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

Steve Rooke sar10538 at gmail.com
Wed Jun 23 05:42:03 UTC 2010

What load do you have it running into? It maybe that you need to load
it into 50 ohms.


On 23 June 2010 16:54, Leigh L. Klotz, Jr WA5ZNU <Leigh at wa5znu.org> wrote:
> 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
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Steve Rooke - ZL3TUV & G8KVD
The only reason for time is so that everything doesn't happen at once.
- Einstein

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