Purpose of variable capacitors C2A, C7A?

[lygra]

@HugeMan

there is also a easy way to calibrate C2A and C7A,just input a square wave signal instead of sine wave , for C2A,with p-p 0.5 v and any frequency you like,and adjust V/div to 0.1V/div, then ,ajust the C2A to make sure the wave in your DSO is the same as the signal input ;for C7a , ,with p-p 3 v and any frequency

Thanks for sharing your information with us.

The square wave is the correct input for circuit frequency compensation. This is because perfect square edges of a square wave requires an infinite number of odd harmonics. You adjust for no overshoot and no undershoot of the leading edge. Adjust as close to a square leading edge as possible.

Attached is a screen capture of the pictures found at:

seeedstudio.com/blog/2009/12 … -dso-nano/

Can you remember if the caps are under the number “6” in the picture?

Thanks