Homemade FM Radio Transmitter Experience
I recently made a small FM radio transmitter, which can broadcast within a range of 100m-200m with a 12V power supply, and the audio quality is decent. The microphone of this transmitter is very sensitive, and with a 5pF-30pF variable capacitor, it can be tuned between 87.5MHz and ~98MHz. However, at higher frequencies, the transmitter becomes less stable and more difficult to tune.
During use, I found that the transistors and capacitors of the transmitter would change frequency due to heat, mainly caused by the Q2 transistor. Although the heat change is small, it still affects the stability of the transmitter.
I referred to the three circuit diagrams in the link, using a TL062 dual op-amp instead of a TL061 single op-amp, and added a switch to select between microphone and audio jack input. Moreover, I replaced R4 (100K) with a variable resistor to observe how its value change affects audio quality. The result shows that the value change of R4 has little or even negative effect on audio quality.Schematic source
