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
The principal problem with conventional DDS implementations is phase
truncation spurs which can occur close to the desired carrier.
Virtually all commercial DDS chips produce such phase truncation spurs.
It is possible to eliminate such spurs if one implements a custom DDS
using an FPGA and an external DAC.
In this case the performance is limited by the DAC.
Another approach is to use a cascaded mix and divide technique
<http://www.karlquist.com/FCS95.pdf> to restrict the effective tuning
range of the DDS.
The amplitude of DDS generated spurs is thereby significantly reduced.
Bruce
_______________________________________________
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
