Hello Time Nuts, I have been on the group and have promoted it to other
folks for a while but this may be my first post here.
I am microwave amateur radio operator and I have question to pose
relating
to the use of GPSDO's with amateur radio for microwave communication.
First, the more generic question. A friend was discussing using a eBay
purchased Trimble 57963-D for providing a 10 MHz refrence for his
portable
microwave station (primarly at 10GHz). He wants a clean high stability
10
MHz refrence mainly to lock the station LO. First I think a GPSDO is
overkill for this application and I am thinking that a good surplus
ovenized crystal oscillator should get him to within a few Hz after warm
up
and a Rb could do better but may have short term stability that may
degrade
phase noise of the LO. I am concerned that a GPSDO is not designed for
portable operations. Moving it should probably force a new site survey
which may take a day or more to complete before it goes into
disciplining
mode so you would loose any potential benefit of a GPSDO by moving around
frequently. If he wants to do this I think he should leave it connected
at
his home location for an extended time (several days at least), then when
he want to go portable (roving), he should
disconnect the GPS antenna entirely to force the unit into holdover
mode
maintain continuous power with battery backup which should maintain the
internal OCXO very close to the target frequence and allow the holdover
algorithm to compensate for OCXO for aging and best it can. I would
guess
that if he chooses to used the GPSDO with the antenna connected it would
probably never exit the site survey mode and you would have the output
default to the last known good DAC value when it was been disciplined so
it
would be operating as a OCXO only (although potentially starting from a
very accurate starting point, if it had been in use at a fixed location
for
a good while) before going portable. Is this a correct view of the
situation? Any recommendations?
I also know of a fellow who has developed some excellent open source
Linux
software to drive an Ettus Research USRP microwave SDR transceiver for
amateur radio microwave applications. His code also has features to
calculate antenna baring and with other available code compensates for
satellite Doppler shift and/or synchronize digital communication modes
using the GPS coordinates and timing data. He has a built in interface
for
a Trimble Thunderbolt for this purpose. I think it also might be a
better
solution to use a OCXO for 10 MHz and a cheap USB GPS sensor for
location?
Is there a cheep USB GPS that provides PPS? Any recommendations?
I have seen simpler GPS controlled 10 MHz sources like the Miller design
that divides down a 10 MHz ref and compares it to a 10 KHz output from a
Jupiter T GPS to tweak the ref freq that may or may not be better suited
to
this application as it may add phase noise to the LO but would be more
real
time in it's GPS correction to the reference frequency.
Regards,
Norman Eric Haskell
KC4YOE
Keller, TX USA
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