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2016

Do you want to accurately measure signals with minimal effects from your oscilloscope probe? The oscilloscope probe is a critical link in the quest for accurate signal measurements. The probe is more than just a connection between the circuit under test and the oscilloscope. It can affect both your measurement results and the circuit under test. It’s important to select the right probe, understand its loading effects, and factor in signal fidelity impacts. This blog will cover probe selection, circuit loading, and factors effecting measurement accuracy.

 

Step 1: Selecting the Probe Type

Higher bandwidth is an obvious advantage of active probes over passive probes. Oscilloscope probe users often neglect the effect of the connection to the target. They tend to focus on the probe’s published specifications and do not factor in the effect of the probing accessories attached to the tip of the probe. The real-world, probe to system connection can drive the probe’s performance downward. Keeping this in mind, the system bandwidth of the probe and oscilloscope together should be three to five times greater than the frequency of the waveform being measured. This rule of thumb ensures adequate bandwidth for signals such as square waves with high-frequency content. In addition, when measuring rise times, you should ensure that the rise time of the oscilloscope and probe system should be three to five times faster than the pulse being measured. You may not always know the bandwidth of the target being measured but may know the rise time of the fastest signals to be measured. In these cases, there is a handy relationship for determining bandwidth: BANDWIDTH × RISE TIME = 0.35. Below is a chart to help with passive vs active probe selection.

 

Passive Probes

Active Probes

Features -

Features -

Higher Resistance

Low Loading

High Dynamic Range

High Bandwidth

Rugged

High Bandwidth

Low Cost

Least Intrusive

Tradeoffs -

Tradeoffs -

Bandwidth Limited to 500MHz

Higher Cost

Heavy Capacitive Loading

Limited Input Dynamic Range

Figure 1: Passive vs Active Probes

 

 

Step 2: Factoring Circuit Loading Effects –

All probes have a somewhat similar impact on the circuit under test. A probe draws a portion of the circuit energy and supplies this energy to the oscilloscope. All probes present a capacitive, resistive, and inductive loading element to the circuit under test. The challenge is to ensure that these effects do not impact the circuit and change the signal from its original state.

 

Capacitive - Capacitive loading is the main culprit of measurement errors. For general-purpose measurements less than 700 MHz, passive high-impedance resistor divider probes are good choices. These rugged and inexpensive tools offer wide dynamic range greater than 300 V and high input resistance to match an oscilloscope’s input impedance. These probes often come with the scope when you purchase it. But, they begin to impose heavier capacitive loading as the frequency of the signal being measured goes up. The input capacitance of the probe and oscilloscope combine to create an impedance between the signal being measured and ground. As the frequency of the signal goes up, the impedance created by the capacitance drops. If the impedance drops too low it can affect your signal being measured, this is known as capacitive loading. For example, a capacitance of 10 pF presents only 100 Ohms of impedance to a 150 MHz signal so it is important to know the input capacitance of the passive probe and the scope you are using. As a rule, high-impedance passive probes are a great choice for general purpose debugging and troubleshooting on most analog or digital circuits.

 

Resistive - Resistive loading is not as troublesome because it is the least likely to induce nonlinear behavior in your circuit. Most common is resistive loading consisting of the circuit’s output resistance and the probe’s own resistance forming a voltage divider circuit. This divider circuit distorts the signal being measured because the probe is seen as a load to the circuit under test. Even a probe with a small output capacitance of 1 pF can substantially affect the measured circuit.  A 1 pF probe looks like a 160-ohm load at 1 GHz, which is the highest frequency associated with a 0.5-ns rise or fall time.

 

Inductive - Many people think that probe input impedance is a constant number. Probes specifications may state that the probe has a kiloΩ, megaΩ or even a 10 megaΩ input impedance, but this is not constant over frequency because input impedance decreases over frequency. At DC and low frequency ranges, the probe’s input impedance starts out at the rated input resistance, say 10 MΩ for a 10:1 passive probe, but as the frequency goes up, the input capacitance of the probe starts to become a short, and the impedance of the probe starts to drop. Figure 2 below shows the limitations of the passive probe relative to an active probe. Note that the active probe in red tracks the non-loaded signal in blue very closely. But the passive probe in green has a slower slope or impulse response. This slower impulse response reflects the lower bandwidth and the ripple or lack of flatness because the displayed signal is missing the high frequency components needed for this measurement.

Figure 2: Passive vs Active Probe loading effects

 

Step 3: Understanding Signal Fidelity Impacts

Ground lead – Ground lead issues are a constant concern because of the difficulty in determining a true ground reference point. This difficulty arises from the fact that ground leads have inductance and become circuits of their own as signal frequency increases. In addition to being the source of ringing and other waveform aberrations, the ground lead can also act as an antenna for noise. Suspicion is the first defense against ground-lead problems. Always be suspicious of any noise or aberrations observed on the signal. The noise or aberrations may be part of the signal, or they may be the result of the measurement process.

 

Ground length – All probe ground leads have inductance, and the longer the ground lead the greater the inductance. Ground lead inductance combined with probe tip capacitance forms a resonant circuit that causes ringing at certain frequencies. In order to see this ringing or any other aberrations, the oscilloscope system bandwidth and the probe bandwidth must be high enough to handle the high-frequency content of the aberrations.

 

Accessory Impact – The convenience offered by some accessories can be attractive, but may come at a cost. Any accessory that is placed between the probe and the target system has the potential to adversely affect the target because of the extra capacitance and inductance of the accessory. Extra capacitance will load the target system and the larger or longer the accessory, the more impact it will have on the measurement being made. Therefore, it may be prudent to reserve the use of larger or longer accessories for slower signal measurements.

 

Passive probes usually specify an input capacitance of 10-13pF. The inductance will be mainly determined by the ground connection and as we have stated, the shorter the ground connection, the smaller the inductance. Reducing this inductance will increase the resonance frequency to a range outside the bandwidth of the oscilloscope. Figure 3 below shows the impact caused by increasing a ground accessory from a one half inch ground blade to a 9 inch ground. The best response is the blue line with a sharp rise and a small overshoot but flat response from there on. The longer ground lead, in red, causes over a 3 nS delay resulting in a lower bandwidth solution. A good rule of thumb when looking at data like this is that .35 divided by the total rise time equals the bandwidth of the probing solution. In this case the 2 nS difference reduces the calculated bandwidth by 350 MHz relative to the blue signal bandwidth. Also note the additional ripple in the red signal. This is due to the resonant circuit created by the added capacitance and inductance due to the longer ground lead. 

Figure 3: Loading Caused by a longer ground lead

 

Damping - As we have stated earlier, the performance of the probe and the resulting signal on the oscilloscope is highly affected by the probe connection. As signal speeds increase you may notice more overshoot, ringing, and other aberrations because probes and their accessories form a combined resonant circuit. If the resonance is within the bandwidth of the oscilloscope probe you are using, it will be difficult to determine whether the measured aberrations are due to your circuit or the probe resonating circuit. To reduce this effect, damping with the appropriate tip resistor can help. Figure 4 below shows the effects of damping on the measured signal reducing the aberrations on the output signal in blue. The resonance of the undamped circuit causes the oscillation seen in blue in the first screen shot using an active probe with a bandwidth greater than 1 GHz. However, this oscillation will not be seen using a 500 MHz passive probe because the frequency of the oscillation is above the bandwidth of the passive probe.

Figure 4: Undamped vs Damped waveforms

 

Summary:

The purpose of these 3 steps was to walk you through the path to accurate probing measurements. The resultant waveform on the oscilloscope will now most accurately represent the signal from your device under test. It is important to remember that there will always be a tradeoff between measurement flexibility, usability, and resulting bandwidth. As we discussed, added capacitance and inductance create RC circuits that resonate within the measured bandwidth of your signal. These circuits will reduce your bandwidth and the ripple will impact your amplitude readings as well as the loading effect. Remember, your probing solution should not load the signal source. It should pass on the signal accurately to the oscilloscope and be immune to noise. In addition, an oscilloscope probe is not a wire. Sure it’s an electrical connection between a node on a board and the oscilloscope. But its resistance, capacitance and inductance can have serious consequences as signal speeds increase.

 

Additional probing resources:

See all of Keysight’s oscilloscope probes

Learn more about Keysight probes with the Oscilloscope Probe Resource Center

See oscilloscope probing solutions in action on the Keysight YouTube channel

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

Previously, I wrote about the hardware comparison between the Keysight S-Series oscilloscopes and the Rohde & Schwarz RTO2000 oscilloscopes. In that review, we saw that the S-Series offered better hardware with much higher ENOB and lower noise, you can read the full article here.  Excellent hardware is nice, but you are probably also looking at other features when you are trying to find the right scope for you.  So let’s take a look at some of the other aspects of these two oscilloscopes that we found in our comparison.

 

Let’s start with the basics.

 

The RTO2000 goes up to 4 GHz bandwidth while the S-Series goes all the way up to 8 GHz bandwidth.  In terms of list price, they are very similar.

Keysight   S-Series

US List Price

R&S RTO2000

US Price (4-channel models)

500 MHz

$17,709

600 MHz

$17,455

1 GHz

$21,251

1 GHz

$21,110

2 GHz

$25,805

2 GHz

$25,480

2.5 GHz

$28,335

3 GHz

$28,555

4 GHz

$37,442

4 GHz

$36,955

6 GHz

$53,634

--

8 GHz

$68,813

--

 

Additionally, the S-Series is easy to upgrade.  Keysight offers bandwidth upgrades, memory upgrades, and MSO upgrades in software which means there is little to no downtime with your S-Series oscilloscope.  The RTO2000 requires upgrades in hardware for each of these (bandwidth, memory, and MSO), which means you’d have to send your scope in, wait for the hardware to be installed, and wait for it to be shipped back to you. 

 

Standard memory is also similar between oscilloscopes.  S-Series and RTO2000 both have 100 Mpts with 2 channels and 50 Mpts with four channel operation. However, the RTO2000 does offer one additional step which gives you 200 Mpts with 1 channel operation.

 

Both oscilloscopes have the same memory upgrades up to 800 Mpts.  But again, the RTO2000 offers one additional upgrade that provides 1 Gpts with 4 channels and 2 Gpts with 2 channels.  But keep in mind that these are hardware upgrades for the RTO2000 and software upgrades for the S-series.

 

Maximum sample rate on the S-Series is 10 GSa/s with 4 channels and 20 GSa/s on 2 channels on all bandwidth models.  To get this same maximum sample rate with an RTO2000 you have to purchase the 4 GHz model.  All other RTO2000 bandwidth models (from 600 MHz – 3 GHz) only have 10 GSa/s on each channel. 

 

Now that we’ve seen how the basic specifications compare, let’s take a look at the GUI.  We have heard a lot of customer feedback about the oscilloscope display. We know you want the entire display available to view your waveform so you can get a large, detailed view of your signal.  That’s why all of our menus, results tabs, and drag-and-drop measurements can be minimized.  You can use the S-Series’ full 15 inch display to view your waveform.  In comparison, the RTO2000’s display is 38.3% smaller than the S-Series’ display.  Additionally, they have a side menu that cannot be minimized, so the available area you can use to view your waveform is even smaller.

Figure 1 - these screenshots are to scale

 

There are also many instances where it takes several more clicks on the RTO2000 than the S-Series to make basic measurements.  An example of this is a voltage peak-peak measurement. On the S-Series, you don’t have to go into any special drop downs or pop-up menus to get this measurement. Just drag and drop from the vertical measurements options always available on the edge of the screen.  Measurements like Vpp, Vmax, Vmin, Amplitude, Vrms, and a variety of time measurements are only one click away on the S-Series.  As to overall GUI experience, I suggest you try out both oscilloscopes to find out what suits you better. I felt that the GUI on the RTO2000 was difficult and confusing; however, as a Keysight employee using the Infiniium GUI every day, I may be biased. Ultimately, GUI preference is up to the user, so try them out.  Download a free trial of the Keysight Infiniium GUI.

 

The S-Series also has the largest range of oscilloscope applications and probes to help you measure and analyze your signals. These include 16 compliance tests available on S-Series that are not available with the RTO2000. 

Protocol Decode options available on both the S-Series and the RTO2000:

  • 8B/10B
  • I2C
  • SPI
  • MIPI CSI-3 (M-PHY)
  • USB 2.0
  • CAN
  • LIN
  • FlexRay
  • I2S
  • MIL-STD-1553
  • ARINC 429
  • 10/100 Ethernet
  • UART/RS-232
  • MIPI RFFE
  • CAN-FD

 

But it doesn’t stop here for the S-Series.  There are an additional 15 protocol decode capabilities that are only available on the S-Series:

  • DVI
  • HDMI
  • JTAG
  • MIPI UniPro
  • MIPI DigRF v4
  • MIPI LLI
  • PCIe Gen1 and 2
  • SATA/SAS
  • SVID
  • USB 3.0
  • USB 3.0 SuperSpeed Inter-Chip (SSIC)
  • USB 3.1
  • USB-PD
  • Universal Flash Storage (UFS)
  • eSPI and Quad eSPI

 

There are only 4 protocol decodes available on RTO2000 that are not available with S-Series.  They are:

  • SENT
  • Manchester
  • MDIO
  • SpaceWire

 

Additionally, there are 16 compliance tests that are available with S-Series that are not available with RTO2000. You can see the side-by-side comparison in the table below.

Table 1 - Compliance App Comparison

 

Another important factor in in your oscilloscope purchasing decision is probing. You know that your measurements are only as strong as the weakest part of your measurement setup, and that includes probes.  So having the right probes is key for making the best measurements. With that in mind, let’s look at the probing options for each oscilloscope. There are about 100 Keysight probes compatible with the S-Series. The RTO2000 lists about 18 compatible probes.

 

This difference highlights some major gaps in the RTO2000 probe portfolio:

  • No low noise, low attenuation probe for measuring small signals such as power supply noise
  • Lack of specialty probes such as extreme temperature probes, high sensitivity current probe, and power rail probe
  • Only one compatible high voltage differential probe (100MHz, 1kV), which limits their ability to make power measurements
    • Keysight offers 6 high voltage differential probes in various bandwidth and input voltage range
  • No high bandwidth, 50 Ohm terminated, zero impedance passive probe

 

This means that you can’t make sensitive power integrity measurements that usually need mV of sensitivity and you don’t have the ability to measure supply drift, periodic and random disturbances (PARD), high frequency transients and noise, or perform electrical product validation at extended temperatures.  All of this can be accomplished using the Keysight N7020A power rail probe with the S-Series oscilloscope.

 

It also means you can’t make high-sensitivity, low-level current measurements. Depending on the sensitivity of your current measurements, the noise of the RTO2000 may be too high to view it on the oscilloscope even if they did offer a probe.  You can however make high-sensitivity, low level current measurements on the S-Series oscilloscope.  With the Keysight N2820A probe you can measure down to 50 uA and up to 5A. 

 

In summary, the S-Series offers a much wider solution base with the number of available applications and probes.

 

Up to this point, I haven’t said a lot that puts the RTO2000 in a good light.  So what does the RTO2000 do better than the S-Series?

 

For one, they do their digital down converting in hardware, which means their FFT has a faster update rate.  This allowed them to implement a pretty feature called spectrogram.

Figure 2 - RTO2000's Spectrogram

 

It plots frequency vs time vs power.  I can’t deny that is a feature that might be appealing to RF focused engineers, although the S-Series does offer a nice FFT solution with a number of FFT measurements like channel power, power spectral density, occupied bandwidth. Plus you can set detector types like in a spectrum analyzer, run a mask test on the FFT, and mark a specified number of peaks on screen.

 

Figure 3 - S-Series' FFT measurements

 

The RTO2000 also has 1,000,000 waveforms/second update rate.  However, you only get this update rate under very specific conditions. The oscilloscope must be set to certain settings, and you can’t have much going on besides viewing the waveform.  As soon as you add a measurement or turn on special triggering, that speedy update rate starts to plummet.  For example, in this first screenshot, you can see we have the RTO2000 operating at 1,000,000 waveforms/second, which allows us to observe an infrequent glitch.

 

Figure 4 - Signal with Infrequent Glitch viewed on RTO2000

 

With glitches like this, it is useful to turn on zone triggering in the area of the glitch so that you can capture and observe the glitch.  But when we turned on zone triggering, the waveform update rate plummets.  It drops to 360 scans/second.  In the screenshot below we have applied a zone trigger and you can see that we waited for over a minute to capture the glitch and the scope still hadn’t triggered on it yet.  So the frustration with the RTO2000 is that you might be able to see the glitch, but you might not be able to trigger on it.

 

Figure 5 - Zone Trigger Looking for Infrequent Glitch using RTO2000

 

The spectrogram and the waveform update rate were the two major features of the RTO2000 that stood out in our analysis as an advantage over the S-Series oscilloscopes. 

Our key take-aways after evaluating the S-Series and the RTO2000 side by side are as follows:

  • S-Series has excellent signal integrity, thanks to:
    • Leading edge, low noise front end design
    • System ENOB up to 8.1
    • 10 bit ADC
  • S-Series offers a wide range of measurement capabilities
    • ~100 available probes
    • ~70 protocol decode, compliance, and analysis software applications
  • S-Series is an oscilloscope will meet your measurement needs for years to come
    • You can start with a 500 MHz scope and upgrade bandwidth and capabilities as needed, all through software

 

In conclusion, the Keysight S-Series provides more reliable, accurate measurements thanks to its superior hardware design, and offers a greater number of test solutions thanks to the number of available applications and probes.

 

 

Specifications pulled from “R&S RTO Digital Oscilloscope Specifications” data sheet version 04.00, June 2016.

Measurements and analysis were made on an S804A with firmware version 5.70 and an RTO2044 with firmware version 3.30.1.1

RTO list prices found at www.testequity.com