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

Robert Atkinson robert8rpi at yahoo.co.uk
Wed Jun 23 06:43:18 UTC 2010

Hi Leigh,
I can't comment on the crystal filter option., but I've found that the packaged filters and filter/balun modules from old 10Mb/s Ethernet cards work well. See this excellent little page by Dave G4HUP http://g4hup.com/DA/Filters%20Transformers%20and%20DC%20Converters.pdf
Robert G8RPI.

--- On Wed, 23/6/10, Leigh L. Klotz, Jr WA5ZNU <Leigh at WA5ZNU.org> wrote:

From: Leigh L. Klotz, Jr WA5ZNU <Leigh at WA5ZNU.org>
Subject: [time-nuts] FRS-C TTL / sine outboard filter question
To: "time nuts" <time-nuts at febo.com>
Date: Wednesday, 23 June, 2010, 5:54

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?


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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