I stumbled across a relatively simple FM radio transmitter online.
Credit: makezine.com
I wanted to understand how it works (it is called "super simple" on the website), but several hours I have nothing but a vague understanding.
I believe R2, L1, C3 form an oscillator, and then the transistor Q1 is used to modulate the signal. C1 would just be used to remove any DC offset. However, this leaves R1, C2, C4, C5, and R3 and, for these components, I have no clue what they do.
Could someone please explain how this "super simple" transmitter works?
Answer
This is a variation on a Colpitt's oscillator.
L1 and C4 make up the main LC resonant circuit.
C5 is used to provide feedback to the emitter of the transistor which amplifies the signal and makes up for losses and the power radiated from the antenna.
The transistor is in a configuration known as "common base" or "grounded base" as the signal to be amplified is fed into the emitter and comes out of the collector, the base is the common terminal.
R2 and R1 provide the bias for the transistor to ensure it has the right operating conditions. R3 provides a path for the transistor current back to the battery but still allows the signal to be injected into the emitter by C5.
This circuit is somewhat unusual in that the antenna is connected to the emitter, often is connected to the collector but they both have the oscillating signal present. C3 is a decoupling capacitor to provide a stable power supply at high frequencies.
Although the frequency of the oscillation is mainly determined by L1 and C4 it is also affected somewhat by the characteristics of the transistor that change with the audio signal injected into the base of the transistor through C1. These changes modulate the frequency of the oscillation in time with the audio.
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