FM Transmitter oscillation problem

Question

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:

1. 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.
2. 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?
3. 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.

Answer 1

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.

Answer 2

On another forum I fixed in 5 steps an FM transmitter that was the exact same circuit as yours:

1) The audio transistor was saturated when the battery was new and was cutoff when the battery was aged so I added a low dropout 5V voltage regulator IC.

2) The bass response was very bad so I increased the value of the audio coupling capacitors.

3) The treble sounds were muffled when heard on an FM radio so I added pre-emphasis (treble boost) to the audio transistor to match the de-emphasis (treble cut) used in all FM radios.

4) The radio frequency changed as the battery voltage ran down so I powered the oscillator from the 5V regulator, reduced its emitter resistor value and increased its base bias resistor value.

5) The radio frequency changed when something moved towards or away from the antenna so I added an RF buffer transistor to isolate the oscillator from the antenna. The buffer transistor also increased the range of the transmitter.

Since a radio circuit will never work when built on a solderless breadboard I built it compactly on a stripboard. It works fine and sounds great.

EDIT: My C5 is 1/21 the value of yours because yours cut all medium and high audio frequencies.

Answer 3

• Did you try finding a 4-6pf capacitor for C4, or using two 10pf caps in series?
• I have never used the uH values of inductors, but wound my own inductors based on the width and length of the coil, and the number of turns (usually from 3 to 6 turns). Using 0.7uH when 0.1uH is asked for is going too far out of the required range.
• Almost any scope will affect such a sensitive circuit as there is no buffering stage. At 1x, a scope probe can have 20-30pF loading capacitance (definitely more than enough to overload the circuit and prevent oscillation). You might get away with x10 setting on the probe to greatly reduce loading, but it will still affect the circuit.
  While your scope may have worked with "a few MHz" signals, those were not as easily affected as the FM frequencies of around 100MHz. The higher the frequencies, the trickier it becomes.
• You could also use a 10nF capacitor between the power rails, close to the LC combo and the emitter of the transistor, because larger value caps may not ground the very high frequencies.
• I was able to successfully make an FM transmitter very similar to yours by using solid bare copper wires of maybe around 16AWG (1.5-2.5mm2) as straight pieces bent at their ends and punctured through a plain cardboard. The main thing is to keep them short and straight.
• You could and should use a 1nF cap for C3, and another 1nF cap from base to ground.
• Finally, I would use a radio receiver to check if the circuit works. When you come across your signal, it will be silent, and by tapping on your circuit with something will make a sound on the radio.

Answer 4

you may need a 0.1uf bypass cap, from the top of the inductor to bottom of R7 (1Kohm).

Often the VDD/gnd path is part of the circulating-energy path.

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In this circuit, looks like the emitter resistor is more of a "constant current source", and the RF-circulation-path is the 4.7pf thru the large bipolar's Cemitterbase and back to VDD thru the base-collector bypass.