[time-nuts] FRS-C TTL / sine outboard filter question
rfnuts at arcor.de
Wed Jun 23 15:50:46 UTC 2010
>> 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.
> It sounds like you made the changes to the A4 oscillator board, but
> not the ones to the A3 power supply board (several inductors,
> resistors and caps). I found that using a 1 uF for C16, instead of the
> documented 0.1 uF, gives a better signal. See the top of page A-15 in
> the manual.
Exactly that's probably the culprit.
If you need a sine wave output but don't want to change anything inside,
just add a lowpass filter.
A 5th order LPF (three inductors/ 2 caps or vice versa) should give you
a clean sine wave output.
Just add a coupling cap to remove the DC component.
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