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

Leigh L. Klotz, Jr WA5ZNU Leigh at WA5ZNU.org
Wed Jun 23 00:54:01 EDT 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?

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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