I connect a 10MHz ref signal to a scope and detected one 10MHz and 20Hz signal. Why?

Dear all,

I have several measurement equipment that provides 10MHz reference signal output. When I connect this signal into the oscilloscope, I detected two signals. One is the actual 10MHz and another a 20Hz signal. Both this signal are clean sine wave. Why is this so? In my limited knowledge, two signal cannot be mixed and yet appears as two distinctive signal in the scope.

The level for both sine wave is the same. In my limited knowledge that means, even if one is a carrier signal, I shall still see the 10MHz signal together with the 20Hz signal. Do appreciate help here.

Thanks.

KL

If you are using a digital sampling oscilloscope it sounds like you are looking at aliasing of the high frequency signal with the oscilloscope sampling frequency. The 20Hz is the difference between your clock and the sampling clock (or a multiple)

Keith

Hi Keith,

So that means a typical Digital scope samples signal at several different frequencies? Is the different frequencies determine by my setting of the time division? All the while I thought that the scope samples at a fixed sampling rate and the time division setting is just view adjustment. Thanks for the information!

Merry Christmas.

If you slow down the timebase the sampling rate will change. Usually this is shown on the display somewhere. There are ways of avoiding this depending on the oscilloscope. I always run mine in 'envelope' mode where it samples at the maximum rate and then takes the max & min values in a certain timeframe to create the points on the display and reduce the data for the selected timebase.

Keith