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2017

I’ll bet you have quite the stock of random probes on your bench. And, you’ve inevitably reached for a probe and made a measurement without even knowing which probe you were using. I’m guilty, too. Probe selection can seem awfully confusing, so we often resort to this “random selection.”

 

The fact is, probe selection can significantly affect your measurement, so let’s break this down and walk through the one simple thing every engineer should know about oscilloscope probes: what the difference is between passive and active. Use this guide as the first step to better, more accurate measurements.

 

The Basics

Passive probes

Passive probes (Fig 1) are the most widely used type of oscilloscope probe. They are rugged and economical and they are what you’ll typically find shipping with your oscilloscope. They’re what you’re bound to find on every engineer’s test bench. These high impedance probes are what we consider “general purpose.”

 

Figure 1: A typical passive oscilloscope probe

Active Probes

Active probes (Fig 2) are a level above passive probes in terms of performance, complexity, and typically cost. You’ll generally purchase these probes separately from an oscilloscope, for a specific measurement application. Compared to passive probes, they need to be handled carefully due to the active circuitry inside the probe head.

 

Figure 2: An active probe with headlights for increased visibility

 

The Big Differentiator: Power

Passive probes contain no active circuitry such as transistors or amplifiers and therefore require no power. A typical 10:1 passive probe and oscilloscope combined circuit is represented in Fig 3.

 

Reading from left to right, you’ll see a 9 Mega Ohm resistor that is one of two parts that make up the 10:1 divider of the probe. Continuing to the right, there’s an adjustable compensation capacitor which you can mechanically adjust to “match” the oscilloscope capacitance. This helps to make sure the probe and oscilloscope are equally compensated and thereby setting you up for an accurate measurement.

 

The oscilloscope makes up the second half of this combined circuit. The 1 Mega Ohm resistor completes the 10:1 divider ratio. In other words, the 9 Mega Ohm and 1 Mega Ohm resistors “divide down” the signal by a factor of 10. The input capacitance is the standard capacitance of your scope, typically printed on the front panel.

 

Figure 3: Oscilloscope-Passive probe combined circuit layout

 

Active probes differ from passive probes because they have “active” circuitry, typically in the form of transistors instead of resistors as well as an amplifier. You can see the typical active probe configuration in Fig 4.

 

Active probes differ from passive probes because they have “active” circuitry, typically in the form of transistors instead of resistors as well as an amplifier.

 

Figure 4: Typical active probe circuit layout

 

The physical look of an active probe is also much different from a passive probe. The head of the probe contains the active circuitry, where the filtering and conditioning are done. The pod interface that plugs into the scope tells the scope which probe has been plugged in, what we call “auto-sensing.” After sensing which probe is plugged in, there is auto-configuration that happens (i.e. settings are auto-adjusted based on the probe). This isn’t to say this isn’t possible with passive probes, it is just not as common.

 

Performance Difference: How to Select a Probe

Passive probes are great for making general purpose measurements. They have a wide dynamic range and bandwidth as high as 500 MHz when connected to the 1MOhm input of the oscilloscope. They work well if you’re working in the DC and low-frequency range. They can also be sufficient for making quick quantitative measurements, such as if a clock is running or if the source is on- simple “yes or no” questions where a high degree of accuracy isn’t required.

 

Active probes are often more expensive than passive probes, but they offer a superior level of performance that may be essential in certain circumstances.

 

Active probes are often more expensive than passive probes, but they offer a superior level of performance that may be essential in certain circumstances. The real driving factor is when you need a high degree of signal integrity, you should use an active probe. But why? It really has everything to do with probe loading.

 

Signal Integrity: What Probe Loading Has to Do With It

When you touch a probe to your DUT, the probe loads the signal and this is called probe loading. To have high signal integrity, you need the least loading possible. The amount of loading on your signal is determined by the probe’s input impedance in relation to the source impedance.

 

To have high signal integrity, you need the least loading possible. The amount of loading on your signal is determined by the probe’s input impedance in relation to the source impedance.

 

The probe’s input impedance is a function of frequency. It stays pretty flat from DC up to a certain frequency but as frequency continues to go up, the probe’s input impedance goes down, as the capacitance of the probe starts to come into play. The more capacitance, the lower the impedance. As the frequency goes up above the crossing point of ~10kHz, this is where we can really see a difference in performance between an active and passive probe. The active probe has a low capacitance value, leading to higher impedance and less loading. You can see this effect plotted in Figure 5.

 

This effect trips up a lot of engineers.

 

This effect trips up a lot of engineers. You might think that a passive probe is just fine because it worked well at low frequencies, but as soon as you try to use it to measure a higher frequency signal (past the crossover point of 70 MHz in the example in Figure 5), there’s a significant performance degradation where you’d be better off using an active probe.

 

Figure 5: Impedance vs Frequency comparison of active and passive probes

 

Key Takeaway

Probe selection does affect measurement accuracy and signal integrity. Next time you go to make a measurement, consider your signal’s speed and the type of measurement you’re trying to make (quantitative vs qualitative) before deciding between a passive and active probe. Probe “random selection” will be a thing of the past.

 

Check out the video version of this blog -> Active vs. Passive Probes- Take the Mystery Out of Probing - YouTube  

Access free probing resources ->  Take the Mystery Out of Probing | Download Probe Training Kit  

If you’re an Infiniium user, you’ll definitely want to keep reading. The 6.1 software update just launched and it’s packed with new features that will improve your testing efficiency. Use this as a guide to make use of the new tools and enhancements that you’ll find in this software update.

 

These updates include:

  • New Jitter Decision Feedback Equalization
  • Improved Clock Recovery and Mask Testing for PAM4 and NRZ
  • New Software Update Analysis Tool
  • New Impedance Warning Function
  • New Power Integrity Analysis Application

 

Jitter Decision Feedback Equalization

Technological advances towards achieving greater Ethernet speeds presents two design possibilities, NRZ and PAM4, and each comes with a unique set of challenges. NRZ (Non-Return-to-Zero) has evolved over 50 years to 100G (25/28G, 4 lanes) and 400G (56G, 8 lanes). From a time domain perspective, NRZ consists of 1’s and 0’s and can be referred to as PAM2 (pulse amplitude modulation, 2-level) with two amplitude levels that contain 1 bit of information in every symbol. The NRZ eye diagram provides timing and voltage used to measure link performance and contains a single eye. But this single eye technology requires advanced technology in order to achieve the higher 400 Gb/s data rate.

 

The current 400 Gb/s challenges include totally closed eyes, shorter unit intervals (UI), tighter jitter requirements, and the mandatory use of forward error correction (FEC). These closed eye issues require enhanced receiver equalization such as continuous-time-linear equalization (CTLE) and decision feedback equalization (DFE) to correct. Moreover, new communication standards are requiring increased receiver sensitivity (down to 50 mV) and jitter budgets are even tighter for 400G at 17ps.

 

This software update adds a Jitter Decision Feedback Equalization function (DFE) to meet these increasing demands.

 

Improved Clock Recovery and Mask Testing for PAM4 and NRZ

If you’re working in 400G, you’ll be glad to hear of some major enhancements to the existing PAM4 solution. In the new software update, an improved PAM4 clock recovery algorithm was added along with jitter measurements on Decision Feedback Equalization (DFE) for NRZ signals.

 

A “Draw Mask” feature was also added to allow users to draw their own polygon mask specific to their eye pattern, enabling exact mask tests to customer specified limits.

 

New Software Update Analysis Tool

If you’ve ever had difficulty determining if an application or protocol is compatible with your scope’s software, you’ll be interested in the new Software Update Analysis tool rolling out in this software update. This tool will help you quickly determine if all applications and protocols on an oscilloscope are compatible with the latest software. This feature also allows software updates to oscilloscopes in secured laboratories that cannot have access to internet-supplied software.

 

New Impedance Warning Function

Safety is always a top concern. We are always working to improve product quality and reliability, and with this comes a new Impedance Warning function. With the new software installed, you will be notified when you have selected a lower voltage 50 Ohm input, and you'll see the max input level allowed on screen. This will help to reduce the chances of user-damage to the oscilloscope input.

 

New Power Integrity Analysis Application- N8846A

The new Power Integrity Analysis Application-N8846A was added to Keysight’s already industry-leading set of N8833A/B cross talk applications. This application was specifically designed to target power supply-induced cross talk.

 

Download Now > Infiniium 6.10 System Software for Windows 7 | Keysight   

Knowing the quality of the scope’s measurement system is paramount when you need to have accurate measurement results.  While banner specs like bandwidth, sample rate, and memory depth provide a basis of comparison, these specifications alone don’t adequately describe oscilloscope measurement quality.  

Figure 1: Keysight Infiniium S-Series Oscilloscope

 

Seasoned scope users will also compare a scope’s update rate, intrinsic jitter, and noise floor, all of which enable better measurements.  For scopes with bandwidths in the GHz range, another quality metric involves characterizing a scope’s ENOB. 

 

What is ENOB in the first place?  It stands for Effective Number of Bits and is really the measure of how well your oscilloscope accurately represents the captured waveform. 

 

The higher the ENOB, the better the oscilloscope sees the signal the way the components in your design see the signal.

 

Bits of Resolution and Effective Number of Bits

The ADC is the most recognized component on the oscilloscope. It converts the analog data to digital data. It drives the oscilloscope’s bits of resolution.  It is defined by its sample rate and its signal to noise ratio.  Typically, scopes have 8 bits of resolution, although recently oscilloscopes have added 10 and 12 bit ADCs.

 

Effective number of bits (ENOB) is a measure of the dynamic performance primarily associated with signal quantization levels of your oscilloscope.

 

While some oscilloscope vendors may give the ENOB value of the oscilloscope’s ADC by itself, this figure has no value. ENOB of the entire system is what is important.

 

While the ADC could have a great ENOB, poor oscilloscope front-end noise would dramatically lower the ENOB of the entire measurement system.

 

Oscilloscope ENOB isn’t a specific number, but rather a series of curves. I am often asked, “what is the ENOB of a specific Keysight oscilloscope?”  Many vendors simply state a specific single number for ENOB, for example, an ENOB of 5.5.  The reality of the situation is this is just not how effective number of bits work. They are frequency dependent. So, it may be 5.5 at one specific frequency setting but is probably not 5.5 across the entire bandwidth of the oscilloscope.

 

ENOB was established by IEEE in 1993 as a measurement of an oscilloscope’s signal integrity and measurement accuracy. 

 

 

It directly correlates to an oscilloscope’s signal to noise ratio.  A higher ENOB will provide better oscilloscope measurements for Jitter, eye height and width, and amplitude.  ENOB is a metric, and does not indicate what is causing signal integrity issues.

 

Effective number of bits is directly related to the ADC within an oscilloscope.  In general, the bits of resolution within the ADC determines the quantizing levels for your oscilloscope as shown in Figure 1.  

 

 

Bits of Resolution

Quantizing Levels

At 1 Volt, Full Scale

 1 LSB =

At 16 mV, Full Scale

1 LSB =

8-bit

256

3900 uV

62.5 uV

10-bit

1024

976 uV

15.6 uV

12-bit

4,096

244 uV

3.9 uV

14-bit

16,384

61 uV

1.0 uV

 Figure 2: Oscilloscope specification comparison

 

Increasing the number of ADC bits makes each quantizing step size smaller, so the maximum error is minimized.

 

ENOB is measured as a fixed amplitude sine wave at varying frequencies.  Each curve is created at a specific vertical setting while frequency is varied. ENOB calculations are easy to make. 

 

  1. First, input a perfect sine wave, capture it on a scope and measure the deviation from the result vs the input.                                                                                                                                                                          For example, input a sine wave from a PSG at 1 GHz into the scope and measure the 1 GHz sine wave. 
  2. Then fit it against a perfect 1 GHz sine wave. 

 

The difference between the data record and best fit sine wave is assumed to be signal error.  ENOB considers noise, ADC non-linearities, interleaving errors, and other error sources. 

 

What erodes the bits of resolution?

ENOB is primarily impacted by noise and distortion.  Noise of course effects your signal-to-noise ratio and distortion impacts the total harmonic distortion.   If the base noise of an oscilloscope is greater than the quantizing levels of the ADC, then there is no way for the scope to accurately represent the digital signal level to the least significant bit. 

 

ENOB values will always be lower than the oscilloscope’s ADC bits.  In general, a higher ENOB is better. However, a couple cautions need to accompany engineers who look exclusively at ENOB to gauge signal integrity. ENOB doesn’t consider offset errors or phase distortion that the scope may inject.  So, it is also important to look at the base noise of an oscilloscope as well as its frequency response (amplitude flatness), phase linearity and gain accuracy to get a complete picture of the accuracy of an oscilloscope.  

 

In general, by choosing an oscilloscope with superior ENOB, you are choosing a scope with better signal integrity.

 

You not only impress your colleagues but you also get more accurate waveform shapes, more accurate and repeatable measurements, wider eye diagrams and less jitter.

 

Figure 3: ENOB of the S-Series DSOS104A 1 GHz real-time oscilloscope from 100 MHz to 1 GHz.

 

For more information on determining measurement quality, check out the Scopes University S1E4 video, Determining Oscilloscope Measurement Quality.

Eye diagrams are extremely helpful in testing the physical layer fidelity of clock or serial data, but many engineers don’t know:

  • What they are
  • Why they should use one
  • How to easily set one up

 

They actually aren’t that complex when broken down.

Eye diagrams can quickly give you insight into your signal, along with any jitter or anomalies that may be present that you might know exist.

 

What they are

What is an eye diagram? Eye diagrams are a layered view of every bit transition combination. There are eight of these in total. You can see in Figure 1 how each of these are layered to make up the eye. This provides a composite picture of the overall quality of a system’s physical layer characteristics like amplitude variation, timing uncertainties, or infrequent glitches.

 

Figure 1: Bit transition combinations

 

Why you should care

An eye diagram is used to detect jitter, but what is jitter and why is jitter bad?

 

Jitter can cause errors in the data that you are trying to transmit. If there is too much of it riding on your signal, the data that is sent will be interpreted incorrectly by the receiving end because the edge crossings aren’t occurring when they should be.

 

Figure 2: Jitter causes errors in the interpreted waveform.

 

How to create an eye diagram

One of the first things many people think when they see an eye diagram on an oscilloscope is “how do you get it to look like that?”

 

This layered view of bit transitions is not something that a normal trigger would be capable of displaying. The answer is, the oscilloscope utilizes a built-in clock recovery system. Clock recovery is actually pretty straightforward. Some signals have an explicit clock signal, and some have an embedded clock. An explicit clock can be driven right into one of the oscilloscope channels, but embedded clocks have to somehow be de-embedded, or recovered, hence “clock recovery.”

 

Figure 3: Clock recovery dialog

 

There are three different ways to utilize the clock recovery system, which all depends on how well you know the bitrate of your signal (the width of each bit):

  1. Fully automatic
    1. The oscilloscope will calculate the ideal bitrate (or nominal data rate) of your signal.
    2. This should be used when you have no idea what the bitrate is and you need the oscilloscope to figure it out.
    3. However, this method is only about 80% accurate.
  2. Semi-automatic
    1. In most situations, you should have a rough idea of what the bitrate should be. You can very easily make a bitrate measurement on the oscilloscope to find this estimation. This method allows you to enter your rough measurement and then use that information as a seed as it calculates the exact ideal bitrate.
    2. This method is significantly more accurate.
  3. Manual
    1. This method should be used if you know the exact ideal bitrate of your signal.
    2. This is the most accurate method of clock recovery.

 

There are a few other settings within the clock recovery menu. To learn more about these, make sure you check out episode 3, How to set up an Eye Diagram, and episode 5, How to Measure Jitter, of Scopes University video series.

 

Once you have your clock recovery system set up, all you have to do to set up the eye is press “auto setup”. You will see in just a few seconds that the eye diagram has begun to form. Over time, you will be able to see if there is any jitter or anomalies in your signal. Generally, you will want to let the test run for a longer period of time.

 

The longer you let it run, the more data is collected, and the more jitter, anomalies, or any infrequent events you can see.

Figure 4: Eye diagram with jitter

 

From here, you can analyze the eye diagram further by using the color grading key. This allows you to visually analyze the frequency of each edge crossing. You can also very easily turn on a histogram on the eye to determine whether the jitter is deterministic or random. This will help you decide if this is something you can fix with a phase locked loop filter or if you have to redesign the component. Perhaps this is a topic for the next blog, though!

Figure 5: Histogram of the eye diagram

 

To learn more about what I talk about here, check out these Scopes University videos.

Episode 3: How to set up an Eye Diagram

Episode 5: How to Measure Jitter