[time-nuts] frequency reference for portable operation
Bob Camp
lists at rtty.us
Sun Mar 10 14:00:57 EDT 2013
Hi
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.
Bob
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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