>> Learned Gentlemen, >> Both the HP 106 and 107 have a post oscillator crystal filter. There >> is also a 10 MHz crystal filter used in my Tracor 527E FDM. >> So the question I have is there anything to be gained by adding 10 >> MHz crystal filters to the 10811 and similar OCXO's? They are very >> inexpensive to purchase. >> Regards, >> Perrier
Some of the commercial ultra-low phase noise OCXO today are using crystals filters at (or just in front of) the output buffer stage. These filters are usually heavily overdriving the crystal into the non-linear region, therefore the long-term reliability of such a circuit is suspect. The purpose is to improve the far-off noise floor, which works fine, but sometimes the close-in noise gets worse, as Rick said before. Cheap monolithic filters (duals) are much more sensitive to high drive levels, and thus are not recommended at all. Another effect of a crystal filter in the output is, that its output impedance outside the passband is far off from 50 Ohm. So it fools the power splitter at the input of cross-correlation based phase noise test equipment, and may display lower unrealistic phase noise "dBc/Hz" values at the noise floor, sometimes lower than the thermal noise floor of a 50 Ohm system. Regards Bernd _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
