[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.
Cheers,
Magnus
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