Re: Tek 2465b two probes showing slightly different voltages and phases


Harvey White
 

I think there should be ways to do this.  What I was considering was a 7000 series preamp which is the 7A26.  It's a 2 channel vertical amplifier.  There's a gain control for each amplifier. It is, and must be, before the channels are combined through the switch/chop/add circuitry.

On the 2465 I cannot say specifically because I don't have one.

However, you're kind of mixing two things at once.

Firstly, the probes have different bandwidths and phase delays because they aren't the same model of probe, and the lead lengths are different.  It does make a difference.

Secondly, setting the scope input to 50 ohms is a red herring since both probes are designed to work into a 1 meg style input.

Thirdly, IIRC, and the manual for probes and scope should show, I think the resistor tolerances are 1% in both the scope input and probes, which kinda limits the response at DC.  You get high enough in frequency, and all the capacitances in the probe and scope input become not only significant, but override the resistor values.

Phase response/time delay is proportional to bandwidth of the probe.  I wouldn't expect the two *different* probes to have the same frequency or phase response.

What I was suggesting is that you, *IF* you have to use both probes to do a differential measurement of a waveform with significant high frequencies, adjust them the best you can, individually.  Then equalize the DC gain for one channel by going into uncalibrated mode for one of them.  (yes, they should be the same and frequently are not).  Going into uncalibrated mode for ONE channel while doing a (ch1 -ch2) mesurement allows you to temporarily mess up one channel until it's as close to the other as you can get.  Whichever channel is the one left on "cal" is the standard.  It'll be one or the other.

so....

50 ohm input impedance on 1 meg standard x10 probes tells you nothing.

two probes with different lead lengths (look at the bandwidth and response for each of the probes with respect to cable length) are not expected to have the same frequency response or phase delay. Phase delay in scopes is done with matched coax cables, not probes

Probes are only so accurate, 1% resistors likely, check both probe and scope manuals.

at high frequencies, the compensation drastically affects the division ratio of the probe when looking at sine waves.  These scopes are optimized for looking at pulses, not sine waves, there are compromises involved.

As a test, take a probe, look at a good pulse, then change the compensation and see what happens.

Look at a high frequency sine wave, change the compensation, and see what happens.

You could use the other probe as  reference.

These are qualitative adjustments, not quantitative.

Harvey

On 12/30/2020 9:31 PM, Mark Hatch wrote:
Harvey,

I see in the manual that there is a way to adjust the phase delta on channel 2 (unfortunately, that facility is defaulted to being disabled so have to figure out how t get into technician mode to enable it).

But in #3, you mention that there was a way to adjust the gain of a channel? Did not see that in the user manual. Is that also in the technician mode?

Regards,

Mark





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