Practical techniques and tips for probing and de-embedding

Agenda Case study 1: How probe correction works (and why it matters) – Untangling the terminology: calibration, correction, compensation, de-embedding, loading – Showing what it means in practice Case study 2: A different way to look at de-embedding – Using a frequency-domain view of your signals to help inform what you see in the time domain – An example: determining filter bandwidth Case study 3: A probing problem that isnt – LPDDR2 application example – Correcting for less-than-optimal probe placement

How probe calibration works (and why it matters)

Lets start with a quiz Whats the difference between: – Calibration – Correction – Compensation – De-embedding What do they have in common? – They all have to do with how the measurement system accounts for the effect of intervening components (such as a probe in this case) on the signal under test

Ideal probe vs real probe Ideal probe: – Perfectly flat magnitude response – Perfectly linear phase response – No loading (infinite impedance) Real probe: – Non-ideal magnitude response – Non-ideal phase response – Some loading (finite impedance) V signal V meas

Calibration Measure the probes response using a precision instrument (VNA) This measurement shows how the probes output signal differs from a reference input (assumed to be ideal) Use this data to build a correction filter so that the displayed signal matches the reference

Compensation We want to view the oscilloscope/probe combination as a single integrated measurement system The scope channel and probe have both been calibrated – Each has its own correction data Combine the channel correction with the probe correction to obtain a calibrated system performance

Use model to remove effects of trace, connector and cable De-embedding Using known information about a physical interconnect or component (typically S-parameter models from measurement or simulation) to remove its effect from a measurement. PCB Want to measure here DUT

PCB Virtual probing Used when you want to virtually move a probe from one point in a circuit to another. Requires models of other components which affect the signal. PCB Want to measure here PCB S-parameter models

Loading We have to take some energy from the signal to measure it This means the probe tip must have a finite impedance across the frequency range of interest Obviously we want to keep this impedance as high as possible

Another quiz Question: When youre looking at a probed signal on your scope, does the waveform on screen: – Show the signal with the probe loading? – Show the signal as if the probe wasnt there? Answer: It depends…

Back to Calibration Measure the probes response using a precision instrument (VNA) This measurement shows how the probes output signal differs from a reference input (assumed to be ideal) Use this data to build a correction filter so that the displayed signal matches the reference VNA Port 1Port 2 Fixture

The importance of the reference signal The measured probe response will depend on the reference signal used in the VNA measurement.

Two different ways to calibrate The measured response is used to obtain the probe correction, which matches the displayed signal to the reference. Source referred: Displayed signal matches source signal (not including probe loading). Input Referred: Displayed signal matches probe input signal (including probe loading).

How does this look in practice?

Eye diagram comparison – direct cabled (3 Gb/s PRBS7) UnloadedLoaded

Source-referredInput-referred Eye diagram comparison – probed (3 Gb/s PRBS7)

Eye diagram comparison (3 Gb/s PRBS7) Direct cabled, unloaded Direct cabled, loaded Probed, source-referred Probed, input-referred

Probe calibration – the conclusions Be aware of the terminology There are two ways to calibrate a probe: – Source-referred – Input-referred Probes which have been calibrated in different ways will show the signal differently – But its possible to convert between them Neither way is right or wrong – but knowing which one you are using is critical to understanding what youre seeing and measuring

De-embedding from another angle

A virtual probing sample application Pattern Generator ISI channel Point A: Wed like to see the signal here Point B: This is the point we can actually access This is not an ideal configuration

A virtual probing sample application Pattern Generator ISI channel

Eye diagrams at Point A and Point B Point APoint B

Channel insertion loss (S21)

Signal spectral content

Ideal de-embed filter

Using the ideal de-embed filter

De-embed filter with 8GHz bandwidth limit

De-embed filter with automatic bandwidth limit

Spectral view of de-embedding: the conclusions The ability to view and understand a situation in both time- and frequency-domains is critical Looking at both domains helps to understand what the de-embed process is doing to your signal The spectral view serves as a visualization of the signal-to-noise ratio

A probing problem that isnt

Some lousy-looking LPDDR2 signals

So whats going on here? PCB Memory Controller DRAM

So whats going on here? Controller DRAM Z 0 = 50Ω R T >> 50Ω VAVA VBVB VAVA VBVB T1T1 T2T2 T3T3

Measuring the propagation delay

Now we have a simple model Z 0 = 50Ω R T >> 50Ω VAVA VBVB T D = 400ps Controller DRAM

Signals at the Virtual probe point

LPDDR2 probing problem: conclusions Be aware of: – What you want to measure – What youre actually measuring If these two things dont look the same, its not necessarily the probes fault Given some knowledge of your system, it is possible to virtually move the probe to a more appropriate location

Thank you