In addition to Matt's answer let me add a few remarks. A few years
ago, while working with Tom Clark at Goddard Space Flight Center (Tom
is an astrophysicist), I was asked to look at some Kalman Filtering
routines for his VLBI work. MANY years later, I desperately needed to
make widely placed, but otherwise unconnected, oscillators and receivers
as coherent as possible. When I realized he had solved this problem I
asked him how. His VLBI work has been used to do extremely accurate
geodesy and done such things has measure the change of rotation rate of
the earth due to El Nino current changes, etc. I knew from the Kalman
Filtering work that he was doing coherent signal processing on recorded
signals. These recorded signals were from instruments thousands of
kilometers apart and then brought back to Goddard for processing
sometimes months after the collection. How was this possible? He tuned
and transported Hy masers to his sites and then locked all oscillators
to the masers. Even that was not enough. He also had to continuously
calibrate the path/ transfer function from the antenna through the
receive apparatus. Using this same maser as the reference source, he
turned on a pulse train generator and combined it with incoming signals
at the antenna.
Most of us here do not need this level of complexity. We can do with
much less. For example, I do coherent signal processing at HF, VHF,
UHF to do timed difference of arrival geolocation with receivers
covering a few hundred square kilometers by doing the same function with
GPS receivers taming an oscillator. The GPS tamed oscillator is used to
cohere the systems and to generate the pulse train that aids in
calibration of the transfer function in front of the digitization
process. I can geolocate a reasonable SNR signal at (say) 10 MHz using
this system to a few meters and carrier phase is definitely in the
calculation.
So one of the first lessons I learned in my travels was "Who is on first
and What is on second". These players include:
NEVER use a PLL'd thing as the oscillator driving a system when you need
high phase stability. It is okay to use a temperature compensated
voltage controlled high quality crystal oscillator. You want the phase
stability of any system guaranteed by a high quality oscillator
(crystal, etc.).
If you also need ACCURACY, you use your Hy maser or you GPS tamed
reference oscillator to tune the high quality VCO to get on
frequency. Your design problem following this realization is to
optimize which of the two different systems dominates the Allan Variance
at what kind of time offsets. Short term phase stability is determined
by the high stability (crystal) oscillator. Long term stability is
determined by the Maser or GPS tamed beast used in a Tom Van Baak style
mechanism to keep the VCO pulled in. The latter is provided by (say)
the Reflock II boards available from TAPR.
Cheers,
Bob
Matt Ettus wrote:
This raises a question I've always had about phase-coherence of PLL
oscillators that use a common
clock source. Will they be coherent "enough" for things like
astronomical interferometry?
Yes.
The individual PLLs will still have "close in" phase noise components
that are unrelated to
on another. I suppose that given that you typically integrate over
several seconds, such
artifacts get cancelled out. But at very fine timescales, I imagine
that PLLs locked to
a common clock are *not* a good way to get decent coherency?
The close in noise is from the COMMON reference, so there is nothing to
cancel, it is perfect. It is the further out phase noise components
which are not the same between the 2 PLLs, since those components are
from the individual VCOs. These integrate out fairly quickly, and are
much smaller anyway.
Matt
--
Laziness is the number one inspiration for ingenuity. Guilty as charged!
_______________________________________________
Discuss-gnuradio mailing list
Discuss-gnuradio@gnu.org
http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
