[time-nuts] DDS in GPSDO design?

[Michael Tharp]

Greetings,

I've been pondering topologies for a custom GPSDO design and two obvious 
choices seem to present themselves. The first, and seemingly more 
popular by far, is to use a "pullable" oscillator as many OCXO and Rb 
oscillators are and discipline it using a slow but precise DAC. But 
unfortunately my Rb is not pullable so I would have to get another 
oscillator. So I have a very stable but off-spec local oscillator, which 
has to somehow be combined with the pulse-per-second from the GPS. If 
there's a palatable analog way to do this, I'd love to hear, because it 
would probably be simpler than the other idea.

The second obvious idea is to use the local oscillator to clock a 
frequency synthesizer (DDS). These can apparently tune a frequency very 
finely and depending on how much one spends will produce a pretty clean 
sine wave even at 10MHz. Since these also tend to require a FPGA it also 
fits nicely with the nanosecond-level phase comparator I've been toying 
with, and the whole mess (microcontroller, DDS, phase comp) can all be 
clocked from some multiple of the LO without worrying about unwanted 
phase correlation. Having the GPSDO be a black box that can transform 
any undisciplined 10MHz reference into a disciplined one is very appealing.

Does anyone have any comments or experience with DDS-based frequency 
references? Are they too jittery for this type of application? It will 
certainly require quite a lot of creative filtering -- one page I read 
mentioned the pitfalls of tempco of phase shift -- but that's just a 
good excuse to brush up on my analog design.

-- m. tharp