[time-nuts] femtosecond jitter anyone?

[Bruce Griffiths]

Hej Magnus

Magnus Danielson wrote:
> 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.
>
>   

Whether that is easy to do is another question.
Some manufacturers (eg Philip's) had purple NP0 surface mount caps
whereas X7R surface mount caps etc were light brown.

> 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.
>
>   

Placing the series tuned LC circuits in shunt with the LC low pass
filter shunt capacitors would be convenient although some adjustment of
the low pass element values would then be required.
Filter design software would be useful for this, however real
measurements and plus more realistic models for the inductors would be
useful particularly is the series resonances of the inductors are
located in the filter stop band.
> Cheers,
> Magnus
>
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>   

Bruce