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8 Posts authored by: JohnnieHancock Employee

Sniffing the air

Dogs do it all the time. But there is much more in the air than just smells. There are RF signals of all kinds all around us. How do we 'sniff' these signals out of the air so that we can observe them on an oscilloscope?

Sniffing where you can’t probe

Probing electrical voltage signals in a circuit is typically achieved using an active or passive voltage probe. If you need to measure current, most engineers use a clamp-on Hall-effect current probe that converts the magnetic field around a conductor, created by the current flowing through it, into voltage. But what if you need to monitor and verify RF signals between two sealed devices (nothing to probe), such as signals transmitted from your key fob to the receiver in the car? Or perhaps Near Field Communication (NFC) signals between your mobile phone (transmitter) and a tag (receiver)? For this you can use a RF loop antenna — sometimes called “sniffers”.

 

Download the "6 Essentials for Getting the Most Out of Your Oscilloscope" eBook.

 

Although RF loop antennas are typically used for spectrum analysis measurements, they can also be used for oscilloscope measurements. Loop antennas come in various sizes and are typically tuned for specific ranges of frequencies. In this post, I’m going to show you very briefly how you can capture key fob signals using a small RF loop antenna, based on amplitude shift-keying (ASK) modulation with a carrier frequency of 434 MHz. Detailed resources are listed at the bottom of this post.

RF_loop_antenna

Figure 1. A typical RF loop antenna

 

Sniffing and decoding automotive key fob RF signals

So, which oscilloscope would you need for the application? Since the carrier frequency in this measurement application is 434 MHz, I’ve used a 1.0 GHz bandwidth Keysight InfiniiVision X-Series oscilloscope (DSOX3104T). In brief, the steps to decode RF signals from an automotive key fob with a scope includes:

  1. Connecting the loop antenna to the scope’s Channel 1 input, terminated into 50
  2. Positioning the loop antenna near the key fob while one of its buttons is pressed to capture the single-shot burst of RF-modulated data packets (channel-1, yellow trace shown in Figure 2)
  3. As decoding the RF-bursted packets requires demodulation prior to digital decoding, you’ll also need to setup the scope to digitally demodulate the signal (hardware-based within the scope, channel-2, green trace shown in Figure 2)
  4. Decoding the digitally demodulated waveform. This can be achieved with the oscilloscope’s user-definable NRZ/Manchester trigger and decode option. Figure 2 shows the Manchester-decoded bits at the bottom of the trace display
  5. Screen display of the Keysight DSOX3104T scope

Figure 2. Screen display of the Keysight DSOX3104T oscilloscope that displays the captured single-shot burst RF-modulated signal (yellow trace), demodulated signal (green trace) and Manchester-decoded bits

 

Sniffing Near Field Communication (NFC) signals from a mobile phone

In Figure 3, I’m showing you the setup for how you can capture NFC signals generated by a mobile phone, using a larger PC trace loop antenna. Since the carrier frequency in this case is just 13.56 MHz, a 100-MHz bandwidth oscilloscope is sufficient for the measurement application.

 Capturing NFC signal from mobile phone

Figure 3. Setup to capture NFC signals from a mobile phone with a PC trace loop antenna and a 100-MHz bandwidth oscilloscope

 

Creating your own RF ‘sniffer’

What if you need a simple ‘sniffer’ that doesn’t have to be precision-tuned? Well, you can create a non-precision loop antenna yourself! Simply connect the ground clip of a standard high-impedance passive probe to the probe tip (shown in Figure 4) and – voilà – you have created an oscilloscope RF ‘sniffer’! Sure, it may not be tuned for a particular carrier frequency, meaning that the voltage levels that you measure on the oscilloscope may not be an accurate representation of the actual RF field strength. But you can still “sniff” signals out of the air to verify proper modulation and timing of your RF-modulated signals.

DIY of RF loop antenna using high-impedance passive probe

Figure 4. DIY your own RF loop antenna using a standard high-impedance passive probe

 

Detailed ‘sniffing’ resources

If you’re interested to learn in greater detail about ‘sniffing the air’ to verify modulated RF signals on an oscilloscope, here are excellent resources to get you started:

 

Decoding Automotive Key Fob Communication based on Manchester-encoded ASK Modulation – Application Note

Decoding Automotive Key Fob Communication based on Manchester-encoded ASK Modulation – YouTube Video

NFC Device Turn-on and Debug – Application Note

NFC Testing Using an Oscilloscope Part 1: Benchtop R&D Measurements

 

Keysight continues to invest in the very popular InfiniiVision X-Series oscilloscopes with new oscilloscope models, new options, and customer-requested enhancements. Every 6 to 8 months, Keysight releases new firmware that our customers can download into their InfiniiVision scopes at no cost. The latest release of firmware (version 7.10) for the InfiniiVision 3000T, 4000, and 6000 X-Series oscilloscopes includes three new licensed “pay for” options as well as several FREE upgrades/enhancements including the following:

 

  • User-definable Manchester/NRZ Trigger & Decode Option
  • Frequency Response Analyzer (FRA) Option
  • CXPI Trigger & Decode Option (new on 6000X, existing on 3000T & 4000)
  • DVM/Counter Option – Now standard!
  • Education Training Kit Option – Now standard!
  • On-screen grid scaling factors – Standard!
  • FFT RF power measurements -Standard!

 

 

User-definable Manchester/NRZ Trigger & Decode Option (DSOXT3NRZ, DSOX4NRZ, & DSOX6NRZ)

 

Keysight’s InfiniiVision X-Series oscilloscopes support triggering on and decoding a broad range of today’s most popular serial bus protocols including I2C, SPI, UART, CAN, USB, etc.  But what happens if you are working with one of the less popular — or perhaps proprietary — serial bus protocols that isn’t supported? Available now on InfiniiVision 3000T, 4000, and 6000 X-Series oscilloscopes is a “user-definable” Manchester- & NRZ-encoded serial bus option. This new trigger and decode option allows you to define the specifics parameters and structure of your particular serial bus including:

 

  • Encoding type (Manchester or NRZ)
  • Baud Rate
  • Start edge
  • # of Synchronization bits
  • Word size
  • Header field size
  • Data field size
  • Trailer field size
  • Decode Base (binary, hex, ASCII, or unsigned decimal)

 
Also available are three new application notes that provide step-by-step instructions on how to set up triggering and decoding for three specific Manchester-encoded serial bus measurement applications including the automotive PSI5 sensor bus, automotive RF-modulated key fob signals, and NFC-F signals. Figure 1 show an example of triggering on and decoding the automotive PSI5 sensor bus, which is often used in airbag systems.

 

Figure 1: Triggering on decoding the Manchester-encode PSI5 sensor bus using Keysight’s user-definable serial bus option.

 

In the above PSI5 measurement example, after detecting the 2-bit Start field, the scope triggered on and decoded a 10-bit payload field followed by a parity bit. The scope also automatically detected a Manchester timing error in the frame that immediately followed the trigger frame (155h). To learn more about triggering on and decoding the PSI5 serial bus, download the new application note on this topic.


 


Figure 2 above shows an example of decoding automotive key fob signals.

 

Decoding RF-modulated key fob signals requires hardware digital demodulation. The new application note on this topic shows you how to capture the RF signal with a “sniffer” probe, how to demodulate the captured signal in hardware, and then how to decode each “code-hopping” RF burst. To learn more about how to set up the scope to demodulate and then decode RF-modulated key fob signals, download the new application note on this topic.


  
Figure 3 above shows an example of decoding NFC-F signals.

 

With NFC-F signals, the scope automatically demodulates the captured RF-modulated waveform if based on a baud rate of either 212 kbps or 424 kbps. When the new Manchester trigger and decode option is used along with the recently-introduced NFC automated test software, the designer of NFC-enabled device now has a very powerful set of debug and test tools. To learn more about how to set up the scope to decode NFC-F signals, download the new http://literature.cdn.keysight.com/litweb/pdf/5992-2337EN.pdf on this topic.

 

 

Frequency Response Analyzer (FRA) Option (DSOXT3FRA, DSOX4FRA, & DSOX6FRA)

 

Using the InfiniiVision oscilloscope’s built-in function generator (WaveGen), these scopes can now perform automatic frequency response analysis (Bode gain & phase plots) as shown in Figure 4 below.

 

 

Figure 1: Triggering on decoding the Manchester-encode PSI5 sensor bus using Keysight’s user-definable serial bus option.

 

Frequency Response Analysis (FRA) is often a critical measurement used to characterize the frequency response (gain & phase versus frequency) of a variety of today’s electronic designs including passive filters, amplifier circuits, and negative feedback networks of switch mode power supplies (loop response). Engineers typically use network analyzers or standalone low-frequency FRAs to perform these types of measurements today. To learn more about the new FRA option, download the data sheet.

 

 

Free InfiniiVision Enhancements with Firmware Upgrade

 
In addition to the new “pay-for” options listed above, there are also free enhancements available for InfiniiVision X-Series oscilloscopes with an upgrade to the latest firmware (v7.10). The built-in DVM and hardware counter option shown in Figure 5 will no longer be an option. These measurement capabilities will now be standard features of all InfiniiVision oscilloscopes.

 

 

Figure 5: The DVM/HW Counter option is now a standard feature in all InfiniiVision X-Series oscilloscopes.

 

The measurement example shown above shows output ripple riding on top of a 5 V dc output from a switch mode power supply (SMPS) with a switch rate of approximately 2 MHz. The scope’s built-in DVM shows that the actual DC output measures 4.97 V and the actual switching rate is 2.0112 MHz as measured by the 5-digit hardware counter.

 

Another new free enhancement is on-screen vertical and horizontal grid scaling factors along the left vertical axis and lower horizontal axis, which is also shown in Figure 5. This has been a very popular feature in Keysight’s higher-performance Infiniium oscilloscopes. Labeling each grid with scale factors can help you quickly estimate voltage levels and timing of your signals. 


In addition to making the DVM/Counter option standard, the education training kit (EDK) option is now standard as well on all InfiniiVision 3000T, 4000, and 6000 X-Series oscilloscopes with the latest firmware upgrade. Using the built-in training signals shown in Figure 6 along with available online training guides, users unfamiliar with the operation of Keysight InfiniiVision oscilloscopes can get up-to-speed quickly. To learn more about the EDK training kit and to download the training guides, go to the InfiniiVision http://www.keysight.com/en/pd-1952427-pn-DSOXEDK/educators-training-kit-for-infiniivision-2000-and-3000-x-series-oscilloscopes?cc=US&lc=eng webpage.

 

 

 Figure 6: The education training kit (EDK) is now standard and includes multiple built-in training signals and online training guides.

 

Also available at no cost in this latest firmware release are new automatic RF power measurements that can be performed on FFT math functions. Figure 7 shows an example of an Adjacent Power Ratio measurement performed on an RF-modulation sideband measurement. Other new RF measurements include Channel Power, Occupied Bandwidth, and Total Harmonic Distortion (THD).

 

Figure 7: New FFT analysis measurements.

Professor BodeWhen I was an electrical engineering student back in the 1970’s at the University of South Florida — go bulls! — two of my favorite classes were Control Systems and Analog Circuit Fundamentals. One reason I loved these classes so much was because we got to create Bode plots. I know, that sounds weird. I really enjoyed finding the theoretical poles and zeros and drawing Bode plots on my green engineering graph paper by hand (pencil, paper, and a ruler). Thank you Professor Bode; you are my hero! But when it came time to go into the lab to verify the frequency response of something like a passive or active filter design that we were assigned to build and test, there were no frequency response analyzers (sometimes called network analyzers) to be found.

 

In those days network analyzers were highly specialized and expensive multi-box systems from test and measurement vendors, including Hewlett-Packard (Keysight’s predecessor). Without

access to one of these expensive instruments in my EE lab, the testing process consisted of taking multiple VIN, VOUT, and Δt measurements on an oscilloscope while changing the input sine wave frequency on a function generator. After making 15 or 20 measurements, I would have enough measured data points to convert to gain (20LogVOUT/VIN) and phase shift (Δt/T x 360) using my trusty slide rule. I would then plot the results back onto that green engineering graph paper alongside the theoretical plots for comparison.

 

The days of plotting theoretical results by hand are over. Most engineering students today use MATLAB® to do that. And certainly the days of taking multiple VIN and VOUT measurements in the lab using an oscilloscope and a function generator set at discreet frequencies must be over, right? After all, the test and measurement industry now offers a broad range of frequency response analyzers (FRA) and vector network analyzers (VNA) that create gain and phase plots automagically. But those days aren’t over! Most undergraduate EE teaching labs are not equipped with frequency response analyzers. Almost all EE students today use the same tedious method of testing a circuit’s frequency response that I used back in ancient times. Why is that?

 

FRAs and VNAs are still considered by many to be a specialized instrument — especially in the university environment. In addition, the price of these instruments start at around US $5,000 and go up from there. This may not sound like much for someone in the high tech industry that depends on this type of instrument to get to their testing done quickly, but almost all universities have to operate on a tight budget. A typical student lab bench (consisting of an entry-level 2-channel oscilloscope, function generator, digital voltmeter (DVM), and power supply) can be purchased for about US $2,000 today. To equip an entire student teaching lab with an FRA at an additional US $5,000 per test station would blow most EE lab budgets out of the water.


 1000 X-Series Oscilloscope

But now the process of making multiple VIN and VOUT oscilloscope measurements to create Bode gain and phase plots are about to be over for many EE students – at least for students at universities that equip their labs with a new Keysight oscilloscope. Keysight just introduced a family of low-cost oscilloscopes with an optional built-in function generator (Figure 2). And the best part is that automatic frequency response analysis (Bode gain and phase plots) can be performed on these student oscilloscopes at no additional charge on models that come equipped with the built-in WaveGen function generator and frequency response analysis (EDUX1002G and DSOX1102G). All of this functionality (oscilloscope, function generator, and frequency response analysis) can be had for just over US $600. Let’s take a look at a measurement example of characterizing a passive bandpass filter using this new oscilloscope.

Passive RLC bandpass filter

Figure 3 shows the schematic of a simple RLC circuit that we will test. At lower frequencies, the 1-µF capacitor dominates the impedance (XC = 1/2πfC) of this circuit and blocks most of VIN from getting to VOUT. At higher frequencies, the 10-µH inductor (XL = 2πfL) blocks most of the input from getting to the output. But in the mid-band frequencies, the 50-Ω load resistor dominates such that most of VIN reaches VOUT (~0 dB). By definition, this is a bandpass filter. We will now test it with Keysight’s new oscilloscope with the built-in function generator and frequency response analysis capability.

 

We begin by connecting the output of the generator to VIN and also probe VIN and VOUT with channel 1 and channel 2 of the oscilloscope, respectively. Figure 4 shows the frequency response analysis setup menu, which displays a block diagram to assist us in making proper connections. This is also where we can define which of the oscilloscope’s input channels is probing VIN, which channel is probing VOUT, the minimum test frequency, maximum test frequency, and test amplitude. For this test, we will use all the default settings. When we select Run Analysis, the oscilloscope executes the one-time  test, sometimes called a "sweep."

Figure 5 shows the test results. The blue trace represents gain (in dB) with scale factors shown on the left side of the display, while the orange trace represents phase (in degrees) with scale factors shown on the right side of the display. A pair of markers is also available to measure gain and phase at any frequency. The oscilloscope even optimizes magnitude and phase scaling factors automatically. But you also have the ability to establish your own scale factors manually after completion of the test. This is probably the easiest-to-use frequency response analyzer on the market today. At least that’s my opinion. And it has to be the least-expensive FRA because it comes standard (US $0) with the purchase of an EDUX1002G or DSOX1102G oscilloscope, which is only US $200 more than the baseline 1000 X-Series model (US $449). But performance is not sacrificed. Using a proprietary measurement algorithm, this instrument can achieve up to 80 dB of dynamic range based on a 0 dBm (224 mVrms) input.

 

Using this new Keysight oscilloscope capability has sure saved me a lot of time in my job. And I bet it will save EE students a lot of time as well so they can complete their lab assignments on time!

 

 

MATLAB is a registered trademark of MathWork, Inc.

Performing parametric measurements on digital modulation is often required to ensure reliable wireless transfer of data, such as secured financial transfers. One example gaining acceptance in today’s mobile technology industry is Near Field Communication (NFC). Most of today’s newer smart phones come standard with NFC technology. It won’t be long before our physical credit cards meet their demise and become a thing of the past — just like the VHS video tape. In the future — if not already — you’ll walk up to a payment terminal at your nearby supermarket, tap your smart phone on the payment terminal, and just like that the money’s gone!

 

Testing the analog quality of NFC digital modulation is possible with an oscilloscope. The key to performing automatic parametric measurements on digital modulation is to first strip the modulation away from the RF carrier. But let’s begin by taking a look at NFC communication between a payment terminal and a mobile phone as shown in Figure 1.

 

NFC communication captured on the oscilloscope

Figure 1. NFC communication captured on the scope.

 

The 13.56 MHz carrier and the first burst of digital modulation is generated by the payment terminal. The payment terminal periodically sends out “pings” or “polls” such as this to see if it can get a reply. The second burst of digital modulation with a much lower modulation index, is a reply from a mobile phone saying, “I’m here. Want to take my money?”, or something like that. Then there is additional handshaking that takes place after these first two bursts of communication before your money actually vanishes.

 

The NFC analog test specification calls for specific measurements to test the analog quality of both poller modulation (payment terminal in this example) and listener modulation (mobile phone in this example). Perhaps this is to ensure that a few extra zeros are not inadvertently added to your bill due poor signal quality. Let’s walk through making a few analog quality measurements on just the modulation generated by the payment terminal (poller).  

 

The first task is to trigger on NFC communication. If you happen to own a Keysight InfiniiVision 3000T or 4000A oscilloscope, then you’re in luck. Keysight just recently introduced an NFC trigger option for these oscilloscopes. If you are using another model oscilloscope from Keysight or any other vendor, pulse-width trigger based on a specific time parameter along with trigger-holdoff might work for you.

 

The next task is to demodulate the modulated RF. For that we will first turn on the oscilloscope’s horizontal zoom timebase mode to zero-in on one set of pulses near the beginning of the first burst of modulation generated by the payment terminal. We will then use one the oscilloscope’s waveform math functions called “Envelope” to demodulate the captured RF waveform as shown in Figure 2.

Zooming in and using “Envelope” math function to demodulate the captured waveform.

Figure 2: Zooming in and using “Envelope” math function to demodulate the captured waveform.

 

The “Envelope” waveform math function is a frequency-domain operation based on a Hilburt transform. The purple trace in Figure 2 is the resultant envelope waveform, which closely tracks the positive extremes of the 13.56 MHz carrier. The reason that it appears noisy is because the Hilburt transform is based on both positive and negative extremes of the carrier. And in this case our RF carrier and modulation are not perfectly balanced. To remove the noise, we can apply a second math function (Low-pass filter or Smoothing filter) on top of the first math function (Envelope) as shown in Figure 3.

 

Filtering the Envelope waveform with a 5-MHz low-pass filter.

Figure 3: Filtering the Envelope waveform with a 5-MHz low-pass filter.

 

By applying the 5-MHz low-pass filter onto the somewhat noisy envelope waveform, we now have now successfully demodulated the RF waveform and are ready to perform some required parametric measurements to insure that poor signal quality doesn’t add zeros to our e-payments. One of the required measurements is the delta-time that modulation is below 5%. For that we can use the oscilloscope’s automatic negative pulse width measurement (-Width) based on custom measurement threshold settings as shown in Figure 4.

 

Using the scope’s automatic parametric measurements to measure the time below 5% modulation.

Figure 4: Using the scope’s automatic parametric measurements to measure the time below 5% modulation.

 

Although the oscilloscope’s automatic pulse width measurement is normally based on 50% measurement thresholds, most of today’s oscilloscopes will allow you specify user-defined measurement thresholds, such as 5%. In this measurement example we measured 2.17 µs. The NFC specification for this particular measurement is 1.8 µs to 2.3 µs.  

 

Note that there are many other required measurements not covered in this article to ensure proper analog signal quality of NFC communication. Along with the ability to trigger on NFC communication, Keysight’s NFC option for InfiniiVision oscilloscopes also provides automated NFC test software for comprehensive testing of NFC-enabled devices. To learn more about testing NFC with an oscilloscope, see below.

 

NFC test software for 3000T X-Series oscilloscopes (DSOXT3NFC)

NFC test software for 4000 X-Series oscilloscopes (DSOXT4NFC)

NFC Testing Using an Oscilloscope Video Part 1: Benchtop R&D Measurements

NFC Testing Using an Oscilloscope Part 2: Automated Measurements

See more on testing NFC

The most versatile tool in the test & measurement world is the oscilloscope. Similar to a multi-purpose pocket tool, not only can a scope be used to view time-domain waveforms (voltage vs time), which is the primary cutting blade of an oscilloscope, but many of today’s scopes have additional blades that can perform measurements that were formerly relegated to specialized test gear, including spectrum analysis (FFT), DVM, counter, logic analysis, serial protocol analysis, and arbitrary waveform generation.

 

The latest blade that Keysight has added to this multi-purpose tool is frequency response analysis. With frequency response analysis, a voltage sine wave source is swept from a lower frequency to an upper frequency while Vin and Vout are measured and the ratio is plotted (Gain in dB = 20 x Log(Vout/Vin). This is the primary function of Vector Network Analyzers (VNA), which are sometimes called Frequency Response Analyzers (FRA).

 

If you can remember back to your college engineering days, you may recall the dreaded Bode plots of gain and phase versus frequency. For my EE class assignments I used a slide rule (I’m ancient), a pencil with a really big eraser, and traditional green engineering graph paper to predict theoretical frequency response results. And then to verify predicted results in the lab, we used a 2-channel analog oscilloscope along with a sinewave generator to perform multiple measurements at discreet frequency settings. We then plotted the results manually on paper for comparison.

 

Most of today’s EE students use off-the-shelf PC apps such as MATLAB or LabVIEW to generate their theoretical Bode plots. But since most universities can’t afford to purchase and fully equip EE teaching labs with specialized test equipment, the method of verification is often the same method that I used 40 years ago.   


Frequency response measurements (Bode plots) are not just something that you are required to do in college. Many electronic designs, including filters and amplifiers, must meet frequency response specifications. One common example where frequency response testing should be performed is when testing the stability of switch mode power supplies.

 

All power supplies have a feedback amplifier network. If the output load of a power supply suddenly increases (sudden increase in current), output voltage will momentarily drop until the feedback amplifier responds to pull it back up. If the feedback amplifier responds too quickly, the net result could be excessive overshoot/undershoot with significant output ringing, or even worse, oscillation. To insure stability, the feedback network of power supplies should be tested. But even in industry today, VNAs and FRAs are often hard to come by and also are not that easy to use. However, almost every test bench has an oscilloscope. And if it’s a Keysight InfiniiVision 3000T or 4000 X-Series oscilloscope, problem solved.

 

Figure 1 shows an example of the setup menu used to perform a power supply Control Loop Response measurement (Bode gain & phase) using a Keysight InfiniiVision 3000T X-Series oscilloscope with the power measurements option (DSOX3PWR).

 

Control Loop Response (Bode) setup menu on a Keysight oscilloscope

Figure 1. Control Loop Response (Bode) setup menu.

 

In this example the oscilloscope uses its built-in waveform generator to sweep an input test signal (sine wave) from 100 Hz to 20 MHz using a fixed amplitude of 200 mVpp at each test frequency. Note that amplitude profiling is also possible. When “Apply” is pressed, the oscilloscope begins the one-time sweep and produces the gain and phase shown in Figure 2.

Gain and phase plot of the feedback network of a switch mode power supply

 Figure 2. Gain and phase plot of the feedback network of a switch mode power supply.

 

The blue trace represents the gain plot with its scaling factors shown on the left vertical axis, while the orange trace represents the phase plot with its scaling factors shown on the right vertical axis. At the completion of the sweep, the gain and phase plots are automatically scaled for optimum display resolution, but these plots can also be scaled manually. Also shown in this plot are automatic measurements of the phase margin at the 0 dB cross-over frequency (PM = 41.52ᵒ at 62.21kHz) and gain margin at the 0ᵒ cross-over frequency (GM = 9.89dB at 130.8kHz). These are important measurements that give you an indication of the stability of your power supply’s feedback amplifier. Note that you can also manually slide the measurement markers along the plots to measure gain and phase at any frequency.

 

To learn more about power supply Control Loop Response testing using an oscilloscope, download Keysight’s application note on this topic.

DSO stands for Digital Storage Oscilloscope. DPO stands for Digital Phosphor Oscilloscope. A DPO is also a DSO. And a DSO can also be a DPO. So what exactly is a DSO and what is a DPO?

A DSO is typically a real-time sampling oscilloscope. Real-time sampling simply means that the scope is able to capture signals in a single acquisition utilizing a high sample rate analog-to-digital (ADC). In other words, a DSO doesn’t utilize repetitive acquisitions to “build-up” sufficient samples to represent the signal under test (equivalent-time sampling), although this is a not a hard-and-fast rule.

As mentioned before, a DPO is also DSO. But a DPO adds one additional element that allows it to better represent the signal’s third dimension. The first two obvious dimensions are voltage and time. The third and less obvious dimension is frequency-of-occurrence, which is represented by trace intensity on a scope’s display. If you can beckon back to the old analog scope days you may recall that these oscilloscopes were able to display a range of trace intensities. This can provide valuable insight into the true analog characteristics of a signal under test. This is especially true for complex-modulated analog signals as shown below, as well as for digital signals that contain varying degrees of noise and/or jitter.

With older analog scope technology, trace intensity variation was a natural phenomenon based on how much time the electron beam remained within a XY region on the inside face of the cathode ray tube (CRT). The inside face of CRTs of analog oscilloscopes are coated with a material called phosphor. When electrons strike the phosphor, the phosphor begins to glow. The more electrons that strike the phosphor in a given region of the CRT for a given amount of time, the brighter the phosphor glows.

When DSOs were birthed in the early 1980’s, this third dimension of trace intensity was initially lost as shown in the screen image below.

As technology progressed, oscilloscope vendors developed a technique that could closely emulate the display quality of analog oscilloscopes utilizing digital signal processing to bring the third dimension back from the grave as shown in the screen image below.

Basically, by counting the number of hits (digital samples) in particular XY regions of a bit map — sometimes called buckets — pixel intensity could be digitally modified to represent trace intensity modulation of phosphor. This is where the term Digital Phosphor Oscilloscope (DPO) came from.

So why doesn’t Keysight have DPOs? Actually, we do. But we don’t call them DPOs. Nearly all of Keysight’s DSOs employ trace intensity modulation. In fact, Keysight’s oscilloscope display technology provides the highest quality trace intensity modulation due to the fact that Keysight scopes have the industry’s fastest waveform update rates with deep memory acquisitions. This provides more hits in XY regions (buckets) in a shorter amount of time to provide a higher level of statistical information for which to base pixel intensity upon.

So why doesn’t Keysight call them DPOs? Keysight believes there is enough confusion concerning different names for the same basic instrument. My large screen flat-panel television that I watch Rockies baseball games on is still just a TV even though it utilizes an entirely different technology than older CRT-based televisions. And besides, why use an old analog technology term when many of today’s younger engineers have never used an analog oscilloscope and don’t have a clue what phosphor has to do with an oscilloscope? Same goes for the term “sweep”. Refer to one my previous blog posts titled, “Oscilloscope Triggering: When is Normal not so Normal?”.

Maybe we should call them DSOWDPTMSCDSAAMDMCs (Digital Storage Oscilloscope with Digital Phosphor Technology, Mixed Signal Channels, Digital Signal Analysis, and Mixed Domain Measurement Capabilities). But in my eyes, it’s still just a scope! And if you are an Aussie, it will always be a CRO (pronounced “crow”).

For more information, see this application note on Oscilloscope Display Quality.

JohnnieHancock

The Crow

Posted by JohnnieHancock Employee Sep 1, 2016

 

During my nearly 37 year career at HP, Agilent, and now Keysight, I have presented lots of oscilloscope seminars and workshops to our current and potential customers. Back in the late 1980’s I did my first oscilloscope seminar tour in Australia with a focus on explaining the differences between analog oscilloscopes and digital oscilloscopes. During this seminar tour I kept hearing the Aussies refer to the scopes as “crows”. In my mind I was picturing an annoying bird and thought that this must be some kind of derogatory term. During this era digital oscilloscopes were in their infancy, and as such had some quirky behaviors relative to their analog predecessors. So this made sense to me that these guys might be frustrated with digital scopes and would call them names. After all, I sometimes call my instruments names if I can’t get them to behave properly. Although it is usually a pilot error on my part when this happens. But never “crow”. Finally during one of the seminars, I asked, “Why do you guys keep calling these things “crows”? It was explained to me that they weren’t calling the them “crows”, but were referring to them by the acronym CRO, which stands for cathode ray oscilloscope.

To this day, Aussies still affectionately refer to their scopes as CROs, even though oscilloscopes no longer have cathode ray tubes. But I guess we still refer to rolling up our windows in our cars and dialing a number on our phones. So we’ll forgive the blokes down under for calling their scopes “crows”. Which brings up another thought. Why is Australia considered “down under” instead of “up over”? Who decided north was up and south was down?

What else are scopes called? There’s digital storage oscilloscope (DSO). There’s digital phosphor oscilloscope (DPO). There’s mixed signal oscilloscope(MSO). There’s mixed domain oscilloscope (MDO). And there’s sampling oscilloscope. What’s the difference?  Perhaps this will be the topic for a future blog. G’day.

Oscilloscopes have two primary modes of triggering: AUTO and NORMAL. However, NORMAL is not the normally used mode of triggering. AUTO is. The default trigger mode in all of today’s oscilloscopes is AUTO. There is a lot of confusion these days among oscilloscope users as to exactly when to use which mode of triggering. Let’s first define what these terms mean and then discuss how these modes of triggering came to be called what they are.

AUTO simply means “automatic”. In the AUTO trigger mode, the scope will trigger on the signal under test if a trigger condition is met, such as a rising edge. But if a trigger condition doesn’t occur within a predetermined amount of time, the scope will begin to generate its own automatic triggers, which are not synchronous to the signal under test. This means that the scope will show of blur of waveforms when this happens. So if AUTO is always the default trigger mode, why would you ever want to see a blur of waveforms? One reason is that a blur of waveforms will show you where the signal is relative to your trigger level. Perhaps you have the trigger level set above (too high) or below (too low) the signal under test. With AUTO trigger you can see what’s wrong and make adjustments. Setting up an oscilloscope is an iterative process of seeing what’s there and then making adjustments (V/div, sec/div, trigger level, etc.) until it is right. Another reason the AUTO trigger mode is the default mode of triggering is that you may want to simply view the DC level of a power supply. Scopes can’t trigger on DC, unless the DC includes lots of switching noise, in which case it is not purely DC.

The NORMAL trigger mode means that the scope triggers if and only if a trigger condition is met. If you’ve got your trigger level set above or below the signal under test, then you’ll be looking at a blank screen on your scope. So when should you use NORMAL triggering? If the signal you want to trigger on occurs very infrequently, perhaps once every three seconds, then you should use the NORMAL trigger mode so that the scope will display synchronized representations (waveforms) of your signal only when trigger event occurs, and not generate automatic and asynchronous triggers between qualified trigger events and thereby show you blurs of waveforms.

So why is this trigger mode call NORMAL? I can only guess. Back in the old analog scope days, this trigger mode was not called NORMAL triggering. It was call the TRIGGERED sweep mode, which makes sense. When a trigger qualification was met, such as a rising edge, the analog scope would trigger a linear sweep of an electron beam across the scope’s cathode ray tube (CRT). But when digital storage oscilloscopes (DSOs) came along about 30 years ago, the representation of waveforms on the scope’s display changed from the sweep of an electron beam that excites phosphor on the CRT to the digitization and storage of discreet waveform points using an analog-to-digital converter (ADC) and then represented as pixels on a scope’s display. Since newer technology scopes stopped sweeping, most oscilloscope vendors began calling it a “trigger” mode instead of a “sweep” mode. And if they had kept using the same old analog scope terminology it would have become the TRIGGERED trigger mode, which sounds redundant. So some genius marketing guy must have said, “Let’s call it the NORMAL trigger mode — maybe because it was the trigger mode that he or she normally used.

Note that some DSOs still call it an AUTO and TRIGGERED sweep mode. I feel sorry for the younger engineers that have no idea what a sweep is.

In my opinion the AUTO and NORMAL trigger modes should be called AUTO On/Off. To me, this makes more sense. But I know that’s not going to happen, just like Australians will never stop calling their oscilloscopes their “crows”, which I think will be the topic for my next blog.

Anyone out there know for sure how this mode of triggering came to be called NORMAL?