Bdale:
I am using an atom bomb to smash an ant:
I have a PTS-160 synthesizer. It is locked with a rubidium oscillator
OR a GPS tamed oscillator. I use the rubidium when I am moving (at
W3CCX or in the car or on 10 Ghz cumulative weekends, etc.) Moving GPS
sucks for taming an oscillator! I have not yet had time to do all of
the necessary evaluation of these approaches. I feed the output to a
distro box that has a few outputs and one CAN be the USRP. I have not
yet done a thing beyond proving it works. The PTS-160 is a marvel and
it has very good phase noise at these frequencies (64 Mhz). With this
lashup, I do not have to use the PLL.
PLL's are great, so far as they go. We would have to know a lot more
about this particular one before we could say if your idea is a good one
or a bad one. How do we ask the right questions? We need to know what
the transfer function of the loop filter in the PLL. What you are
proposing is commonly used to frequency lock an oscillator but
introduces more phase noise (MUCH MORE) than the crystal you are trying
to replace. If your system has a certain phase noise at 10 Mhz, you can
be certain that it will have many dB more at 64 Mhz. The very best
approach for what you are asking is to find a 64 Mhz TC-VCXO and to use
your GPS in a system to pull the TC-VCXO on frequency since you are
going to have one heck of a time beating the phase noise of the crystal
oscillator in TC-VCXO. Phase noise will be a killer deal in your EME
applications as you know. You can take TAPR's new Reflock II and tame
the oscillator frequency. Valpey-Fischer makes such an oscillator but
they do not sell one and do not sample. We would need to come up with a
description of what it is we would want to do, get the Reflock II in
our hands, tame the TC-VCXO with GPS and then suggest a group buy on
these oscillators which will very likely be the best way to go. I have
an interest in locking 2 then 4 then . . . USRP's. This will be to
build a phased array. I also need to figure out to synchronize the
blocks at the sample "name". One step at a time . . .
Bob
Bdale Garbee wrote:
Looking at options for locking all the clocks that are part of the EME
station I'm building to my HP 58503A GPS-synced clock. Searching the
list archives, I found this note from Matt regarding the external clock
input on the USRP:
http://lists.gnu.org/archive/html/discuss-gnuradio/2005-03/msg00030.html
Has anyone played with this?
Of particular interest is whether anyone has hacked the FPGA code to use
the internal PLLs to retain a 64 Mhz sample clock using a 10 Mhz
reference clock input to the board.
Bdale
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