[time-nuts] femtosecond jitter anyone?
bruce.griffiths at xtra.co.nz
Wed Apr 8 23:18:41 UTC 2009
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 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.
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