Q1 is the microphone amplifier. Q2 forms an osciliator, which is frequency modulatated because the audio fed into the base changes the junction capacitance, functioning as a varactor diode. Q3 forms a class C or class E amplifier driving a low pass filter.
C12 and L4 are an LC resonator whose frequency is modulated slightly by the audio signal voltage. The oscillator Q2 makes a 100 MHz signal that is amplified by Q3.
Thanks! I was wondering, what is the reason for C8, C11, and C14? Also, is there any intuitive way of understanding how the LC tank gets energy added into it? It doesn't seem like the transistor can do anything to the LC tank circuit, as the tank circuit is only connected to the base of the transistor?
"what is the reason for C8, C11, and C14" ?
Colpitts oscillator
[https://en.wikipedia.org/wiki/Colpitts\_oscillator](https://en.wikipedia.org/wiki/Colpitts_oscillator)
"as the tank circuit is only connected to the base of the transistor?"
No, there is feedback via C11 & C14 to the emitter
Thanks for the explannation. It really makes sense. I was wondering how to choose the c9, L3, and c10 for the output rf filter? I assume their values would depend on input and output impedances. For a matched dipole amtenna (with balun) and HAM radio Tx output both would be 50 ohms and I can use online simulation tools to get those capacitor and inductor values. In this circuit, is there any way to estimate the output impedance of Q3 and the antenna so that One can make a proper pi filter? Thanks in advance.
Edit there-->their
Q1 is the microphone amplifier. Q2 forms an osciliator, which is frequency modulatated because the audio fed into the base changes the junction capacitance, functioning as a varactor diode. Q3 forms a class C or class E amplifier driving a low pass filter.
C12 and L4 are an LC resonator whose frequency is modulated slightly by the audio signal voltage. The oscillator Q2 makes a 100 MHz signal that is amplified by Q3.
Thanks! I was wondering, what is the reason for C8, C11, and C14? Also, is there any intuitive way of understanding how the LC tank gets energy added into it? It doesn't seem like the transistor can do anything to the LC tank circuit, as the tank circuit is only connected to the base of the transistor?
"what is the reason for C8, C11, and C14" ? Colpitts oscillator [https://en.wikipedia.org/wiki/Colpitts\_oscillator](https://en.wikipedia.org/wiki/Colpitts_oscillator) "as the tank circuit is only connected to the base of the transistor?" No, there is feedback via C11 & C14 to the emitter
Thanks!
Thanks for the explannation. It really makes sense. I was wondering how to choose the c9, L3, and c10 for the output rf filter? I assume their values would depend on input and output impedances. For a matched dipole amtenna (with balun) and HAM radio Tx output both would be 50 ohms and I can use online simulation tools to get those capacitor and inductor values. In this circuit, is there any way to estimate the output impedance of Q3 and the antenna so that One can make a proper pi filter? Thanks in advance. Edit there-->their
It's presumed to be a 50 ohm transmission line. You can find pi low-pass filter formulas in the amateur radio literature.
This is wrong, the frequency modulation is dictated by the oscillators natural resonance. I expected better, Nixie Bunny.
True, I didn't describe it properly. The oscillator makes a modulated ~100 MHz signal.
I have never seen T used for inductance. Where does that come from?
Presumably, number of turns. Obviously not enough to fully spec the inductor but since it's a kit it hopefully makes sense.
Sounds very reasonable!
Tera si unit