2008/10/24 Bruce Griffiths <[EMAIL PROTECTED]>: > 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. > 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 see. > 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 -- Steve Rooke - ZL3TUV & G8KVD Omnium finis imminet _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
