PXI-6608 specifications

Question

Hi,
I have a question about the performance verification of the PXI 6608.
This is what I was able to perceive from the manual.
We have two modules:
CTR0: referenced to an external clock that has a minimum uncertainty of 0.75 ppb and generates a 400 s pulse to CRT1.
CTR1: is referenced to the internal 10 MHz OCXO and counts the number of cycles in that 400 s window: 400 * 10 MHz=4*10^9 . Any result different from this means an error.
Now to my understanding the performance verification is comparing the 10 MHz OCXO with the external clock. So by looking at the manual we have a uncertainty of 75ppb when the 10 MHz OCXO is on the slot 2: 0.75ppb * 10 MHz = +/- 0.75 Hz. BUT the performance verification assumes a tolerance of 0.1 Hz (10,000,000.1 < Hz < 9,999,999.9 Hz)
The calibration execute has the same approach as the performance verification,
Why does the PXI 6608 have a tolerance different from the manual?
Are we not testing the 10 MHz OCXO?

Answer 1

Hello,
In order to set the counter source to the timebase, you can use the the get/set/reset CI_CtrTimebaseSrc in order to set this in CVI. Please consult the DAQmx C reference Help for more information. Here are the functions if you can't find it in the help.
int32 __CFUNC DAQmxGetCICtrTimebaseSrc(TaskHandle taskHandle,
const char channel[], char *data, uInt32 bufferSize);
int32 __CFUNC DAQmxSetCICtrTimebaseSrc(TaskHandle taskHandle,
const char channel[], const char *data);
int32 __CFUNC DAQmxResetCICtrTimebaseSrc(TaskHandle taskHandle,
const char channel[]);
As for performing a loopback test to test its accuracy, this isn't the best way to do this because the source is creating a signal with relatively the same accuracy as you are reading in back on the same device. Knowing that this signal is accurate down to .7ppm, we need to apply a source for the counter that is much more accurate than this. First way to do this is to change the timebase source to the 10MHz clock like I mentioned above. The other way is to place the PXI-6608 in slot 2 so that it overwrites the 10MHz clock on the backplane. This 10MHz clock is used with PLLing the 80MHz clock that is normally used in counter tasks. Which slot do you have the PXI-6608 in? Could it be placed in this slot?
Jim St
National Instruments
RF Product Support Engineer