[time-nuts] DDS in GPSDO design?

Bob Camp lists at rtty.us
Sun May 27 23:41:13 UTC 2012


Hi

Simple answer - good resolution is a challenge to get. It's much easier to modify the Rb to make it pull able. 

Bob

On May 27, 2012, at 6:08 PM, Michael Tharp wrote:

> 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
> 
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