25 Mhz noise seen on 5112

[asselin]

I have a PC (Win2k) with a 5112, and 7334 (motion controller) and a GPIB card installed. I am running labview 7.0 and have developed an application to acquire signals with the 5112 as the 7334 moves my reciever along 3 axes.

I have noticed that the fourier transform of my signals reveals a very narrowband peak at 25MHz. I have tried to find the source of it, with little luck. I think it is coming from inside the computer, since I still see it if I attach a low pass 20MHz filter on the input of the 5112. I even see it if there is no signal present, but just the trigger (PRF = 1khz). Another piece of information is that it doesn't reduce in amplitude if I average the signal repeatedly, which makes me think it is some sort of clock pulse
in the computer. There is mention of a 25MHz clock in the manual of the 5112.

Any ideas as to what this is? Is this a hardware defect, or a fact of life? Has anyone else noticed this?

[Josh H]

There is no inherent 25 MHz spur in the 5112, so the peak must be coming from noise in the computer or one of the other boards. The first thing you should do is take all of your cards except the 5112 out of your PC and perform an FFT. This will at least show you if one of the other cards (7334, GPIB, etc.) is giving you the 25 MHz spur. If you still see the peak, try the board in another environment and/or computer. Please let me know your results.