Purpose of trimmer capacitors CV1-CV4

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Purpose of trimmer capacitors CV1-CV4
« on: February 23, 2017, 09:18:55 PM »
Hi,

what is the purpose of the trimmer capacitors CV1-CV4? They have a range between 1.4 - 3 pF.

And how are they calibrated?

Thx.

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Dejan

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Re: Purpose of trimmer capacitors CV1-CV4
« Reply #1 on: February 25, 2017, 03:38:33 PM »
Trimmer capacitors are part of the voltage divider circuit in the input stage. Depending on the input vertical scale setting, a relay switches between 1000:47 and 1000:953 voltage divider. To get a flat frequency response, the values of RC constants in the input divider must match. If this is the case, then the divider behaves like a simple resistive divider. The trimmer capacitors are needed to adjust the value of capacitance in the RC divider in order to match the RC constants.

The calibration of trimmer capacitors CV is similar to probe compensation. For calibration use 1kHz square wave input signal and observe the signal. First select 100 mV/div vertical scale range (inputs will connect to 1000:953 divider) and adjust CV3 and CV4 until you see a clean square wave signal. Then select 200 mV/div and adjust CV1, CV2 to get the same response for the 1000:47 divider. After calibration, check that both input channels have the same response.