instruments, you’re not just buying the hardware. You’re buying into the manufacturer’s ecosystem.

 

In addition to the dependable hardware, the Keysight Streamline Series also comes with our intuitive UI to give you the same experience as full-size instruments. This means that the P937xA has the same SCPI commands, GUI, and measurement science as both our benchtop and modular vector network analyzers. You can also run our common benchtop applications and software upgrades.

 

There are a lot of USB instruments out there now. When shopping for small form factor equipment, it can be hard to see the difference between each vendor when they have somewhat similar specs on paper.

 

Let’s dive into some of these specs and the differences between two of the most popular compact USB vector network analyzers on the market: Keysight’s P937xA and Anritsu’s Shockline MS46122B.

 

In RF measurements, the name of the game is minimizing interference. In today’s wireless world, RF signals and devices must fight all kind of interference. It can feel like the universe is against you when testing RF equipment – and it actually kind of is – thanks to interference from lightning, solar flares, and Earth’s magnetic field.

 

Fortunately for you, modern VNAs are built so precisely that even smaller USB instruments boast impressive specifications. You can see the banner specs of the MS46122B and the P937xA summarized in Table 1 below.

 

Performance / Spec

Keysight P937xA

Anritsu MS46122B1

Min Frequency

300 kHz

1 MHz

Max Frequency

4.5 / 6.5 / 9 / 14 / 20 / 26.5 GHz

8 / 20 / 43.5 GHz

Number of Ports

2 / 4

2

Dynamic Range2 @ 10 Hz IF BW

@4 GHz

115 dB

100 dB

@20 GHz

110 dB

100 dB

Trace Noise

@4 GHz

0.003 dB [1 kHz IF BW]

0.006 dB [10 Hz IF BW]*

Power Range

@4 GHz

-40 to +7 dBm

-20 to +5 dBm

@20 GHz

-40 to +2 dBm

-20 to -3 dBm

Stability dB/deg. C (typical)

@4 GHz

0.02

0.02

IF Bandwidth

10 Hz to 1.2 MHz

10 Hz to 300 kHz

*Performance is characteristic, not typical

1 All Anristu specifications were found in the ShockLine™ Compact Vector Network Analyzers MS46122B datasheet (PN: 11410-00995 Rev. F) on June 20, 2018.

2 Standard (not typical)

Table 1. Keysight P937xA and Anritsu Shockline MS46122B banner specifications.

 

It’s easy to just throw numbers around, so I’m going to explain why you should care about these numbers.

 

Dynamic range, power range, and IF bandwidth are the holy trinity of noise reduction. Flexibility with these specs gives you space to separate your signal from the noise.

 

Dynamic range is the power range of simultaneous input signals that can be accurately measured. This is critical for applications like characterizing filters where the stopband and passband power levels can vary greatly. A wide dynamic range provides more room to set wide IF bandwidth to speed up your measurements.

 

A good power range is essential for characterizing nonlinear devices with a power sweep. It also allows you to run tests without an external power amplifier, saving bench space and keeping your cost of test down.

 

IF bandwidth is one of the most important network analyzer parameters. It offers control over the trade-off between noise reduction and measurement speed. As you can see, the P937xA gives you about a half-order of magnitude more frequency to work with.

 

It sounds great on paper, but how does it look?

 

We all want our products to get to market as fast as possible. An intuitive user interface is critical in speeding up the development process. Easily move through each phase with quick access to common tools and menu options.

 

Note: All Anristu screenshots below were taken on a MS46122B by a Keysight employee on February 21, 2018.

 

User interface of Keysight P937xA vs. Anritsu Shockline MS46122B.

Figure 2. User interface of Keysight P937xA vs. Anritsu Shockline MS46122B.

 

Setting a trigger is one of the most basic needs in making measurements. Having the trigger button right on the main menu of hardkeys makes it so you are always one button press away from the full trigger menu. No need to dig through various menus to set up one of the most critical components of a measurement.

 

Trigger menu on Keysight P937xA vs. Anritsu Shockline MS46122B.

Figure 3. Trigger menu on Keysight P937xA vs. Anritsu Shockline MS46122B.

 

Adding a trace, undoing or redoing an action, or taking a snapshot has never been easier. The P937xA user interface has shortcut icons along the top of the screen for common quick actions.

 

Quick action buttons on Keysight P937xA vs. Anritsu Shockline MS46122B.
Figure 4. Quick action buttons on Keysight P937xA vs. Anritsu Shockline MS46122B.

 

Another helpful capability of the P937xA is the setup wizard. Having all the fundamental steps and parameters in one place makes setup a breeze. There is no need to dive into multiple menus when you have everything in one place.

 

Main parameter setup on Keysight P937xA vs. Anritsu Shockline MS46122B.

Figure 5. Main parameter setup on Keysight P937xA vs. Anritsu Shockline MS46122B.

 

The P937xA also supports context menus with a press and hold (or a right click). These menus give you quick access to different features depending on where you click.

 

Right click quick access menus on Keysight P937xA.

Figure 6. Right click quick access menus on Keysight P937xA.

 

Another feature that will come in handy is the drag and drop for different trace views. This lets you simply drag each trace and place it in a portion of the screen that makes it easier to view the data. This helps make the view more customizable to your specific tests rather than being restricted to a pre-defined menu.

 

Easy drag and drop traces on Keysight P937Xa vs. display options on Anritsu Shockline MS46122B.

Figure 7. Easy drag and drop traces on Keysight P937Xa vs. display options on Anritsu Shockline MS46122B.

 

As we’ve seen, the P937xA’s interface is full of intuitive features to make your measurements easier.

 

In summary, we’ve seen that the P937xA offers excellent measurement capabilities with the same UI you would get from a high-performance benchtop instrument. The P937xA’s stability and very low trace noise mean that once you calibrate it, you can be confident in your measurements. The intuitive user interface ensures you’ll test more efficiently and speed up your development phases. Altogether, the new Keysight Streamline Series USB VNAs pack big performance into a small package so you can fit even more functionality on your bench.

 

Compact form. Zero compromise.

 

Click here to learn more about Keysight’s Streamline Series USB vector network analyzers. Or check out the video demo.

In the previous edition of The Four Ws, I reviewed the fundamentals of adjacent channel power (ACP). This time I’m discussing the WHAT, WHY, WHEN and WHERE of harmonic distortion measurements. Measuring harmonic distortion will help you validate the proper functioning of your device’s components and, in turn, avoid interference with systems operating in other channels.

 

What is harmonic distortion?

From simple continuous waves (CW) to complex digitally-modulated signals, every real signal has some amount of distortion. One type of distortion to consider is the total harmonic distortion (THD). The THD value indicates how much of your device’s signal distortion is due to harmonics. These harmonics are energies created at various multiples of the frequency of your signal where none previously existed or should exist. This extra energy is frequently caused by nonlinearities in the transfer function of a circuit, component or system. In practical systems, nonlinearities are due to gain compression, transistor switching or source-load impedance mismatches.

 

An 850-MHz signal with obvious harmonics on both sides.

Figure 1: A basic swept measurement made with an X-Series signal analyzer shows an 850-MHz signal with obvious harmonics on both sides.

 

To calculate THD you need to determine the ratio of the sum of the power of all surrounding harmonic components to the power of your device’s fundamental signal:

To calculate THD you need to determine the ratio of the sum of the power of all surrounding harmonic components to the power of your device’s fundamental signal.The resulting THD is stated in dBc.

 

Why and When to measure THD

THD is typically characterized during design validation and troubleshooting when you are confirming that your signal is behaving as expected. Your THD will indicate if your device’s surrounding harmonics will affect your signal quality or interfere with another device.

 

You want the THD to be as low as possible. This implies that your device has a nearly pure signal making it unlikely that it’s harmonics will cause interference. On the other hand, a high THD means that you may need to rework your design because the distortion could negatively affect your signal quality or create interference in other channels.

Measuring THD can also be an effective indicator of overall signal performance. In an amplifier, for example, excessive THD indicates issues like clipping, gain compression, switching distortion, or improper transistor biasing or matching.

 

An example of Where distortion shows up and how you measure it

A simple, real-world example of harmonic distortion is found in audio speakers. Let’s say you’re playing a song from your phone and you hook it up to a speaker. If the speaker’s internal components – amplifiers and filters – give us an accurate reproduction of the song, then the speaker has a low amount of distortion. On the other hand, if the speaker’s internal components give you a misrepresentation of the song then it has a high amount of distortion. Therefore, you want your device’s THD value to be as low as possible to maintain good signal quality.

 

Another issue harmonic distortion can cause is interference with other signals. Since harmonic distortion is unwanted energy at the harmonics (integral multiples) of the fundamental frequency, the distortion can interfere with another device that is operating in the same band as the harmonic. Therefore, a low THD value is also a good indicator that interference is less likely to occur.

 

Seeing your signal’s harmonics can be difficult to observe and measuring them can be quite time consuming if done manually. You’d have to identify all the harmonic power levels, sum them, and then find the ratio to the power of your device’s signal. That is a hassle.

 

However, some signal analyzers provide a built-in measurement that will automatically calculate THD for you. This can shorten your measurement time and ensure an accurate calculation.

 

The built-in harmonics measurement calculates the THD and results for up to 10 individual harmonics.Figure 2. The built-in harmonics measurement on an X-Series signal analyzer quickly calculates the THD for the same 850-MHz signal seen in Figure 1. In addition to THD, the measurement shows results for up to 10 individual harmonics.

 

Using the harmonics measurement shown in Figure 3, you can calculate the total harmonic distortion and the results for up to ten harmonics, automatically.  All you have to do is set the fundamental frequency and the measurement takes care of the rest.

At each cycle, the analyzer performs an accurate zero-span measurement of the device’s signal and each of its harmonics. It calculates the level of each harmonic, as well as the total harmonic distortion of the signal, both of which are shown in dBc. The harmonic distortion measurement used in our example supports signals from simple CW to complex multi-carrier communication signals.

 

Wrapping up

Knowing the total harmonic distortion of your signal can help you evaluate if your device will cause any interference with its own signal or with systems operating in other channels. If you identify troublesome harmonics, you’ll have to rework your design and use something like a filter to tune them out.

THD is just one of nine RF power measurements made easy with PowerSuite, a standard feature on the X-Series signal analyzers. If you’d like to learn more about power measurements, check out the PowerSuite page and the Making Fast and Accurate Power Measurements application note.

 

I hope my fourth installment of The Four Ws provided you with some worthwhile information. Please post any comments – positive, constructive, or otherwise – and let me know what you think. If this post was useful give it a like and, of course, feel free to share.

This week’s post is guest authored by Charlie Slater, Business Development and Operations Manager for Keysight Services.

 

These days, most organizations operate within one of two scenarios: cutting costs while delivering the same topline, or holding costs steady while increasing revenues. The third, less-common scenario is investing more to create a giant leap in output. If you’re in this fortunate group, confidence in future growth usually opens the door to major investments in plant, property and equipment (PPE)—and the “E” in PPE includes test equipment. Optimizing the management of test assets can help you create some semblance of order within the chaos.

 

Uncovering some unexpected side effects of rapid growth

Surprising problems can arise when your organization is moving at high speed. Several months ago I met with a manager in a high-growth company. Our purpose was to plan for onsite delivery of calibration services. When creating such a plan, key baseline information includes the location and condition of all in-hand test assets.

 

As we talked, it became clear that he had incomplete data about his company’s installed base of test equipment. Further discussion revealed the unexpected cause. The company’s engineers had extremely high purchasing authority and pallets of new network analyzers and spectrum analyzers were coming in every day. The manager had virtually no idea what was arriving and limited visibility into what his engineers were actively using or even if the equipment was in working order.

 

Gaining control of test assets and getting more from each one

During chaotic growth, sticking to the basics can help contain spending and restore order to an organization. For the company described above, accurate tracking of all new and existing RF equipment helped get its inventory under control. Today, better monitoring enables compliance with internal and external quality standards, and this includes staying up to date with test-asset calibration.

 

The underlying solution is real-time tools that provide centralized visibility. This enhances productivity by letting managers and engineers find and reassign unused instruments rather than waiting for delivery of new ones.

 

For any organization, real-time monitoring can pinpoint instruments that are underused or idle. In many cases, the most cost-effective way to refresh a languishing-but-viable test asset is an update or upgrade—and new functionality may be just a download away. For hardware upgrades that require installation, the turnaround time is usually shorter than the lead-time for a new instrument.

 

Exploring all three scenarios

To learn more, check out our latest resources, including a white paper about how to best enable 4G to 5G migration and a case study about how one company improved the health of their test assets.

 

Please chime in with any and all comments. How have you tried to optimize your situation? What worked best and why?

In a rock band, the drummer keeps the beat steady and the other musicians follow the rhythm. The drummer keeps the entire band in synch. The same concept is true when you integrate multiple instruments into a test system. The individual instruments need to be synchronized, especially when you are making multi-channel RF measurements. Like a drummer, a trigger and a reference clock communicate the “beat” to synchronize the instruments so they can make precise, time-aligned measurements. Let’s take a closer look at multi-channel measurements and how to achieve an accurate multi-channel test setup.

 

Multi-antenna RF techniques

Most modern wireless systems, whether in commercial applications or aerospace and defense, have adopted some kind of multi-antenna technique, such as MIMO (multiple input, multiple output), beamforming or phased-array radar. These techniques improve:

  • Spectral efficiency (bit/sec/Hz)
  • Signal quality
  • Signal coverage

 

For example, MIMO increases data rates by using two or more streams of data transmitted with multiple antennas. The antennas transmit the data on the same frequency and at the same time without interfering with one another, as shown in Figure 1. Spectral efficiency is improved using the same bandwidth.

 

Simplified 2x2 MIMO system

Figure 1. A simplified 2x2 MIMO system with two transmitters and two receivers.

 

Keys to synchronize multiple instruments

While MIMO and other technologies deliver increased data rates, they also increase the number of antennas in a device. And, as the number of antennas increases, test complexity increases significantly. For example, the latest IEEE WLAN technology, 802.11ax, use up to 8x8 MIMO. That means your test setup must have eight transmit channels and eight receive channels! And, it’s crucial that they are synchronized. 

 

To synchronize your test system, there are three key elements: the trigger, the sampling clock, and the event-signal effects.

 

An easy method to synchronize multiple instruments is to use a trigger. A trigger is a coordination signal that is sent to each instrument in a test setup. When the trigger signal is detected, each instrument performs its task. Using a trigger signal ensures all your instruments are in synch. However, there are two sources of error that must be addressed:

  1. Sampling clock: Even when all the instruments being triggered are identical, for example your signal generators, the initial phase of each instrument’s sampling clock is random. To align the sampling clock of each instrument, use the same reference frequency for all the instruments.
  2. Event-signal effects: Cabling and external devices can affect how long it takes your trigger signal to reach each instrument. This is called trigger delay. These event-signal effects need to be accounted for so that your instruments still transmit or receive at the same time. Using a channel skew control on your master instrument allows for precise time synchronization between all channels.

 

Figure 2 illustrates two arbitrary waveform generators (AWGs) that are in time alignment. Here’s a quick review of the setup:

  • First, use a common frequency reference to synchronize the timing clocks for all instruments.
  • Second, connect the master's "trigger out" to the slave's "trigger in" connector. The AWG will start generating the signal after a trigger event is detected.
  • Finally, remove the effects of master-to-slave trigger delay to align the two waveforms. The trigger delay can be measured with an oscilloscope or a digitizer. Then, add the delay time to the master AWG.

 

This process also applies to analyzers. You can use one splitter to distribute signals to a multi-channel analyzer and measure the time differences among the channels. The relative delays of each channel can be compensated by application software. Having the timing synchronized between the instruments allows you to build a multi-channel RF test system.

 

Two AWGs configured to generate time-aligned signals.

 

Figure 2. These two AWGs (master and slave) are configured to generate time-aligned signals. To remove the effects of master-to-slave delay, it is necessary to delay the signal generated by the master.

 

Modular instruments can make implementation easier

While the number of synchronized channels increases, the cabling between the instruments becomes much more complicated and achieving proper time-synchronization can take a significant amount of time. Modular instruments are based on standard instrumentation buses such as PXI, AXIe, and VXI. These instruments can share clocks and trigger signals through a backplane bus. This makes it easier to implement synchronization and makes the trigger events more repeatable because the test environment is controlled with minimal cabling.

 

For example, a PXI trigger bus consists of eight trigger lines spanning the backplane connectors. The trigger lines are divided into three trigger bus segments, slot numbers 1-6, 7-12 and 13-18. Figure 3 shows an easy PXI trigger setup with Keysight IO Library software.

 

PXI triegger setup using Keysight IO Library software

 

Figure 3. PXI trigger setup using Keysight IO Library software. In this example there are eight trigger lines (0-7) and three bus segments. The trigger routing direction between the segment of each trigger line can also be configured.

 

Figure 4 below shows two PXI chassis being used as a WLAN 802.11ax test solution that fully supports 8x8 MIMO. The PXI backplane bus routes trigger signals to target modules for eight-channel signal generation and acquisition. This system takes advantage of the PXI standards that minimize a chassis’ slot-to-slot trigger time and clock skew to hundreds of pico seconds.  This results in very accurate timing synchronization so you don't need to make any adjustment for MIMO transmitter and receiver testing.

 

WLAN 802.11ax test solution for 8x8 MIMIO.

Figure 4. WLAN 802.11ax test solution that fully supports 8x8 MIMO configuration in two PXI chassis.

 

Trigger and Time Synchronization Lead to Better Testing

To effectively test today’s multi-channel devices, you must perform tightly synchronized, multi-channel signal generation and analysis. With accurate triggering among the instruments, you ensure that all measurements start at precisely the right time. (If you require carrier phase coherency, you will also need to use a common local oscillator (LO) reference.) To simplify your test synchronization, consider a modular test system that allows easier integration of multiple instruments into a multi-channel test system.

 

If you’d like to know more about instrument interactions, refer to the following application note: Solutions for Design and Test of LTE/LTE-A Higher Order MIMO and Beamforming.

 

If you like this post, give it a like and feel free to share. Thanks for reading.

As you walk through your lab, take a look at each RF bench. How old are your signal and network analyzers? How often are they kludged together to create one-off measurements? How recently have your engineers bugged you about getting new equipment that can actually test your latest RFIC?

 

I’m here to help you make a stronger case when your team’s success depends on timely access to better RF instruments. This post introduces language, concepts and solutions that will help you influence purchase decisions and improve your chances of getting the right tools at the right time. When you apply these ideas, your newfound business sense may surprise—if not impress—your boss or boss-squared.

 

Understanding your current reality

Day to day, you deal with competing objectives: delivering excellent results while staying within stringent constraints. From a high-level business perspective, there are three ways to do this: cut costs and deliver the same topline; hold costs steady and increase revenues; or invest more to create a giant leap in output. These days, most organizations operate within the first two scenarios while fast-growing companies chase the third.

 

Getting the right tools at the right time (and place)

Whichever situation you face, one of your biggest issues is likely to be test equipment. In fluent “manager speak,” “test assets” are often your organization’s most “underutilized assets.” Why? Because it’s difficult to confidently determine two crucial bits of information: the location of every instrument and how much each one is truly being utilized.

 

For you and your team, easy access to the right tools enables everyone to do their best work and stay on schedule. Applying manager-speak once more: for “technical staff,” “highly available” test equipment can be a “high-leverage asset.”

 

Pushing for better decisions in less time

An accurate view of location and utilization is essential to making credible decisions in less time: Do you need to purchase or rent additional equipment? Is it better to redeploy, upgrade, trade in or sell some of your existing gear?

 

A few basic changes can provide three big benefits: better visibility, improved utilization, and reduced expenses (capital and operating). The starting point is a solution that puts real-time information at your fingertips. Relevant information about test-asset location and utilization is essential to greater availability and improved productivity.

 

Taking the next steps

Being able to make quick, thoughtful decisions on how to best equip your engineers with the right tools is the foundation for a successful organization. To learn more, check out our latest resources to better understand how to drive down your total cost of ownership.

 

Please chime in with any and all comments. How difficult has it been to get the test tools your team needs? What techniques have you used to help make it happen?

Prove yourself as an engineer! The Schematic Challenge is the perfect opportunity to test your skills. On March 12, 13, and 14, we will be posting a new schematic or problem-solving challenge. If you, as a community, are able to answer questions 4, 5, and 6 correctly by Thursday, March 15 at 11:59 PM MST, we will add three 1000 X-Series oscilloscopes to the overall Wave 2018 giveaway! Answers should be posted in a comment on the #SchematicChallenge posts on the Keysight Bench or RF Facebook pages. Work with your family, friends, coworkers, or fellow engineers in the Wave community to solve these problems. If you haven’t already, be sure to register for Wave 2018 at wave.keysight.com.


Question 4:

By Ryan Carlino

 

Status: SOLVED! (A=1 and B=2)

 

Week2 Q4 Schematic Challenge Wave2018

Given this circuit and assuming an ideal op-amp powered by +/-5V and ideal resistors, calculate the output voltage with respect to the input. Vin will be limited to +/-1V.

Express this transfer function like this:
Vout = A*Vin + B

The answer being posted should be a single number AB. For example, if A=4 and B=7, the answer you should post is 47.

 

Question 5:

By Jonathan Falco and Lukas Mead

 

Status: SOLVED! (90 MHz)

 

What integer frequency in MHz should the LO be set, to allow the RF input range to be seen on OUT?

 

.


Question 6:

By Barrett Poe

 

Status: SOLVED! (4-10-8-8)

 

You are asked to design the front end of a 10 MHz oscilloscope. The “front end” refers to the internals of an oscilloscope between the probe and the analog to digital converter. Your system requires you take a +/- 10V signal input, and output a 0-3.3V signal to the ADC input, which is terminated at 50 Ohms. Your circuit must scale, offset, and filter the incoming signal, then rescale it to the full range (within 10%) of the ADC’s reference voltage without clipping the sampled signal.

 

Oh no! You also just discovered your supplier has discontinued your favorite ideal operational amplifier (opamp). Your next two best choices are:

  • Opamp with 1 pF of capacitance on the inputs
  • Opamp with 10 pF of capacitance on the inputs

Make sure your design works with both of these back-up options. However, note that you will only use the same parts together. Meaning, you will only ever have two 1 pF opamps OR two 10 pF opamps, never one of each.


Also keep in mind – opamp output voltage cannot exceed the supply rails.

Output Voltage = 0 to 3.3V; Ensure Vout is +0%/-10% of ADC range for max input across bandwidth

Frequency = DC to 10 MHz

 

Assign a value to variables a, b, c, and d. The final answer to be posted on Facebook should be expressed as: a-b-c-d. For example, if a = 8, b = 6, c = 12, and d = 10, then the answer should be expressed as: 8-6-12-10.

 

HINTS:

The variable “a” is equal to one of these three options

  • c-1
  • 4
  • 4b

The variable “b” is equal to one of these three options

  • c+1
  • 4
  • 10

The variable “c” is equal to one of these three options

  • b/2
  • 6
  • 2*a

The variable “d” is equal to one of these three options

  • 8
  • (b+2)/3
  • 10

Helping You Achieve Greater Performance and Fast Measurement Speed

At an exhibition demo booth, an engineer complained to me about the measurement speed of a PXI oscilloscope. To make a measurement, he programmed the data acquisition and post-analysis himself. The test took him over a minute to get each result. I told him that he didn’t have to do all of that; all he needed to do was setup the measurement on the oscilloscope and fetch the measurement item result directly. The process should only take a couple of microseconds. An on-board ASIC helps minimize data transfer volumes and speed-up analysis!

 

Like an oscilloscope has on-board digital signal processing, RF signal analysis tools also have on-board processing to accelerate measurement speed.

 

RF Measurement Challenges

For RF signal analysis, it's common to frequency-shift the RF signal to an intermediate frequency (IF) so that you can use a high-resolution digitizer for a high dynamic range signal acquisition. This then gets sent to a PC for data analysis. However, the complexity of this analysis increases with today's wireless communication systems, such as 5G technologies, 802.11ax standard and so on. Measuring these systems can include complex modulation schemes (e.g., orthogonal frequency-division multiplexing, OFDM), carrier aggregation, or MIMO (multi-input multi-output) signals.

 

These complications require significant signal processing, which in turn slows the measurement speed. This is a challenge as measurement throughput is critical in most applications, especially in high volume production testing.

In most signal analyzers, a digitizer is an indispensable component. For wider bandwidth analysis, you need a high-speed digitizer to capture signals. At the heart of a high-speed digitizer is a powerful FPGA or ASIC that processes data in real-time. This allows data reduction and storage to be carried out at the digital level, minimizing data transfer volumes and speeding-up analysis.

 

A key feature often available on digitizers is real-time digital down conversion (DDC). In frequency domain applications, DDC allows engineers to focus on a specific part of the signal using a higher resolution, and transfer only the data of interest to the controller/PC. It works directly on ADC data providing frequency translation and decimation sometimes called "tune" and "zoom." The block diagram shown in Figure 1 illustrates this basic concept of DDC.

 

Digital down-converter block diagram

Figure 1. Digital down-converter block diagram

 

How DDC Works

The frequency translation (tune) stage of the DDC generates complex samples by multiplying the digitized stream of samples from the ADC with a digitized cosine (in-phase channel) and a digitized sine (quadrature channel).

The in-phase and quadrature signals can then be filtered to remove unwanted frequency components. Then, you can zoom in on the signal of interest and reduce the sampling rate (decimation).

 

Finally, the on-board processor sends only the data you care about (I/Q data) to the on-board memory for further analysis. Most of Keysight's digitizers and signal analyzers have implemented DDC to accelerate measurement speed and for demodulation acceleration.

 

In addition, you can perform FFT with I/Q data in parallel for spectral analysis.  Some signal analyzers can do real-time FFT processing (nearly 300,000 times/second) and use comprehensive spectrum displays (density and spectrum) so that you won't lose any agile signals on the screen, shown in Figure 2.

 

Real-time spectrum analysis at 2.4 GHz ISM bandFigure 2. Real-time spectrum analysis at 2.4 GHz ISM (industrial, scientific and medical) band

 

Benefits and Limitations of a High-Speed Digitizer with DDC

Using a high-speed digitizer with DDC for your RF testing can be significantly more efficient:

  1. The frequency translation (tune) reduces both on-board memory and data transfer requirements. The resulting data is in complex form (I+jQ), which is usable for demodulation analysis directly and accelerates measurement speed.
  2. Digital filtering and decimation (zoom) reduce the wideband integrated noise and improve overall SNR.

 

However, there are some limitations with DDC implementation:

  1. The ADC's sampling rate is limited. It's not possible to digitize the high-frequency carrier directly. A common workaround is to use an analog circuit to bring the carrier to an IF so the digitizer can acquire the signal.
  2. The ADC's dynamic range is also limited. In wireless communication systems, you may need to capture both large and small signals at the same time.

 

New generations of high-speed and high-resolution ADC technologies provide excellent resolution and dynamic range into the tens of GHz, which allows you to capture high-resolution wideband waveforms. DDC accelerates measurement speed and increases processing gain to improve performance.

 

Furthermore, the I/Q data can be processed further for advanced real-time signal analysis or sent to a customized FPGA for user-defined signal processing algorithms. These provide you better RF measurement fidelity, signal integrity and higher measurement throughput.

 

If you’d like to learn more about wideband signal acquisitions, you can refer to the following white paper Understanding the Differences Between Oscilloscopes and Digitizers for Wideband Signal Acquisitions to understand what you should be using for your application.

Like any RF engineer, there comes a time in your product’s design cycle that you need to test your device to make sure it’s behaving as you expect. There are different ways you can view your device’s signal, which brings us to why measuring signals in the time domain and frequency domain is the same, but not. This is because they both convey the same signal, but in a different way.

 

Figure 1. The time domain of a signal on the left, and the frequency domain of the same signal on the right. The time domain displays a signal in respect to amplitude vs. time whereas the frequency domain displays amplitude vs. frequency.

 

By properly combining spectrum, or a collection of sine waves, you can view the time domain of your signal. It shows your signal’s amplitude versus time. This is typically done using an oscilloscope. Why would you want to view your signal in the time domain, you ask? Basically, a time-domain graph shows how a signal changes with time. This lets you see or visualize instances where the amplitude is different.

 

Viewing your device’s signal in the time domain doesn’t always provide you with all the information you need. For example, in the time domain you can decipher that a signal of interest is not a pure sinusoid, however, you won’t know why. This is where the frequency domain comes in. The frequency domain display plots the amplitude versus the frequency of each sine wave in the spectrum. This may help you discern why your signal isn’t the pure sinusoidal wave you were hoping it to be.

 

Figure 2. Harmonic distortion test of a transmitter, which is most appropriately measured using a spectrum analyzer in the frequency domain.

 

The frequency domain can help identify questions about your signal that you wouldn’t be able to see in the time domain. However, this doesn’t mean that you can just scrap measuring signals in the time domain altogether. The time domain is still better for many measurements, and some measurements are only possible in the time domain. Examples include pulse rise and fall times, overshoot, and ringing.

 

But just like the time domain has its advantages, so does the frequency domain. For one, the frequency domain is better for determining the harmonic content of a signal (as seen in Figure 2). So, those of you in wireless communications who need to measure spurious emissions are better off using the frequency domain. Yet another example is seen in spectrum monitoring. Government regulatory agencies allocate different frequencies for various services. This spectrum is then monitored because it it is critical that each of these services operate at its assigned frequency and stay within the allocated channel bandwidth.

 

While measuring signals in the time domain and frequency domain is similar, it is also very different. Each domain conveys the same signal, but from different perspectives. This enables us engineers to get more insight into how our device is behaving and ultimately develop better products for our customers.

 

To build a stronger foundation in signal analysis that will help you deliver your next breakthrough, check out the Spectrum Analysis Basics app note. Please post any comments - positive, constructive, or otherwise - and let me know what you think. If this post was useful give it a like and, of course, feel free to share.

benz

Passing Along the Magic

Posted by benz Feb 20, 2018

  Demystifying technology, and marking five years of The RF Test Blog

For several years, I co-coached two middle school robotics teams. It was a great experience, and I learned at least as much as I taught—though generally about different subjects!

Some of the kids gravitated toward the robot mechanisms, while others found a natural focus on the programming side. I suppose that’s part of the intent of robotics clubs, mixing hardware and software to increase the chances of inspiring kids to pursue STEM studies and careers.

Ironically, our success with ever-more-complex technology may create some barriers to getting kids interested in it. During a club meeting one afternoon, I was vividly reminded of Arthur C. Clarke’s famous quote, “Any sufficiently advanced technology is indistinguishable from magic.” While the kids were working with robots and laptops, virtually all of them were carrying a magical device that was even more advanced: their mobile phone.

These thin slabs of metal, glass, and plastic, invisibly connected to the rest of the universe, could be expected to do just about anything when equipped with the right app. Seeing something so magical being taken so thoroughly for granted, I understood why some kids weren’t all that captivated by the robots.

That realization left me a bit troubled, and I wondered about other ways to get the kids engaged.

A partial answer came later in the semester. My co-coach had the brilliant idea of devoting one club session to the dismantling of technology. She brought in some older devices, working or not, including an early digital camera, a portable CD player, and a slider-type mobile phone. We gave the kids some small screwdrivers and turned them loose to get a glimpse behind the engineering curtain.

I was amazed at the spike in enthusiasm and engagement, especially from some kids who had previously been marginal participants. Once they reasoned out how to open the devices and free the contents, they then delighted in showing others how they thought the parts actually worked. They got an especially big kick out of the tiny motor and attached eccentric that vibrated the phone. It was the one recognizable part of the device that moved!

My take-away: if we want to pass along our interest in creating the magic of new technologies—and solving the attendant problems—we need to keep our eyes open to new approaches to communicate and share.

That’s what we were thinking five years ago when we started this blog. Since then, it has been a delight to learn about RF technologies and share the results with you. I very much appreciate your indulgence as I’ve wandered from Loose Nut Danger (the first post) to MIMO to the technology of furry hoods.

It’s now time to pass along the writing of this blog to a new generation, with their own perspectives, insights, and peculiar interests.

Composite image of new primary writers of this blog: Eric Hsu, Vandana Duff, Nick Ben, and Tit Bin Teo

Meet the new primary writers of Keysight’s Better Measurements: The RF Test Blog, clockwise from upper left: Eric Hsu, Vandana Duff, Nick Ben, and Tit Bin Teo

Nick Ben has already written several guest posts here, and I think this blog will benefit from the new writers’ wider range of interests and experience. I look forward to following where they lead.

As for me, I plan to pursue my interests in a direction that looks more like retirement, with increased opportunities to learn and to teach, coach, and share.

  Fortunately, we can make things better—for your signal analyzer

 

Note from Ben Zarlingo: This guest post comes to us from Bill Scharf, a Keysight engineer with long experience in microwave signal analysis.

 

If you use even a gently aged spectrum analyzer, you may sometimes wonder why its amplitude accuracy above about 4 GHz is slightly worse than when it was new or after it has been freshly calibrated. Personally, I sometimes wonder why I cannot do the things I did when I was 20 years old.

 

In both cases aging is occurring. Although nothing is technically broken, we can make things better without magically locating a certain DeLorean car equipped with a flux capacitor and then driving 88 mph, hoping for a lightning strike, and traveling back in time.

 

What is a preselector, and why would it drift?

If we assume the instrument is a Keysight X-Series signal analyzer, what has probably happened is the preselector, sometimes called a YIG-tuned filter or tracking filter, has drifted a bit—but not enough to cause an out-of-specification situation. In an X-Series analyzer, the preselector is located in the signal path between the input attenuator and the first mixer, and it is used only at tuned frequencies of 3.6 GHz and higher.

 

The filter bandwidth should be wide enough to measure the desired signal, yet narrow enough to reject image frequencies and undesired signals (which may overload the first mixer). Depending on the tuned frequency, the bandwidth of the filter ranges from about 40 MHz to 75 MHz. Filter shape and ripple across the passband also vary with tuned frequency. As the analyzer tunes, the preselector filter tracks the change and provides a “centered” passband at the current frequency, as shown below.

Frequency response or gain/attenuation parameters of a YIG-tuned filter, used as a preselector in signal analzyers to remove undesired image or out-of-band signals and the spurious responses they would create in the signal analysis results

Typical passband response of a YIG-tuned preselector

Instrument software automatically handles most of this preselector tuning; however, careful adjustment of the instrument will help deal with the rest.

 

Ensuring better performance

As the instrument ages, especially its preselector assembly, the filter bandpass will drift. As a result, the signal being measured might fall in an area of passband ripple or on a steeper portion of the filter response. Here are three tips to help ensure the best performance:

  •  For the absolute best amplitude accuracy, the Preselector Center function (accessible via the front panel or SCPI) uses internal calibration signals to vary the preselector filter tuning in real time and obtains the best possible tuning. Be forewarned that this routine is time-consuming. If you need the very best amplitude accuracy using the preselector, then re-center the preselector at each measurement frequency.
  • Every three to six months, apply the Characterize Preselector routine. This performs “preselector centering” at various pre-determined frequencies up to the maximum frequency range of your analyzer. The analyzer stores the tuning values and automatically uses them the next time the analyzer is tuned to those frequencies. One advantage: after this routine runs, you may not need to rely on the slower Preselector Center routine (above). No external equipment is required: simply press System, Alignments and Advanced then select Characterize Preselector.
  • Bypass the preselector filter. If your instrument contains option MPB, microwave preselector bypass, you can select the bypass path and remove the preselector from the signal path. The downside: the instrument is no longer filtering the input signals (i.e., it isn’t “preselected”). Depending on the span setting, you may see image frequencies that are not being rejected by the preselector and so appear at the first mixer. The advantage: the bandwidth is about 800 MHz at the first mixer, preselector drift is no longer an issue, and measurement speed may increase because the instrument is no longer trying to avoid oversweeping the preselector filter.

More detail is available in our preselector tuning application note.

Wrapping up

Three closing comments: The “Y” in the YIG-tuned filter, when inverted, is almost the same schematic symbol as the flux capacitor. If you are more than 20 years old, use a knee brace when running marathons, thereby avoiding future trips to the hospital. Those of you that have an X-Series analyzer can use the Characterize Preselector routine to optimize accuracy between periodic calibrations.