[time-nuts] 5 and 10 MHz crystal filters

[Mike Feher]

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: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] 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
>
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