[time-nuts] 1pps signal presence with no GPS signal.

David Kirkby david.kirkby at onetel.net
Fri Jul 8 22:15:24 EDT 2005

Magnus Danielson wrote:

>>In my case, I intend locking two items to 1pps.
>>a) PRS10 rubidium from Stanford.
>>b) HP 10811A via the Bruce Shera board.
> I assume you mean the Brook Shera board from A & A Engineering:
> http://a-aengineering.com/gps.htm
> http://www.rt66.com/%7Eshera/

Yes, sorry, I meant Brook and not Bruce.

>>As far as I can tell, the PRS10 will not update the rubidium's frequency 
>>unless the 1pps is present, so it would seem to me the PRS10 would 
>>benefit from no 1pps signal rather than the wrong one. But perhaps I am 
> I think you are right. As I recall it, it stops learning from lack of PPS.
> As always, stop assuming and know instead.

I'm pretty sure that is correct, but will double-check the Stanford 
manual. But that was how I understood it.
>>In the case of a 10811A via the Shera board, I am less sure.
> I scanned the article, 

There's a pdf on the web by the way.

> but no real clue. However, if it is missing, it should
> be a small feat to fix the PIC code to do it properly. 

Although I will have to double check, I believe with no pps input, the 
DAC voltage remains fixed, so the voltage applied to the EFC input on 
the 10811A will be fixed.

> Basically, lack of PPS
> within say 1.5 sec of the last one means go into hold-over state. 

Incedentely, when I changed the register value on the M12+ timing 
receiver to stop the 1pps output if there is no lock, it still produces 
it for about 5 seconds after the antenna is disconnected. But given the 
time constants used in the control loops are likely to be hours, a few 
seconds will have no significant effect I would guess (although I have 
not sat and worked it out carefully).

> When in
> hold-over state just keep the same DAC value. If you are fancy, you do more
> (the HP SmartClock technology is neat since it will actively compensate for
> perturbations in environmental variables etc to predict the trimming needed,
> which is cool).

Em, that is being clever. I think I'll just stick to removing the 1pps 
output if there is no lock. I've not read much about TRAIM, but I assume 
that is better than relying on locking to at least one satellite, since 
it will exclude any that are significantly different from the others. 
Although the Motorola manual suggests the reliability is such that a 
false error will occur once every 5.6 days (or some number similar to 
that), which hardly sounded too good to me. I need to understand a bit 
more about this.

A couple of suggestions on how I think the Synergy SynPaQ III could be 
improved, if people choose to mount it inside another enclosure.

1) Allow some decent method of securing the unit inside another box.

I have no doubt voided my warranty by drilling and tapping the case with 
a couple of M3 threads, so it can be secured from underneath. It seemed 
better than making up brackets, or cable ties.

2) Have some method of bringing the drive to the LEDs out of the box, so 
  they can be replaced by LEDs on a front panel. I'd like to get the 
1pps LED to a front panel, but short of soldering wires onto the PCB, 
that is not possible. I might either use a light pipe, or use the 1pps 
output and put it into a pulse stretcher, so it can drive an LED on a 
front panel. I'm not sure of the speed of a 555 timer, but I suspect 
that will do the job of stretching the pulse to something that is 
reasonable for an LED. But it would make life a lot easier if those 4 
LEDs could be bought onto a front panel with not too much hassle.

David Kirkby

> Cheers,
> Magnus

David Kirkby,

Please check out http://www.g8wrb.org/
of if you live in Essex http://www.southminster-branch-line.org.uk/

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