[time-nuts] Question on crystal jumps
bruce.griffiths at xtra.co.nz
Fri Oct 24 03:14:26 UTC 2008
Steve Rooke wrote:
> 2008/10/24 Bruce Griffiths <bruce.griffiths at xtra.co.nz>:
>> The rubidium standard is passive it doesnt oscillate, it merely acts as
>> a high Q filter whose resonance can be probed with an external source.
>> Thus you need to replace the crystal oscillator with another low noise
>> source to interrogate the rubidium resonance.
> So, do I take it you are suggesting that the xtal oscillator is used
> to excite the rubidium standard? I can't see how that works, can you
> please comment further.
The crystal oscillator is used to drive a frequency synthesizer chain
that has an output at 6.8.....GHz.
This output is then used to interrogate the Rubidium resonance at 6.8GHz
using a derivative of the Pound technique.
The resultant modulation of the optical transmission of the Rubidium
cell is used to lock the 6.8...GHz signal to the Rubidium transition.
Caesium beam standards are also passive and a synthesized 9.192....GHz
signal is locked to the desired Caesium 133 transition.
>> To be useful the bandpass filter would need a bandwidth of a few
>> milliHertz or less.
> Well, if the xtals we use are capable of resonating sufficiently well
> to provide us with a freq standard that is acceptable for our uses,
> IE. within a few miliHertz, I can only see that using multiple xtals
> can only improve on the stability of a singe xtal in an oscillator
> circuit. Remember that the output of these GPSDOs, and as I understand
> it rubidium standards, are really the output of the xtal oscillator.
> The centre frequency of that xtal oscillator is relatively slowly
> varied to be correct by it's difference to the GPS signal, or rubidum
> stage, via some form of comparator driving a low pass filter and onto
> the EFC. Hence it is the xtal oscillator which is the signal source we
In practice you cant achieve a crystal Q greater than a few million at
10MHz, so achieving a filter bandpass of a few millihertz is a little
difficult to do.
Using several crystals in an oscillator is a well known technique for
increasing the group delay and improving the oscillator short term
>> The crystals in the lattice filter will also experience similar jumps.
> But it is highly probable that they will not all jump at the same time
> for reasons I have given in a previous posting. It's similar to what
> other people have described about using three or more xtal oscillators
> and having a vote on what the frequency should be. Trying to control
> multiple oscillators, and implement the voting arrangement, would be
> quite complex and has a high probability of being prone to other
> effects. A lattice filter is very simple in comparison, you have the
> same voting scheme of multiple xtals (and you can have many more than
> three) but none of the complexity of the oscillator arrangement. You
> just have to make sure that it is driven with the reference standard
> and all the stages could be manually tuned for peak after a burn in.
> As the xtals change over time the level may drop but the frequency
> would remain as stable as the input reference. Of course, if you used
> just a very simple lattice, it's possible for all the xtals to drift
> in one direction which would make the filter less able to suppress
> noise sidebands but it would need to have some math done to
> investigate the implications of this. Also a series parallel design
> would probably circumvent this.
> Sorry, just thinking out loud.
> 73 - Steve
A jump in the parameters of any of the crystals will still perturb the
phase shift of the crystal filter wehther it is used as a passive filter
or the frequency determining element in an oscillator.
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