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x1 Oscilloscope Probe Shootout

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gf:
Keysight distinguishes two different frequency responses of a probe

* The response with respect to the probe tip voltage as the probe loads the circuit (which is independent of source impedance)
* The response with respect to the voltage at the probe point as if the probe is not there (assuming 25 Ohm source impedance)Nobody here seems to consider #2 :-//
(Only rf-loop mentioned "according to normal practice Probes are classified by measuring the 25ohm source impedance" which suggests that he did mean to the latter one.)

artur0089:

--- Quote from: 2N3055 on September 25, 2023, 11:49:16 am ---So why are you measuring probes if you already know what manufacturer said?

--- End quote ---
Trust but check!

--- Quote from: 2N3055 on September 25, 2023, 11:49:16 am ---What is the point of the topic ?  ^-^

--- End quote ---
Bode plot on this Rohde & Schwarz - not everything is clear...

2N3055:

--- Quote from: gf on September 25, 2023, 12:07:29 pm ---Keysight distinguishes two different frequency responses of a probe

* The response with respect to the probe tip voltage as the probe loads the circuit (which is independent of source impedance)
* The response with respect to the voltage at the probe point as if the probe is not there (assuming 25 Ohm source impedance)Nobody here seems to consider #2 :-//
(Only rf-loop mentioned "according to normal practice Probes are classified by measuring the 25ohm source impedance" which suggests that he did mean to the latter one.)

--- End quote ---

That is something different.

Probe will, by virtue of it's own electrical characteristics, when connected into a circuit, become part of the circuit.
This means that connecting the probe will change voltages, phases, resonant frequencies, currents and whatever in that part of circuit you connected a probe to. That is reason why you cannot measure accurate frequency or sometimes measure at all crystal oscillators etc.. etc.

In some Keysight scopes, they have a software correction mode (Source Estimate) that by presuming 25Ω source impedance it artificially "removes" loading, i.e. it approximates what loading and signal distortion probe caused when connected and tries to approximate how signal looked before. It does that by presuming clean 25Ω resistive source impedance and by knowing what that particular probe loading looks like.

The fancy sounding "True View" is just like normal probe view on every oscilloscope.

This is remotely related to topic here but not relevant. Good read though.

Problem with probes is this:

1. Probes load circuit. By doing that they change real signal in different degrees and levels.
2. Amount of signal "degradation" will depend on signal source impedance and source impedance characteristics.
3. Then we have probe's own transfer function, the measure of how signal on input relates to signal on output, as consequence of it's electrical schematics.
4. Scope input also presents complex schematic but lets pretend that probe output and scope input are already married in probe's electrical schematic to simplify things because probe is supposed to be used that way and this works for us in this case.
5. Combine all of that.

To make possible to have at least some kind of way to compare probes, standardization bodies made standard to test probe BW with 25Ω source impedance.  A terminated 50Ω generator. That allows for some kind of comparison but BW measured that way will not correspond with frequency response of probe when connected to any kind of other circuit.
If you connect probe to high impedance source, your BW will be dominated by RC filter (or RLC if  source is inductive) created from source impedance and shunting capacitance of probe's tip.

One other way to measure probe is with a Bode plot. But that introduces another complication. Namely, Bode plot will plot phase/gain frequency response, but will REMOVE loading to the signal source by the probe. How? Because it will measure delta, difference, between input and output signal as a ratiometric measurement, compensating for changes in input signal.. In effect, it will provide result of measurements as if source impedance of generator was 0Ω. Of course it is not ideal, but as long loading is within dynamic range of inputs it won't care.

Huh? So no more answers but more questions...

So what do we conclude form this? 

One example: audio power amplifiers or power supplies have very low impedance on their outputs for instance. When measuring those, your probe will behave similar to Bode plot (with added benefit that you will also know phase response of the probe).

If you wanted to measure on 600Ω system (audio) you would be prudent to insert impedance converter between your AWG and probe and run a frequency sweep with 600Ω impedance, characterizing it for that use...

I just tried now and with paralleling terminators I dropped source impedance to 12.5Ω. -3dB pint raised from 24MHz to 28 Mhz.
So when measuring some PSU output for ripple or noise, in usual 20MHz BW for that purpose, my simple 60MHz(10x) rated at 6Mhz when 1x will comfortably and accurately perform that task.

Everything else will be anybody's guess...

mawyatt:

--- Quote from: 2N3055 on September 25, 2023, 02:00:19 pm ---
Probe will, by virtue of it's own electrical characteristics, when connected into a circuit, become part of the circuit.
This means that connecting the probe will change voltages, phases, resonant frequencies, currents and whatever in that part of circuit you connected a probe to. That is reason why you cannot measure accurate frequency or sometimes measure at all crystal oscillators etc.. etc.

--- End quote ---

Sorta the Heisenberg Uncertainty Principle in our field, anything and everything you measure is fundamentally "Wrong", how Wrong depends on many factors!! Thus, we must attempt to minimize the things that influence the measurements towards more Wrong!!

Best,

artur0089:
The fewer add-ons, the more truthful the result.
Sweep generator - probe - oscilloscope.
Newfangled things are harmful when they are in large quantities.

--- Quote from: 2N3055 on September 25, 2023, 11:49:16 am ---Take one of those 1x10x probes and experiment with compensation trimmer and see how depending of how probe is set will change response.

--- End quote ---
The changes were ±2 MHz.

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