PXI 6608 Measure Frequency 1Hz with accuracy 1ppm

Question

I need to measure a frequency of 1 Hz with accuracy less than 1 ppm
icant do it with a PXI 6608 its normal ? 
1 Hz is measured 1,000007 Hz by a PXI 6608
in this case, the error is 7ppm but i need less than 1ppm, i should measure 1,0000007 to have 0,7 ppm 
please help its so urgent !!!!!!!!!

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