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 isn’t– LPDDR2 application example– Correcting for less-than-optimal probe placement

How probe calibration works(and why it matters)

Let’s start with a quiz

• What’s 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)

Vsignal Vmeas

Calibration

• Measure the probe’s response using a precision instrument (VNA)

• This measurement shows how the probe’s 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 oftrace, 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

0

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0.01 0.1 1 10 100Im

peda

nce

Mag

nitu

de (O

hms)

Frequency (GHz)

LeCroy Probe ImpedanceDxx05 Probe with Dxx05-SI

LeCroy Dxx05 with Dxx05-SISolder-in Tip (Differential)

LeCroy Dxx05 with Dxx05-SISolder-in Tip (Single-Ended)

Another quiz

• Question: When you’re 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 wasn’t there?

• Answer: It depends…

Back to Calibration

• Measure the probe’s response using a precision instrument (VNA)

• This measurement shows how the probe’s 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 1 Port 2

Fixture

The importance of the reference signal

• Source-referred: Reference measured without probe loading the circuit

Probe response

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

• Input Referred: Reference measured with probe loading the circuit

Probe response

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)

Unloaded Loaded

Source-referred Input-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 it’s possible to convert between them

• Neither way is “right” or “wrong” – but knowing which one you are using is critical to understanding what you’re seeing and measuring

De-embedding fromanother angle

A virtual probing sample application

Pattern Generator

ISI channel

Point A: We’d like tosee the signal here

Point B: This is the pointwe 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 A Point 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 isn’t

Some lousy-looking LPDDR2 signals

Some lousy-looking LPDDR2 signals

So what’s going on here?

PCB

Memory Controller DRAM

So what’s going on here?

Controller DRAMZ0 = 50Ω

RT >> 50Ω

VA VBVA

VB

T1 T2 T3

Measuring the propagation delay

Now we have a simple model

Z0 = 50Ω

RT >> 50Ω

VA VB

TD = 400ps

Controller DRAM

Signals at the Virtual probe point

Signals at the Virtual probe point

LPDDR2 “probing problem”: conclusions

• Be aware of:– What you want to measure– What you’re actually measuring

• If these two things don’t look the same, it’s not necessarily the probe’s fault

• Given some knowledge of your system, it is possible to “virtually” move the probe to a more appropriate location

Thank youpatrick.connally@teledynelecroy.com