[time-nuts] DDS in GPSDO design?
gxti at partiallystapled.com
Sun May 27 22:08:41 UTC 2012
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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