Hi guys. I have a 35MHz oscilloscope. When CH1 and CH2 were fed 1kHz sine waves (90 degree phase diff), it showed a circle in X-Y mode as expected. However, I noticed that when I increased the frequency to 1MHz, it no longer showed a circle. I have to change the phase diff to be roughly 120 degree to get a circle.
It seems to me when the frequency of the signals are increased, the two channels are out of phase. Is this normal? If calibration is needed, which part should I calibrate? (I have no service manual and need to look at the schematics to find the trimmers)