Gain/Phase info sought

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14 Oct 2012
Posts: 19

It might be good to have some special instructions or APC files for gain/phase measurements.

Am I right that the dynamic range of B/A is only about -72 db because the A/D is 13 bits + sign?


Are these the proper settings:

Spectrum mode: gain/phase
Average = On (prevent noise artifacts)
Average mode = peak (to make a plot)
Display mode: sampled or repetitive
Sample duraiton: long enough to give frequency resolution


I got into this doing a PSRR (power supply rejection ratio) on an amplifier.

24 Oct 2012
Posts: 395

Hello Bryan,
The range selected by the scope window amplitude axis is presented to the ADC. With a 14 bit ADC, you will be digitizing this range with 14 bits. To maximize dynamic range you should use all of the displayed range.

The noise from both Channels A and B contributes to polluting the Gain measurement. This is especially important when the B channel is a small value. For example the difference between 1 ADC count on Channel B and 2 counts results in a 6 dB change in displayed value.

In addition crosstalk between the two channels will limit the resolvable gain. At lower frequencies you should be able to do better than -80 dB dynamic range. At higher frequencies (>1 MHz) this will fall off to around -70 dB. Below is a plot of a cleverscope sweeping from 0 to 400 kHz, with Channel B shorted with a 50 ohm plug, and Channel A connected to the rear sig gen, with 5V p-p signal. The Channel A amplitude range was +/-2.5V. The peak at 22 kHz is caused by the internal crossover between DC and AC attenuator matching.

We have made a video on how to do Gain-Phase, and this is on our videos page.

Your settings are correct. Peak Averaging is used to build up the complete picture.
You can use the Filter buttons to reduce high frequency noise if your frequency range is less than the filter cutoff frequency.

Repetitive sampling is only useful on a repeating signal above 1 MHz.

Note that we automatically turn on peak capture for the sample stream if the control panel resolution is larger than 10ns. Use the Filter ON to disable this (even if filters are not turned on in Settings/Acquisition Settings). This will stop high frequency noise in your spectrum.

While I'm at it - make sure that your cleverscope, unit under test and cables are well away from your PC and monitors. The LCD screens we use here create a lot of noise that gets into cables and the unit under test, which will also reduce the dynamic range you can see. (-80dBV is only 100uV after all!).
.

1 Nov 2012
Posts: 1

There is a video on Gain/Phase on the website now.
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