On Thu, Feb 28, 2013 at 03:49:55AM +0100, Magnus Danielson wrote: > On 02/18/2013 10:32 PM, Herbert Poetzl wrote:
>> [ lot of stuff zapped ] > It would be interesting if fractional resolution of the DDS > would be developed using the PHASE interface. > As PHASE ripples, a carry needs to be sent into the phase > accumulator of the AD9830, or alternatively fiddling the > frequency value would help. While continuously adjusting the phase (via PHASEx) would probably work, I was more thinking about changing the FREQx values, which contribute to the phase accumulator. IMHO this should give a much better result than the abrupt phase changes and thus a smoother output with less unwanted noise. One approach might be to set FREQ0 to the last value below the desired frequency and FREQ1 to the first value above (i.e. FREQ0 + 1) and then toggle FSELECT in a pattern to match the fractional part. FSELECT has a pipeline delay of 6 MCLK, but that doesn't matter much if you do micro adjustments and FSELECT can be changed at any time without causing any discontinuity. And the best part: FSELECT is a small cable soldered onto the AD9830 which can easily be rerouted to the PIC. But I haven't found the spare time to dig into yet ... best, Herbert > Anyway, good work! > Cheers, > Magnus > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
