[time-nuts] DDS in GPSDO design?

Tom Knox actast at hotmail.com
Mon May 28 00:27:58 UTC 2012


DDS can be used in areas that require even the highest spectral purity. The Symmetricom Hydrogen Maser offers a DDS unit AOG-110 to generate a wide range of freq's from the 5MHz output.
Their Phase Noise Measurement Test Sets is also based on a DDS design. a good DDS circuit can add as little as 3dB phase noise to a quality ref. The key is the reference. Of course by it's very nature any digitally signals will have some HF noise unless filtered. There are a number of cheap DDS boards on eBay but I have never played with any. I have a few freq standards with variable DSS output that can be real handy.
Thomas Knox



> Date: Sun, 27 May 2012 18:08:41 -0400
> From: gxti at partiallystapled.com
> To: time-nuts at febo.com
> Subject: [time-nuts] DDS in GPSDO design?
> 
> 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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