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

[Didier Juges]

If your concern is to clean up the harmonics, a crystal ladder filter is probably not the best choice, a low pass filter would be easier to design, would probably require no adjustment and be cheaper in parts with less effect on the fundamental signal you are interested in.

If your concern is cleanliness and close-in phase noise (as Mike referred to), then a narrow crystal filter would indeed be a better choice. Keep in mind that ladder filters have an assymetric frequency response. That may or may not help you.

On the other end, if you are going to drive any kind of digital circuitry, a square wave (even distorted) is probably better than a sine wave.

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: Tue, 22 Jun 2010 21:54:01 
To: time nuts<time-nuts at febo.com>
Reply-To: Discussion of precise time and frequency measurement
	<time-nuts at febo.com>
Subject: [time-nuts] FRS-C TTL / sine outboard filter question

At the local flea market, I picked up what appears to be an Efratom 
FRS-C.  It is marked "TTL" internally.  It has the passive connector 
board, but not the active board with the 15 MHz synthesizer on it.

Mine is marked "TTL" internally.  The service manual has a chart showing 
the differences between the sine and TTL options, and I converted it to 
the sine version by changing a jumper to a resistor and populating an LC 
filter with 10uH and 100pF (~5 MHz).  I also terminated the RF 
connection on the connector board with a 47 ohm resistor to ground.

The output now doesn't have the tremendous overshoot it used to have, 
but it's also not very sinusoidal.  That's not surprising given the 
simplicity of the on-board filter.

Instead of a multi-stage LC filter, I wondered about a crystal ladder 
filter: since the output frequency is fixed, the high Q and low cost of 
the crystal filter might be an advantage, but I wasn't sure about how 
effective xtal ladder filters are at suppressing harmonics, as each 
individual crystals would have odd overtone responses, so it might not 
be a good plan.

Does anyone have practical experience with a filter topology for 
cleaning up the output of the FRS-C at 10 MHz?

Leigh.

P.S. Just so that I can be topical, note that the FRS-C has a C-field 
adjustment 0-5V input, so I could use it as the reference oscillator for 
a TPLL.


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