Column Control DTX

Evaluating Oscilloscope Signal Integrity

Application Notes

Why Does Signal Integrity Matter?  

The term “signal integrity” surfaces regularly in the electronic test. Signal integrity is the primary measure of signal quality. The importance of signal integrity increases with bandwidth, the need to view small signals, or the need to see small changes on larger signals. Why does oscilloscope signal integrity matter? 

 Signal integrity impacts all oscilloscope measurements. The amount of impact signal integrity can make on signal shape and measurement values might surprise you. Oscilloscopes themselves are subject to the signal integrity challenges of distortion, noise, and loss.

Oscilloscopes with superior signal integrity attributes provide a better representation of signals under test, while oscilloscopes with poor signal integrity attributes show a poorer representation of signals under test. This difference impacts engineers’ ability to gain insight, understand, debug, and characterize designs. Results from oscilloscopes with poor signal integrity can increase risk in development cycle times, production quality, and components chosen. To minimize this risk, it is a good idea to evaluate and choose an oscilloscope that has high signal integrity attributes. 

There are several oscilloscope attributes that work in conjunction with each other, so they must be considered holistically. There are many banner specifications that are said to give you the ‘best’; resolution, noise floor, jitter, etc. But you need to be mindful that one good specification will not ensure the best representation of the signal. You must consider the entire system to ensure you will be able to make the most accurate measurements possible. Looking at only a single signal integrity attribute in the absence of others can lead a user to false conclusions.  

Table of Contents 

  • ADC Bits and Minimum Resolution  
  • Scaling Impact on Resolution 
  • Noise 
  • Frequency Responses 
  • Correction Filters 
  • Software Filters 
  • ENOB (Effective Number of Bits) 
  • Intrinsic Jitter 

Correction Filters 

Some oscilloscopes have strictly analog front-end filters that determine frequency response, while others apply correction filters in real-time. Correction filters are typically implemented in hardware DSP blocks and are tuned for a particular family of oscilloscopes to create a flat magnitude and phase response. Combining correction filters with frontend analog filters creates flatter magnitude and phase responses versus raw analog filters alone. Oscilloscopes of superior quality include both analogs as well as correction filters to create a uniform and flat frequency response. 

Frequency response shapes generally are named by their roll-off characteristics. Responses that have brick-wall filters are desired as they produce less noise by more quickly attenuating out-of-band noise. For fast edges, out-of-band higher harmonics are quickly attenuated resulting in slight under- and over-shoot. Responses that have a Gaussian roll-off don’t show as much ringing, but with the trade-off is additional noise. 

Software Filters 

In addition to correction filters, oscilloscopes can employ filters in software. Software bandwidth filters typically require the scope to operate at maximum sample rate to prevent aliasing, run slower, and generally don’t have the signal integrity characteristics of equivalent hardware or analog filters. However, these filters can be quite flexible. 

One example of a software filter is the Infiniium PrecisionProbe application. The application is aimed at increasing signal integrity by de-embedding the effects of channels, probes, or cables on the measurement. The application allows for quick 2-minute characterization of a probe or cable’s S21 parameter using the oscilloscope’s internal calibration signal that has a very fast edge. Using this information, the application creates a filter inversely proportional to the cable’s filter up to the roll-off point, allowing the measurement to be show with the effects of the BNC cable extracted.

Intrinsic Jitter 

Jitter describes the deviation from the ideal horizontal position and is measured in ps rms or ps pp. A number of jitter contributors naturally occur in high-speed digital systems. Jitter sources include thermal and random mechanical noise from crystal vibration. 

Additionally, traces, cables, and connectors can further add jitter to a system through intersymbol interference. Excessive jitter is bad. Jitter can cause timing violations that result in incorrect system behavior. Too much jitter in communication systems will cause unacceptable bit error rates (BER) resulting in incorrect transmissions. Measurement of jitter is necessary for ensuring high-speed system reliability. 

If you need to make jitter measurements, understanding how well your oscilloscope will make those measurements is critical to interpreting your jitter measurement results. Oscilloscopes sample and store digitized waveforms. Each waveform is constructed of a collection of sample points.  

A perfect oscilloscope would acquire a waveform with all sample points equally spaced in time. However, in the real world, imperfections in the internal scope circuitry horizontally displace the ADC sample points from their ideal locations and this value is represented in the jitter measurements that the oscilloscope makes. Oscilloscopes themselves have jitter and when they make a jitter measurement, they can’t determine which portion of the jitter measurement result came from the device under test versus the scope itself. 

A perfect oscilloscope would report zero jitters if it took a jitter measurement on a perfect jitter-free signal. However, scopes are not free from jittering themselves. Oscilloscope jitter can come from interleaving errors, the jitter of the ADCs sample clock input signal, and other internal sources. This collection of horizontal error sources produces a total time error called the equivalent sample clock jitter, or simply sample clock jitter. This is also called the intrinsic source jitter clock (SJC). Oscilloscope vendors shorten the term to “intrinsic jitter” and use this term to mean the minimum intrinsic jitter value over a short time record. 

×

Please have a salesperson contact me.

*Indicates required field

Preferred method of communication? *Required Field
Preferred method of communication? Change email?
Preferred method of communication?

By clicking the button, you are providing Keysight with your personal data. See the Keysight Privacy Statement for information on how we use this data.

Thank you.

A sales representative will contact you soon.

Column Control DTX