Communications: using direct digital synthesis  (Page 2/3)

Spectral copies

Spectral Copies: The digital-to-analog converter on the DDS is unfiltered, which means that there is no anti-imaging filterto remove the spectral replicas. To see this, plug the output of the DDS board directly into the vector signal analyzer(VSA), and observe the spectrum. Use 104.9 MHz as the center frequency, and set the span wide enough so that you can seethe spectra of the replicas to the left and right of the 104.9 MHz signal. Use the marker to find the peaks of the otherreplicas, and record their frequencies. Once you've done that, reattach the antenna to the DDS output, and tune thereceiver to the frequencies you just recorded. You should be able to hear your audio on each of the other frequencies.

How to use the dds

The DDS has several different modes of operation: single-tone, unramped Frequency Shift Keying ( FSK ), ramped FSK, chirp, and Binary Phase Shift Keying ( BPSK ). In this lab we will use the DDS in single-tone mode. Single-tone mode is easy touse, and is powerful enough to create many different kinds of waveforms, including FM and FSK.

Fm code

The FM code you just ran (also listed here ) is fairly straightforward. The program first calls the radioinit subroutine. This routine sets the DDS to single-tone mode and turns off an inverse-sinc filterto conserve power. Following radioinit , the setcarrier subroutine is called. This routinesets the frequency of the DDS output by writing to the two most significant 8-bit frequency registers of the 48-bitfrequency-tuning word on the DDS

To set the carrier frequency, we first need to determine what frequency word has to be written to the frequency registers onthe DDS. This can be done using :