Hi The gotcha with the DDS is phase truncation. That pretty much trashes the ADEV.
Bob -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Dennis Ferguson Sent: Monday, October 22, 2012 1:44 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Adjusting HP 5065A frequency On 22 Oct, 2012, at 12:48 , "Bob Camp" <[email protected]> wrote: > The "big bucks" approach probably is to run a good RF ADC on the input and > then do all the offset stuff as DSP math. The VCXO just sits at it's magic > frequency and never moves. More money / no pops. A somewhat cheaper way might be to use a DDS. That is, lock the clock driving the DDS to the input frequency and then program the DDS to correct the measured error of the input clock. A DDS with a 48 bit control word will have an effective resolution of about 4e-15, if my arithmetic is right, which seems adequate for the purpose. The DDS also gives you the option of generating any (corrected) output frequency you want. The output frequency could even be programmable if you don't mind looking at the DDS digital noise in the output, though that could be cleaned up by picking a fixed output frequency ahead of time and adding a cleanup PLL for the chosen frequency following the DDS. I'm not sure why this problem isn't always dealt with this way, actually. Since the corrections are applied in digital arithmetic the precision with which they can be made is limited only by the bit-width of the adders you use to compute each cycle's update and, given that the D/A converter it is driving is probably going to be limited to 300 or 400 MSPS, even an FPGA (let alone semi-custom logic) could carry more bits through the computation than are useful to have. There is probably some catch to this that I don't understand. Dennis Ferguson _______________________________________________ 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.
