[time-nuts] DDS in GPSDO design?

[Bruce Griffiths]

The keyword is good most commercial DDS chips aren't (at least when used 
directly for a fine offset generator ).
The Symmetricom TSC5120A phase noise test set implements the DDS in an 
FPGA wherein which its possible to virtually avoid the phase truncation 
errors produced by commercial DDS chips.
FPGA + external DAC DDS implementations can be very good as long as an 
appropriate design is used.
One can also choose output frequencies for DDS chips that are phase 
truncation free but this precludes fine tuning unless one resorts to 
using the several DDS chips in a diophantine synthesizer.

Bruce

Tom Knox wrote:
> 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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