# GPSDO comparison

**URL:** <https://community.allstarlink.org/t/gpsdo-comparison/12085>\
**Category:** AllStarLink\
**Created:** [August 3, 2015, 6:29pm UTC](https://community.allstarlink.org/t/gpsdo-comparison/12085 "2015-08-03T18:29:46Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![KD7LXL](https://community.allstarlink.org/letter_avatar_proxy/v4/letter/k/ee59a6/32.png) [@KD7LXL](https://community.allstarlink.org/u/KD7LXL)\
**Post date:** [August 3, 2015, 6:29pm UTC](https://community.allstarlink.org/t/gpsdo-comparison/12085/1 "2015-08-03T18:29:46Z")

</div>

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:

> **[2020 10MHZ OUTPUT SINE WAVE GPS DISCiPLINED CLOCK GPSDO + Antenna+ Power supply...](https://www.ebay.com/itm/121530825744)**
>
> 10M OUTPUT :SINE WAVE,3.3Vp. - ACCURACY:above than OCXO 2 order of magnitude. - Power: DC11.7-12.9V,\<= 15W. There are NO EXCEPTIONS!

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](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

---

<div class="post-metadata">

**Author:** ![W9CR](https://community.allstarlink.org/letter_avatar_proxy/v4/letter/w/ecae2f/32.png) [@W9CR](https://community.allstarlink.org/u/W9CR)\
**Post date:** [August 3, 2015, 6:50pm UTC](https://community.allstarlink.org/t/gpsdo-comparison/12085/2 "2015-08-03T18:50:38Z")

</div>

> 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](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:  
> [Some GPSDO Performance Comparisons](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

> **···**
>
> On 8/3/15 2:29 PM, Tom Hayward wrote:
> 
> --  
> Bryan Fields
> 
> 727-409-1194 - Voice  
> 727-214-2508 - Fax  
> [http://bryanfields.net](http://bryanfields.net)

---

<div class="post-metadata">

**Author:** ![Kj6qfs](https://community.allstarlink.org/letter_avatar_proxy/v4/letter/k/ba8739/32.png) [@Kj6qfs](https://community.allstarlink.org/u/Kj6qfs)\
**Post date:** [August 3, 2015, 9:07pm UTC](https://community.allstarlink.org/t/gpsdo-comparison/12085/3 "2015-08-03T21:07:47Z")

</div>

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](mailto: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](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](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](http://bryanfields.net)
> > 
> > * * *
> > 
> > App\_rpt-users mailing list
> > 
> > [App\_rpt-users@ohnosec.org](mailto:App_rpt-users@ohnosec.org)
> > 
> > [http://ohnosec.org/cgi-bin/mailman/listinfo/app\_rpt-users](http://ohnosec.org/cgi-bin/mailman/listinfo/app_rpt-users)
> > 
> > To unsubscribe from this list please visit [http://ohnosec.org/cgi-bin/mailman/listinfo/app\_rpt-users](http://ohnosec.org/cgi-bin/mailman/listinfo/app_rpt-users) and scroll down to the bottom of the page. Enter your email address and press the “Unsubscribe or edit options button”
> > 
> > You do not need a password to unsubscribe, you can do it via email confirmation. If you have trouble unsubscribing, please send a message to the list detailing the problem.
> 
> –
> 
> KJ6QFS  
> Sam Skolfield