I've been trying to get this simple FM transmitter to work over the past few days and I could use some help in a few areas.
First iteration was on a breadboard with all parts included. The exception being I didn't have a 0.1uH coil handy but I had another coil rated at 0.7uH (using my LCR meter). I also didn't have a C4 value of 4.7pF but I had a 10pF value. I figured I could just tune it later. I hooked a scope up to it and got no oscillation. I tried connecting the probe to the gate and the collector separately and together. Still no oscillation.
Second iteration. I removed the audio component of the circuit. Essentially everything to the left of C2. I had gotten that working no problem anyway so I figured I would concentrate on the oscillator. This time I tried different values for the inductor ranging from 20uH to 1mH and different fixed values for the variable cap (10pf, 20pf, 30pf). These were all ceramic caps. No love.
Third iteration. I tested the transistor to make sure that wasn't the issue. Seems fine. PN2222A variety. (Also tried 2N3904 and BC547). I then modeled this up on a piece of perf board. I even found a SMD inductor of 0.1uH so now I thought I would be good. Tested all the pieces on the meter before installation. Still nothing. I'm not seeing anything on the spectrum analyzer either.
Questions:
- Do you think my benchtop power supply is causing a problem? All the various versions of this type of FM transmitter circuit on YouTube and the Internet all seem to use a battery. Any chance the 60Hz main power is messing with things? I've put .1uF and higher caps on the power rails.
- I have a 200Mhz SIGLENT SDS1202X-E scope. I've built and tested other oscillators up to a few MHz before with no issues. Any thoughts on that being the issue?
- I would expect to see something on the scope right? Any chance the probe is dampening the oscillations?
I know there are a lot better FM transmitter circuits out there I was just experimenting with this (and others like it) and couldn't get any to work.
Best Answer
Not on a breadboard. Not ever.
The capacitance between rows is about the same as the capacitors you are trying to use.
The inductance in each row is about the same as the inductors in your circuit.
I think you'll find that all the videos that show that circuit have the circuit built on a piece of perfboard.