[time-nuts] frequency reference for portable operation
EWKehren at aol.com
EWKehren at aol.com
Sun Mar 10 12:36:30 EDT 2013
Having built a few portable frequency sources my recommendation based on
your application and what is reasonably available today would be a FE 5680A
in combination with a 100 MHz clean up XO. We have very good results with a
slightly modified what I call the Dorsten PLL-VCXO by W-H-Rech. I can send
you the article and some plots, the article is in German but you would be
able to understand or I would help you.
Using an AGM battery and a $ 4 up converter I would plug it in to the car
when driving and at home run it off AC and when in the field you will have
plenty of time to operate. 9.8 A 12V Li-ion are available for $ 45.
When home keep it running and use the RS 232 to update Rb frequency.
Bert
In a message dated 3/10/2013 10:23:21 A.M. Eastern Daylight Time,
jimlux at earthlink.net writes:
Asking here on behalf of a friend..
With respect to portable amateur microwave operation.. you want good
close in phase noise (so you can use narrow band filters) AND good
frequency accuracy (so you can find the signal)>
the typical operation is "drive somewhere, operate a bit, drive
somewhere operate a bit" repeated (contacts from different grid
squares/peaks/what haveyou"
My instinct is that this is an application for a nice quiet OCXO on a
battery. Adjust the frequency before you set out against a good
reference and just go from there.
Surplus Rb references are apparently also popular, but I think they keep
those on battery too (that is, you need to be ready to go 10 minutes
after arriving, and I don't know that a Rb is "settled in" that quickly).
So the question from my friend was with reference to GPS disciplined
oscillators. Would that do any better? I'm used to GPSDOs in a fixed
location where you have time to do long term averaging.
And what about truly mobile operation (there are folks in the SF bay
area apparently doing 10GHz mobile ops.. slotted WG radiator on the roof
of the car, etc.)
What sort of 1pps timing accuracy do you get from a GPS "on the move".
I assume it would have the usual 10ns sort of uncertainty (in that the
mfr specs don't say "only with the antenna fixed in one place for N
hours"). 10ns is only 1E-8 of a second. Presumably one can average a
bit over many pps ticks.
I've got a bunch of Wenzel Streamline units, and they typically do
1E-10/day aging and 1E-9 over temp. Assuming the temperature doesn't
vary a "lot", seems like the OCXO is "better" than the GPS, at least in
a 1-2 day time frame. (and, of course, isn't that just what a GPSDO is,
in holdover mode, anyway)
The Rb is good to 1E-11 over the short run (assuming it's been
"calibrated" recently) but I notice that the PRS10 data sheet says 7
minutes to 1E-9, so in the "non continuously powered" mode of operation,
it's not all that wonderful.
The Rb is definitely higher powered.. The PRS10 is 2+ amps at 28V to
start, and 0.6 to run. 15-16 Watts is a lot to keep on a battery.
(Assume you run off a pair of 7Ah 12V batteries.. that gives you 10-12
hours).
The Wenzel is a couple watts (after a 5W warmup). The GPS is a LOT
lower power. The Garmin GPS 18x is 0.45W, of course the 1pps on that
receiver is only specified to 1 microsecond.. A moto Oncore UT is a bit
less than a watt and claims <100ns (with SA.. showing the age of the
datasheet I have).
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