Rick - Unless I misunderstood, I do not believe what you said is correct. A crystal filter can indeed improve the far out phase noise of a crystal oscillator. The oscillator's noise is not only a function of the crystal, but, the combination of the crystal noise and the amplifier noise used for feedback and the Q of that circuit. All of those additional components degrade crystal noise. Consequently, a 100 Hz wide filter at 10 MHz can really do a nice job on just a decent 10 MHz oscillator. Lincoln Labs proved this in their EDM of a satcom system in the early 80's that then I took over in industry to replicate and improve in 1986. We did have a 10 MHz crystal filter in there for cleanup. In our application, the final transmit frequency was close to 45 GHz, and it needed to be very clean, even after an optimal architecture used to get us up there. 73 - Mike
Mike B. Feher, EOZ Inc. 89 Arnold Blvd. Howell, NJ, 07731 732-886-5960 office 908-901-9193 cell -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Richard (Rick) Karlquist Sent: Monday, November 15, 2010 4:44 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] 5 and 10 MHz crystal filters 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. > > _______________________________________________ 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. _______________________________________________ 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.
