Hey all maybe someone out there can help me with this.. I managed to find an old aircraft VHF tranceiver all tube type and the channel changer uses two sets of crystals and a mixer to get to the correct frequency. Well one set of the crystals happen to be right at the FM broadcast band. I found a 97MHz crystal which is very close to my 96.9MHz channel I use. Using a basic oscillator I can tune the crystal like a VXO just fine to my channel.
Now the problem is that since I am using a crystal it is hard to get FM modulation. I know about using a variactor capacitor to modulate FM using crystals, but I don't have any varicaps right now. So I looked around on the net and found that it's possible to build a Reactance Modulator. Problem is I can't find any plans or schematics for FM transmitters built this way. I guess early tube type FM transmitters were modulated this way.
Anyone have any tips, ideas? I'd like to build a FM transmitter this way since I have the xtal . PS: I already tried building a phase modulator on the output of the osc but I can't seem to get the audio loud enough w/o distortion.
Once I figure this out, I will post my plans since there doesn't seem to be any info out there on this.
Likely the crystal is a 5th or 7th overtone type as a 97 MHz crystal is too fragile to handle (though modern SMD makes it possible).
If you can pull the crystal and put it in a breadboard oscillator, try and find the fundimental with a receiver (a frquency counter will get confused, because the overtone will ring much stronger than the fundimental).
When you find the fundimental, just make a VXO with a series inductor and capacitor diode (capacitance and inductance resonant at the fundimental) and add your modulation to the capacitance diode.
Likely the crystal is a 5th or 7th overtone type as a 97 MHz crystal is too fragile to handle (though modern SMD makes it possible).
If you can pull the crystal and put it in a breadboard oscillator, try and find the fundimental with a receiver (a frquency counter will get confused, because the overtone will ring much stronger than the fundimental).
When you find the fundimental, just make a VXO with a series inductor and capacitor diode (capacitance and inductance resonant at the fundimental) and add your modulation to the capacitance diode.
Hope this helps.
Peace!
Thanks. Well I did end up building a simple 50mw tx out of it on a breadboard. I found a page talking about how you can use regular diodes as variactor capacitors. They mentioned how LED lights work really well for this. So I tried it with a small led light and a capacitor and sure enough it worked exactly like a variactor diode but I had to find just the right led to get the most modulation w/o distortion. However, I still can't seem to get the modulation as loud as other stations. I seem to only be able to get about 60% of the audio into it before it distorts and sounds bad. However it does work as long as people don't mind listening to a quieter than normal signal . I can get almost 100% modulation but only when I get the crystal on 96.8MHz, but I need it at 96.9 and it seems to quiet down a bit when tuning the VXO. I will have to post a schematic soon. Next step is to use that stereo generator program on my laptop and connect the audio out to the mono transmitter to get it to generate stereo with RDS
The reason it distorts when the crystal is tuned off its frequency is because of the nature of a crystal... it will be linear on both sides of its resonance only if it's dead-centre on frequency. If it's pulled off, you can only go so far:
Using other diodes as varactors is OK, but you'll need to keep the temprature stable. Without modulation, measure the frequency. Then put an ice cube on your LED and watch her go off into the blue yonder!
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