[time-nuts] FRS-C TTL / sine outboard filter question

[Didier Juges]

Leigh,

A narrow filter around 10MHz is likely to show phase ripple in its passband, and that will most certainly vary over temperature, not a good thing for ADEV, that's why a LPF is preferred.

Also, most crystal filters may not be happy with the output level of a typical OCXO, so you would have to attenuate the signal before the filter and amplify it after, an unnecessary complexity that will not help ADEV.

Crystal filters are not usually used for harmonic suppression because away from the resonance, crystals are good capacitors, so you will probably not find much data, and if you find data, it will probably suck :) In most applications, they are preceded and followed by L-C filters to normalize the out-of-band response.

Didier

------------------------ Sent from my BlackBerry Wireless thingy while I do other things... 

-----Original Message-----
From: "Leigh L. Klotz, Jr. WA5ZNU" <Leigh at WA5ZNU.org>
Sender: time-nuts-bounces at febo.com
Date: Wed, 23 Jun 2010 13:44:37 
To: Discussion of precise time and frequency measurement<time-nuts at febo.com>
Reply-To: Discussion of precise time and frequency measurement
	<time-nuts at febo.com>
Subject: Re: [time-nuts] FRS-C TTL / sine outboard filter question

> ...
>> [snip]
> If you need a sine wave output but don't want to change anything inside,
> just add a lowpass filter.
> A 5th order LPF (three inductors/ 2 caps or vice versa) should give you
> a clean sine wave output.
> Just add a coupling cap to remove the DC component.
>
> Adrian
>
> _

Thank you, Adrian.  I'll pursue both options, internal fixes and external
filter.  A multi-pole LPF is easily understood, and I don't believe I care
about passband ripple, though I wonder a little about the effect of the
coupling on phase changes (i.e., does this have any unfortunate effect on
adev?).

I still don't know if a crystal ladder filter will suppress the harmonics,
but I did get the answers that make me not need to ask anymore (1. fix the
internal filter, 2. use the high Q xtal filter for optional removal of
close-in noise)

Given time I would do all of these things, but I may not get to them all!

Leigh.



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