EMRFD Message Archive 9012

Message Date From Subject
9012 2013-08-27 12:34:41 Ashhar A Phase Modulator that doesn't work
I figured that a half-wave filter could change the output phase of the signal if middle capacitor was varied. This lead me to try out a phase modulator by adding a varactor to the middle capacitor of a half-wave filter cut for appox. 145 MHz. Check the circuit in my folder in the files section. It is in my folder at http://groups.yahoo.com/group/emrfd/files/VU2ESE/

I could hear a faint trace of bob marley from my PC's audio out feeding the modulator when my FT-817 on the bench has its audio turned up to max. When I attached a 50 ohms termination to view the waveform on the scope, the FM completely vanished. Just a dead carrier. Turning the bias around on the varactor had no effect.

The coils are air wound 4 turns on 1/20 bolt. I am getting 0.5 volt of carrier on 50 ohms termination. (That's not so bad).

Can someone enlighten me? I want to know why it won't work, it looks so workable...

- farhan
9013 2013-08-27 13:08:29 kb1gmx Re: A Phase Modulator that doesn't work
Ah, simple the most phase change that will network will produce
is far to small for the detector.

There is a phase shift but not very much. Calculate a change of
say 5 degrees at 146mhz means in Khz. Its quite small.

If you cascade several such stages you may get enough phase change
then, maybe.

Either that or do it at 6mhz and multiply that up (24th harmonic at 144mhz) and see if it sounds better..

Old crystal controlled FM gear did that, oscillator at a few mhz
and a phase modulator then multiplers (typical multiplcati
9014 2013-08-27 13:15:25 Ashhar Farhan Re: A Phase Modulator that doesn't work

I am using an Si570 to eliminate frequency multiplication.

The phase change at the corner frequency should be quite dramatic as it
swings from +ve to -ve sharply. I will try putting a trimmer and see if i
can hit the sweet spot. Otherwise, I guess, I must try the armstrong

If this fails, I guess I will just build an armstrong modulator with an
ADE-1 mixer...

- farhan