[time-nuts] Short-Term Stability
SAIDJACK at aol.com
SAIDJACK at aol.com
Thu Mar 16 15:42:23 EST 2006
The DDS system you describe in your paper is very similar to our FireFox
Signal Generator unit (_www.jackson-labs.com_ (http://www.jackson-labs.com) ).
We included a GPSDO than can achieve parts to the E-011 accuracy in it basic
form and about 10x better (being quantified now) with a slightly more
expensive high-stability double-oven OCXO.
The output ranges from 0.00001Hz to 1640MHz with a 1GHz, 10 bit DDS driving
the 0-400MHz output directly, and the frequencies above 400MHz are generated
by frequency multiplying the DDS output using a low-noise VCO and PLL.
Some results on the DDS output: we achieve DAC generated spurious below
about -52dBc with the 10 bit DAC clocked at 1GHz. Drawbacks of this DDS are some
digital noise crosstalk, and over 2W of power consumption at 1Gs/s! (We are
desparately waiting for a 2GHz, 12bit DDS :)
The big advantage of using this DDS output, as mentioned in the paper, is
the frequency hopping capability up to over 400MHz output frequency
(essentially instantaneous, <<1ms) since its a digital system. The hopping time thus
depends realy only on the output reconstruction low-pass filter.
Another advantage of using a high speed DDS is that it essentially doesen't
increase the phase noise floor like a wide-band VCO would do. So using a
1Gs/s or 2Gs/s DDS you can choose the best single frequency VCO you can get to
drive the DDS, run it with a very small PLL loop bandwidth (say <10KHz) and
thus get extremely low phase noise. The DDS will thank you with even lower phase
noise at its output (20log(fin/fout) relationship).
This compares to having to use a wide-band VCO for multiplying up the
frequency, which inherently has higher phase noise due to its wide band capability.
The VCO doesen't generate wide-band spurs of course like the DDS does, if
designed well.
Another huge advantage of using the DDS is that using a DDS and some tricks
we can get about 49 bits of frequency resolution over the bandwidth. This, in
combination with the internal GPS disciplined OCXO gives a nice capability
to really generate essentially any frequency with extremely high accuracy.
So the trade-off when using a DDS versus PLL frequency multiplying is less
phase noise, but higher spurs.
BTW: we are working with one of the members of this discussion group to try
to get the best accuracy possible from the GPSDO (thanks much for the help!!).
Let me know if you would like to get more detailed info on how we
implemented the GPSDO and DDS, and the caveats we ran into etc.
SJ
More information about the time-nuts
mailing list