NI5112 PFI Reference Clock Input Problem

Question

I'm writing C code to use an NI5112 O-Scope card as a digitizer and I would like to synchronize it with an HP 8648C signal generator (which has the high-stability timebase option) through PFI1. I want to use the signal generator for the reference clock as the NI5112 samples about 40 Hz high at 5 MSamples/s and because I need the two to drift together. When I use the signal generator as reference clock the result is a very "noisy", i.e. with an input sine wave the FFT is noisy, and I the timing is too bad for processing input GPS signals (which have stringent timing requirements). If I use an Agilent 33120A function generator as the reference clock, I get better results, but GPS processing is still compromised. When not using a
n external reference everything looks clean and works fine except for 40 Hz error in the sample rate. The reference outputs from both the 8648C and 33120A work fine with each other and look good and clean with the o-scope, so I can't figure what the problem is. Maybe the card is just extremely picky about external reference clocks. The function call I'm using to initialize this is:
handleErr(niScope_ConfigureClock(vi, NISCOPE_VAL_PFI_1,
NISCOPE_VAL_NO_SOURCE,
NISCOPE_VAL_NO_SOURCE,
NISCOPE_VAL_FALSE));
The program runs normally with no errors, it's just that the results are bad. Any help would be appreciated.

Answer 1

Hello,
If the data that you are acquiring is very noisy, I would start getting suspicious about the input impedance (1MOhm or 50Ohm). Does the impedance of the signal generator match the scope's? How does your signal look in the Scope Soft fron Panel? Does it make a difference if you use the internal clock?
Another thing that I would consider is the following parameter from the niScope_ConfigureClock function: NISCOPE_ATTR_CLOCK_SYNC_PULSE_SOURCE. In order to synchronize your arb to the scope, not only they need to share the same clock (10MHz), but also the slave receives this sync pulse from the master. In you application, if the signal generator has the ability to send a sync pulse, you can route that signal through PFI2 to ensure synchronization.
Hope this helps. Good luck with your application!
Annette Perez