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: [email protected]
To: [email protected]
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
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
