[time-nuts] GPSDO using 100Hz

[Bruce Griffiths]

WarrenS wrote:
> ...
>
>   
>> Also if you would, I'd like to have a better understand of
>> what seems like an over obsessions with Low Noise GPSDO.
>> I do understand the need (or at least the desire) 
>> to have low noise oscillators when using them directly for high 
>> frequency and/or short time scale data taking applications, 
>> and the need for a good stable oscillator where it must maintain 
>> frequency if the GPS signal is lost, 
>> BUT it sounds like this is not what a large percentage of the 
>> second hand GPS trackers discussed here are used for. 
>> It does not seem like it would matter what the noise of the internal 
>> OSC is, if the unit is only being used for averaging things over longer 
>> periods such as looking at phase drift over time when doing freq checks. 
>> Any data taken at a period of say 1000 second to several days is 
>> pretty much limited by the noise of the GPS and not the Oscillator, 
>> so why is there the desire for the low noise oscillators in these cases?
>>
>> Warren
>> *****************
>>   
>>     
> Often the use of a low noise OCXO is mandated by unstated applications
> such as:
>
> 1) The OCXO output is used to drive the external frequency standard
> input of a counter.
>
> 2) The OCXO output is used calibrate other oscillators to 1E-9 or
> perhaps a little better and averaging over 1000 seconds or so during
> adjustments isnt a sensible option.
>
> 3) The OCXO output may be used to control the frequency of a microwave
> transmitter and/or the LO of a receiver which requires low drift and
> phase noise.
>
> 4) Obsession
>
> Bruce
>
> ****************
> Bruce
>
> Thanks, I'd  forgotten about really good counters with resolution better than a1 ns,
>  because I don't have one, Wish I did. 
>
> and I understand the microwave and Obsession needs,
>
> But  as far as #2, calibrating to 1E-9 or better, am I forgetting something else?
> I think even the poorest Osc's, that would ever be put in a GPSDO,  
> have phase noise and drift of under 1 ns, so in 1 sec it could  do 
> 1E-9 or in 10 sec 1E-10.  Are you saying that 1 to 10 seconds may 
> be too long of time to average over?
>
> Warren
>   
No, but one likes to have a local standard at least 10 x better than the
device being calibrated.

Bruce