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

David Smith w6te at msn.com
Wed Jun 23 05:03:52 UTC 2010

Leigh, I have the same FRS-C TTL unit. I do not have a manual.

Would you be so kind as to send the conversation info to me?


Dave W6TE
  ----- Original Message ----- 
  From: Leigh L. Klotz, Jr WA5ZNU<mailto:Leigh at WA5ZNU.org> 
  To: time nuts<mailto:time-nuts at febo.com> 
  Sent: Tuesday, June 22, 2010 9:54 PM
  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?


  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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