[time-nuts] FRS-C TTL / sine outboard filter question
Leigh L. Klotz, Jr WA5ZNU
Leigh at WA5ZNU.org
Wed Jun 23 04:54:01 UTC 2010
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
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