[time-nuts] Darn you people....

Bob Bownes bownes at gmail.com
Tue May 3 19:32:56 UTC 2011


I wasn't aware of the phase of the 1pps vs the 10mhz, I was just
shocked to find the two were indicating as locked yet drifting apart
to the tune of 6hz/minute. I'd be happy if the two were within 0.0625
hz. As I think I said, the ZAZ gpsdo unlocked shortly thereafter. Mine
seems to have a hard time keeping the FLL locked and I suspect an
issue with the 10881.


Was surprised to see the phase difference between the 10Mhz outputs of
the ZAZ controller however.

Bob

On Tue, May 3, 2011 at 1:14 PM, Luis Cupido <cupido at mail.ua.pt> wrote:
> Bob,
>
> You must be aware that the VE2ZAZ GPSDO does not lock the 10MHz phase to the
> phase of the 1pps. It is a frequency counter which uses the frequency error
> to correct the 10MHz VCXO. So it is a frequency locked loop FLL.
>
> By definition on a PLL you will have the phase error being used to
> compensate the 10MHz VCXO
> On a FLL a frequency error is required to compensate the 10MHzVCXO
>
> While having a permanent very very slow OCXO trend (drift or aging)
> a PLL(order 2) will exhibit a constant phase error while the frequency is
> right. With an FLL it will have a constant frequency error.
>
> Furthermore the frequency counting resolution is 16 seconds, so
> the ZAZ gpsdo won't lock better than 0.0625Hz.
> A PLL scheme, thunderbolt, the old Brook Shera's, the 10KHz James Miller's,
> or my reflock (I or II) will get you always on the ballpark.
>
>
> This PLL/FLL aspect is a subtle difference that has absolutely no expression
> whatsoever for radio-ham applications both TX and RX on HF to microwaves
> etc. application to which it was targeted and serve very well.
>
>
> But you will surely start to see these small aspects when you step into the
> next level of timenutness !!!
> Apparently you have both feet there... looking at sub Hz you're infected
> that's a fact ;-)
>
>
> Luis Cupido.
> ct1dmk.
>
>
>
>
>
> Bob Bownes wrote:
>>
>> First it was building a VE2ZAZ GPSDO with an 10881 I happened across.
>> Next came the TICII's followed closely by the 5370B. Then the
>> thunderbolt. Now it's time to break down and get 10Mhz/5Mhz
>> distributed to all the instruments in the lab. (Still looking for a
>> distribution amp)
>>
>> Now the ZAZ GPSDO and the thunderbolt are side by side on the bench.
>> Both said they were locked last night. But they were about 0.10hz
>> apart. Now the GPSDO voltage adjust is all the way to the bottom rail.
>>
>> I have no idea what time it is!
>>
>> Grrr. :)
>>
>> I suspect the 10881. :(
>>
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