[time-nuts] femtosecond jitter anyone?
magnus at rubidium.dyndns.org
Thu Apr 9 00:16:56 UTC 2009
Bruce Griffiths skrev:
> Magnus Danielson wrote:
>> Bruce Griffiths skrev:
>>> 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
Autocalibration will adjust the discharge current, measure the voltage
bias and adjust the linearization data (65 bytes per channel).
>> The crystal filters of that chain is setup in PI-filter style with the
>> crystals in serial mode.
>>> However it may be necessary to control the temperature of these crystals.
>> Indeed. I wonder if he really needs that level of sine purity. An LC
>> tank should be sufficient to get started.
> The filter phase instability is reduced significantly if one uses LC
> filter traps tuned to the unwanted harmonics together with a low pass
> filter rather than using a high Q bandpass filter.
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.
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