GPSDO comparison

App_rpt-users
1

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.

At less than half the going price of Thunderbolt, you can find these
BG7TBL GPSDOs:

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?

Tom KD7LXL

2

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:
10MHZ OUTPUT SINE WAVE GPS DISCiPLINED CLOCK GPSDO + Antenna+ Power supply | eBay

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:
Some GPSDO Performance Comparisons

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

···

On 8/3/15 2:29 PM, Tom Hayward wrote:

--
Bryan Fields

727-409-1194 - Voice
727-214-2508 - Fax
http://bryanfields.net

3

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@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