Lower bandwidth probes for a higher bandwidth oscilloscope - bad?

fazeka

Member
2012-09-02 9:22 pm
Hi all,

I have an older Tektronix 2445A 'scope that needs a new pair of probes. The only work I do is in the audio realm. Highest frequency I would test would be the bias frequency of an analog tape recorder (432 kHz).

My friend has a set of Pomona 6493 probes he would consider selling me. I did some research and found that these are 60MHz. The Tek 2445A is a 150MHz scope.

I've researched online, seems everyone suggests going with probes that are higher bandwidth.

What's issues would I encounter (for audio work, if it matters) with using 60MHz probes with a 150MHz scope?

Thanks!
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
You would have no issues at all using 60MHz probes for audio.

Just make sure that whatever probe you use is correctly compensated for your scope... and this applies only to divider probes such as 10:1 and 100:1... and is set uing the 'CAL' output from the scope and adjusting the probes trimmer for a flat response.

YouTube
 

fazeka

Member
2012-09-02 9:22 pm
Hi,

Thanks for the response.

Good to know. One concern/I think I see a problem:

The original Tek probes had a compensation range of 10 pF to 25 pF. The Pomonas have a compensation range of 15 pF to 40 pF.

Is that going to make my scope be under-

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or over-

electronicdesign_com_sites_electronicdesign.com_files_uploads_2016_10_11_1116_CTE_Keysight_Bog_F5.png


compensated?
 
The manufacturer claims they are 150 MHz passive 10:1 probes, although for some reason they recommend them for 60 MHz scopes:

150 MHz X10 Oscilloscope Probe | Pomona Electronics

When measuring circuits with high-frequency oscillations, you might see a slightly smaller amplitude and slightly different waveform than with even faster probes, but I don't think that matters for audio - usually you want to know whether it oscillates, not with what amplitude and waveform. They are also fast enough for most digital audio waveforms.
 
Whether it will be under-, over- or correctly compensated depends on the input capacitance of your scope and whether you trim the probes correctly. If the input capacitance is in the trimming range, you can trim the probes for proper compensation, if not, you can't. So if it is between 15 pF and 25 pF, both the old and the new probes should work OK, if it is between 10 pF and 15 pF, the new probes will always overcompensate.

Oscillations in audio amplifiers can be anywhere between, say, 100 kHz and 300 MHz. Incorrectly compensated overall feedback loops usually cause oscillations in the 100 kHz ... 30 MHz range, but local oscillations related to wiring parasitics are usually close to the fMAX of the transistors, typically 100 MHz ... 300 MHz for low-power low-frequency transistors.

By the way, if you can't find any information on the scope's input capacitance, you can roughly measure it by connecting the calibrator output to the input via a large resistor and measuring the time constant. The resistor has to be placed at the scope input, because any wiring in between will add its own capacitance. Divide the time constant by the value of the resistor in parallel with the 1 Mohm from the scope and you have the capacitance.
 
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If you are using a passive probe and want to see high frequencies always use the x10 setting. The bandwidth for the x1 setting is very much less. Basically only ever use x1 if you have to because the signal is very small. x1 setting adds significant capacitance and can affect a circuit (prompting or quenching oscillations for example).