[time-nuts] Fast frequency counting question: correction

[Bruce Griffiths]

Bruce Griffiths wrote:
> John Miles wrote:
>   
>>> Probably impractical as the sampling noise is so high that it will
>>> require  tens of thousands of measurements to get the noise down to the
>>> required level and meanwhile the temperature etc will have changed
>>> significantly.
>>>     
>>>       
>> Heh, that's a good point, you can't just sit there and trigger the thing
>> 10,000 times per second for 24 hours...
>>
>>   
>>     
>>> Also the relatively low ADC resolution and nonlinearities
>>> will tend to limit the effective resolution in determining the zero
>>> crossing locations.
>>>     
>>>       
>> I stand by my assertion, though, that all you're going to get with high
>> sampling precision is a lot of well-characterized random noise determined
>> almost solely by outside factors.  It's an interesting question, but one
>> that can't be answered without knowing more about the symptom that started
>> the whole thread.
>>
>> -- john, KE5FX
>>
>>   
>>     
> If the oscillator ADEV were to follow a 1E-12/Tau law into the microsecond 
> region then at 10us the ADEV will be around 1E-7.
>
> A measurement system noise sampling jitter of a little less than a 
> picosecond should suffice to characterise this.
>
> It is unlikely that a GPS TCXO has that low a noise it may be perhaps 
> 10x this.
> Thus measurements of the startup transient characteristics to the 1ppb 
> level in 10us windows are perhaps somewhat futile.
>
> The 16Hz variation in a ~ 200kHz signal is equivalent to noise of about 
> 4E-7 at 40MHz with a 10us gate.
>
> A substantial part of this may well be due to the TCXO and local 
> oscillator phase noise with some contribution due to amplified mixer noise.
>
> More detail on the end users measurement techniques are in order to see 
> exactly how and what they are measuring is required.
> If in fact they want to characterise the zero crossing jitter more 
> accurately during startup subpicosecond (but not 10 femtosecond) system 
> noise can probably be justified.
> This is easy to achieve with a high resolution (14-16 bit) ADC directly 
> sampling the 40MHz signal.
>
> 100x smaller measurement system noise is perhaps only justified if one 
> were observing the startup transients of a much higher Q oscillator 
> however these would last much longer so in practice perhaps it isnt 
> justified.
>
> Bruce
>
>