A friendly warning about crystal filters. Crystals, whether used in an oscillator or filter, have intrinsic phase noise. You cannot improve the phase noise of a crystal oscillator with a crystal filter unless the filter crystals have lower phase noise than the oscillator crystals. In general, post filtering of an oscillator is rarely done for the reasons I mentioned. One place where it made sense was in the HP 8662 with the 640 MHz output option where a 10811 is multiplied to 80 MHz and then filtered to reduce the noise floor. It is further multiplied to 640 MHz and filtered again by a SAW filter, that was made at HP in those days. As others have mentioned, filter characteristic impedance is a function of the crystal physics. There is a specific impedance that you must use for a given bandwidth. Fortunately, it is easy to put transformers at the ports to convert to 50 ohms.
Rick Karlquist N6RK On 11/15/2010 7:44 AM, John Ackermann N8UR wrote:
I'd like to find a couple of 5.0 and/or 10.0 MHz crystal filters, preferably configured for use in-line with 50 ohm coax. I've done some googling but don't find anyone selling these as a stock item. Do they exist? Any pointers would be appreciated. Used/surplus is fine (even preferable). Thanks! John _______________________________________________ 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.
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