[time-nuts] 5 and 10 MHz crystal filters

[John Ackermann N8UR]

I hadn't considered phase noise, thanks for bringing that up!  I know tempo can also be an issue with high Q filters.

These aren't significant issues for my current application, which is to clean up a synthesizer running at a slight offset from the nominal 10.000 MHz frequency.  So I'm starting with much more noise than an OCXO.

John 

On Nov 15, 2010, at 4:43 PM, "Richard (Rick) Karlquist" <richard at karlquist.com> wrote:

> 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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