[time-nuts] femtosecond jitter anyone?

Magnus Danielson magnus at rubidium.dyndns.org
Thu Apr 9 07:59:59 UTC 2009

Bruce Griffiths skrev:
> Hej Magnus
> Magnus Danielson wrote:
>> Hej Bruce,
>> Bruce Griffiths skrev:
>>> Magnus
>>> Magnus Danielson wrote:
>>>> Bruce Griffiths skrev:
>>>>> Magnus
>>>>> For examples of the use of crystals in filters for cleaning up the
>>>>> output of a crystal oscillator look at the circuit schematics for some
>>>>> of the early crystal frequency standards.
>>>>> Crystal filters were used quite liberally in some of these to clean up
>>>>> the outputs.
>>>> It's used in the step-up chain of the SR620 for instance. The SR620 has 
>>>> a 10 MHz oscillator (TCXO or as in my case a Wenzel OCXO) which is 
>>>> stepped up to 90 MHz using a fairly ordinary odd-order stepup and 
>>>> filtering chain. The ECL logic counter frontend use this as coarse 
>>>> counter frequency. The analog interpolators is a bit interesting in a 
>>>> few peculiarities but nothing really exciting.
>>> Except if you believe that they actually use a Z5U capacitor (specified
>>> in the parts list) for the interpolator TDC ramp capacitor.
>>> If so, this would make for some interesting linearity and dielectric
>>> absorption compensation software.
>> The caps listed as Z5U in the part list is not the timing caps. C701 and 
>> C711 both being 100 pF NP0 is the timing caps. If you look careful you 
>> will see it referenced. A 7 us sample pulse will charge a polypropylen 
>> cap with the buffered value for a sligthly later performed 12 bit ADC 
>> conversion.
> The online version of the manual specifies both of these caps as Z5U
> (see attachment).
> This may be an error in this version of the manual, or perhaps early
> versions used Z5U??
> 100pF NP0 surface mount caps have been available for decades.

My manual specifies NP0 for these. Maybe a hardware revision that 
occured between the manuals.

>> Autocalibration will adjust the discharge current, measure the voltage 
>> bias and adjust the linearization data (65 bytes per channel).
> Dielectric absorption and voltage dependence correction for Z5U caps
> would be very challenging.

I do not disagree with you on that, but we do not know how it actually 
works. I could lift the lid and try to identify what is really sitting 
in there.

It could be that the online manual reflects the original mistake.

>> Agree. Any temperature effects will occur with very flat slopes as the 
>> tuning is far away from the frequency of interest.
>> A number of shunting LCRs can be used, infact a suitable crystal could 
>> be used to shunt into ground.
> Series tuned LC shunts are generally better as at resonance their ESR
> can be much lower than that of a crystal, the signal level that they can
> handle without damage is also much higher.
> Although a single LC series tuned shunt wont provide large attenuation
> one can always use one tuned to each significant harmonic per filter
> section.

It needs to be combined with a general low-pass filter of a few poles.


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