How do I control the DDS oscillator in the DDC on the NI-5640R?

[pfarrell]

I am using Labview FPGA to create a QPSK communications link between two NI PCI-5640R boards.  I am trying to use the QPSK demod available on IPNet but it seems I need to send baseband I and Q data into the VI.  How do I control the frequency of the DDS in the Digital Down Converter after the ADC so I can shift my signal to baseband?  I am sending a signal that has a symbol rate of 500kbps at a carrier frequency of 60 MHz.  How do I set up the QPSK demod and DDC to demodulate this signal?

[Speedy_Badger]

Hello,

The NCO of the DDC is controlled by the ni5640R Configure NCO vi which is fourth VI in sequence when you view the QPSK demod (HOST) vi. By default, a constant value of 25 MHz has been wired into this value which correlates to the IF center frequency of the PXI-5600 downconverter. In practice however you can change this to a control and modify the value to 60 MHz to achieve your needed center frequency. The decimation setting of the ni5640R ADC Configure DDC VI is where you will adjust your IQ rate, where IQ rate is equal to the timebase / decimation factor. Be aware however that your decimation values are limited to the following possibilities:

4 <= 2^N <= 32,768

Therefore, your symbol rate pf 500kbps, will be coerced by the decimation possibilities to 781,250 kbps. 

By the way, the QPSK demod example I'm referring to is the one installed with the RF Fixed Point library located here,  in case we are referring to different examples.

Thanks!