AD9953 application

Sorry for repeating the same question sent to the support staff, for the hurries we have in this.

We plan to use DDS AD9953 for a 4800bps shaped-BPSK direct digital modulator taking advantage of fine resolution that we need. AD9953 is recomended for shaped PSK. We have made calculations on interpolation needed to remove alias when modulating the phase at 4800bps in the DDS since we could only filter them with the output SAW which is not very narrow (4MHz BW). We need then to interpolate the signal by >256, but in this case then we could not use the SPI port because the total bus speed needed would be higher than 25Mbps (32*256*4800). Then we would need to use instead the internal RAM, removing the speed problem, and any filter issues. But in the RAM mode we would be limited to a filter with one single symbol lenght, which prevents to apply Nyquist or linear phase filters with some latency. So for filter latency purposes the best would be filtering in the FPGA and then transmit to the DDS via SPI instead using the RAM. This leads us to a incompatibility or difficult compromise.

We have designed in my company many types of modulators but never used DDS for direct PSK. Can you confirm the DDS technique is really recommendable for shaped PSK and/or suggest a more optimum way to implement with it, or it is really comfortable only for non-filtered PSK? If we don't find a good way maybe we would need to change to a integrated I/Q modulator with frac-N synth and abandon the DDS idea.

Thanks in advance

Jaime Martin
Broad Telecom, S. A.

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