[App_rpt-users] GPSDO comparison

Sam Skolfield kj6qfs at gmail.com
Mon Aug 3 17:07:47 EDT 2015


The GPSDO does not interface with the RTCM at all; It does share the the
1PPS from your GPS antenna, and it can be "interfaced" with a PCGM module
to move the clock from 10MHz to 14.4Mhz or other rates.

On Mon, Aug 3, 2015 at 11:50 AM, Bryan Fields <Bryan at bryanfields.net> wrote:

> On 8/3/15 2:29 PM, Tom Hayward wrote:
> > Since asking about transmitter steering, I've been looking into the
> > feasibility of simulcast. The marginal difference between transmitter
> > steering and simulcast (in terms of equipment) is a GPSDO and a
> > transmitter that accepts a 10 MHz reference input. I have some leads
> > on transmitters--Quantars or some Daniels MT-4D. Other transmitter
> > recommendations are welcome, but what I'm really curious about is the
> > GPSDO.
> >
> > It looks like the Trimble Thunderbolt is the gold standard.
>
> It is, and the RTCM interfaces with it.  Spend the money if your time is
> worth
> it, or unless you want the enjoyment of re-engineering stuff.
>
> > At less than half the going price of Thunderbolt, you can find these
> > BG7TBL GPSDOs:
> > http://www.ebay.com/itm/121530825744
> >
> > Some are labeled Trimble and Huawei, so I think it's safe to say they
> > are neither. The question is: are they any good? It looks like KE5FX
> > has done some testing:
> > http://www.ke5fx.com/gpscomp.htm
> >
> > I don't really know what to look for in the graphs, but he says in his
> > conclusion "the BG7TBL unit is actually running at about 9.999 999 999
> > 800 Hz". Is this a deal-breaker for simulcast? If all transmitters
> > have a BG7TBL GPSDO, does it matter that they aren't exactly 10 MHz?
> > Is anyone running these GPSDOs with RTCM simulcast that can comment on
> > their performance?
>
> I doubt it as the PLL is going to divide the signal down to 5 or 6.25 KHz,
> and
> then use it as a reference to compare it to the VCO. (in a quantar for
> example).  This is an error of 200 uHz, but it looks like it's a consistent
> error (it's precision is excellent but accuracy is meh).
>
> This is an error of 0.000020 PPM or .020 PPB.  This is amazingly good.  For
> example this is an error of  0.0185 Hz at 927 MHz.  It's only going to be
> better lower in frequency.
>
> This would be the difference between a true 10.000000000000000 Clock and
> this
> clock.  Assuming all the clocks are the same amount off, your sites will be
> off a few ten thousandths of a Hz from true, but all will be the same
> amount
> off so it's not going to matter in any case.
>
> Keep in mind the Quantar with the UHSO option is like 10 PPB  and still
> used
> for simulcast in some applications at VHF/UHF since it's about 1-2 hz of
> error.
>
> I've got a 220 hamtronics simulcast setup working, not really deployed it
> long
> term yet, but I'd like to.  I have some Pic code posted here about my PLL
> programing.
>
> 73's
>
> --
> Bryan Fields
>
> 727-409-1194 - Voice
> 727-214-2508 - Fax
> http://bryanfields.net
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-- 
KJ6QFS
Sam Skolfield
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