About Oscilloscope Probing Philosophy

Question

Recently, I read Tektronix's Probe Bandwidth Calculations. It point out that Tektronix and Agilent have different probing philosophy:

There are two schools of thought on what an oscilloscope probe should actually display on an oscilloscope. Tektronix subscribes to the philosophy that a probe should measure the unloaded, or original, signal. Agilent subscribes to a different philosophy that says a probe should measure the loaded signal.

Because I've never used Agilent's scope, I want to know if you've noticed this. And what about other scope vendors such as LeCroy, RIGOL, etc, which philosophy they subscribe to.

Answer 1

It would appear that LeCroy follow Agilent/Keysight in this respect (or, at least, Tektronix's presentation of what Agilent's method is). This can be seen from their probe manuals, for example for the ZS4000 active (single-ended) probe. They provide the probe impedance as a function of frequency and advocate that the user corrects for it when interpreting the measurement, using the formula:

$$V_\mathrm{out} = Z_\mathrm{probe}/(Z_\mathrm{probe}+Z_\mathrm{source}) \times V_\mathrm{in}$$

I avoid quoting further from their manual to avoid potential copyright issues (because it would require the whole section to be quoted to reproduce it properly here), but if you follow the link and read the manual, you will find that everything is quite clearly stated.

For the differential probes operating in the 10 GHz range (for example, the WaveLink D1030), their approach is slightly different to either of the ones presented in the Tektronix technical brief. The probes measure the loaded signal, as per Agilent, but they provide equalization software (Virtual Probe) to recover the unloaded signal. One models the circuit impedances and indicates the type and location of the probe, and the de-embedding is done accordingly. They summarize it as follows (quoting from the WaveLink probe manual):

Teledyne LeCroy probes are calibrated at the factory using a Vector Network Analyzer (VNA) to measure a system (probe plus test fixture) frequency response. The test fixture is de-embedded from the measurement using Teledyne LeCroy's Eye Doctor tools so the remaining frequency response is due to the combination of the test signal and the probe loading on the test circuit. The system frequency response is then calculated for these remaining circuit elements.

If you wish to de-embed the effect of probe loading on your circuit, you can use the appropriate equivalent circuit model ... and Teledyne LeCroy's Eye Doctor tools to accomplish this.

You can also use Teledyne LeCroy’s Virtual Probe option. This option allows you to select the probe tip from a list of supported tips. Your selection applies a corresponding s-parameter file that is derived from the equivalent circuit model of the tip.

However, I haven't actually used these probes, so I can't comment on how good the software is.

Answer 2

I am a user of Agilent MSO oscilloscopes. I had never heard about anything like "probing philosophy" before you showed me this document. But I have heard much about "signal measurement techniques".

In short, this document is about marketing rubbish, as you (probably) know that Tektronix and Agilent compete for the market and Tek comes after Agilent here. I will not advocate any of them, they both provide good, state-of-the-art products, but the use of a "compare to the leader" method in a market competition is generally used by the one following the leader, not by the leader itself.

Look for yourself, the document discusses the "probe bandwidth", saying nothing about the components from which that bandwidth results, i.e. capacitance (I found no mention of the "pF" unit in the whole doc) in the first place. There are no physics, only claims in the doc.

The Agilent o-scopes I'm familiar with, have internal switches to select between 10 Megs and 50 Ohms input resistance while input capacitance is mostly a property of a passive probe. There are also active probes to make HF/VHF measurements better/possible.

What to use depends on the appropriate signal measuring technique which, in turn, depends on the situation you are facing. There is no instrumentation task that can be done with Tek while not with Agilent, and vice versa, of course.

Therefore, such "philosophy" is about marketing [rubbish], not electronics [truth :-].

P.S. Also, there is Agilent's answer for Tektronix's question on the philosophy.

Answer 3

Informative only:

• Agilent probes catalog

  Agilent Oscilloscope Probes and Accessories Selection Guide [PDF]

  (here, see also p. 15 for adapter to use Tektronix active probes)

• LeCroy probes catalog

  Oscilloscope Probes and Probe Accessories [PDF]

• Rigol probes catalog

  Probes & Accessories [HTML]

I think theirs philosophy (if any) is simple: "we provide solution for any your problem".

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// please improve this answer by adding items with links to other vendor probes //

Answer 4

@diverger,

From Agilent's Side-by-Side Comparison of Agilent and Tektronix Probing Measurements on High-Speed Signals referenced by me earlier:

Conclusion

Regardless of whether you use Tektronix’ or Agilent’s measurement performance standards for characterizing the performance of high-bandwidth active probes, Agilent’s probes outperform Tektronix probes on all counts as documented in this application note. Agilent’s 1134A InfiniiMax active probes using various probe heads (browser, solder-in) demonstrate less probe loading and more accurate reproduction of the signal applied to the probe tips. Even if you apply a Tektronix standard where you ignore probe loading and compare measured signals to unloaded/unprobed signals, Agilent’s probes still out-perform Tektronix probes when you compare the same probing configurations (Tektronix browser connection versus Agilent browser connection, Tektronix solder-in connection versus Agilent solder-in connection) and when you use identical sampling techniques (real-time), as shown in this application note.

With the introduction of the new 1130 Series InfiniiMax differential active probes, Agilent adopted a new probe architecture/topology where the probe amplifier is physically displaced from the probe head using precision RF transmission line technology for high-impedance connections. This new probe technology enhances usability and measurement performance for high-bandwidth applications. In fact, Agilent’s new 1130 Series InfiniiMax active probing system was recently selected as the 2002 Test & Measurement Product-of-The-Year award sponsored by EDN magazine. To our knowledge, this is the first time EDN has selected an “accessory” for this award in the Test & Measurement category.

If it's not about marketing. What is it about?

IMO, I made a mistake in my first answer having suspected only Tektronix in marketing rubbish. It was be necessary to suspect both of them, remembering however that Agi was the defensing side. (And we need to recognize that Agi won this PR boxing round.)

I think neither Tek nor Agi but the engineer selects the method to measure and the corresponding background to interpret the result. Both Tek and Agi give the instruments for both approaches.

From the both documents I understand the one technical thing: they both beaten capacitance issues so well, they both beaten attenuation issues so well, but they beaten inductance issues not so, one did it better than another. I also assume that one patented some on this earlier and wider than another. It's very probable the philosophy (as PR base) itself and its difference (as PR task) were grown up on that.

As my conclusion, what do you prefer to buy from Tek, Agi, LeCroy, etc: a good thing you need or a puffing story about why other sucks? Tek/Agi drops its shit to the fan directed to the competitor, calling this "philosophy". Let's they taste their shit self, without us.

Good luck.

P.S. Both documents look like a WWF wrestling battle there each word and action are set up long before the battle begins :-]