[time-nuts] frequency reference for portable operation

Bob Camp lists at rtty.us
Sun Mar 10 14:00:57 EDT 2013


The FE Rb's are dirty enough that you may need a second stage of PLL filtering in the system. Possibly a cheap 10 MHz low power OCXO and a sub 1Hz loop. A lot depends on just how good you want the close in noise and spurs to be. 

Put another way - is this PSK-32 at 10 GHz or conventional AM or …? Do we want to tune a bit to find the guy or not? 

The flip side to the fancy stuff would be SSB at 10 GHz. If I'm within a couple of KHz, I can hear you / you can hear me. We can tune to match things up. That drops the accuracy on the mobile end to 100's of ppb. A <$30 eBay OCXO will handle 1/10th of that nicely. A cell phone TCXO might also do the trick over a narrow range. I'd much rather have the OCXO's stability once I tune on frequency. 


On Mar 10, 2013, at 12:36 PM, EWKehren at aol.com wrote:

> 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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