[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.
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 

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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