[time-nuts] 5 and 10 MHz crystal filters

[Richard (Rick) Karlquist]

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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