How does this oscillator work?

[hackdog]

I recently got this FM transmitter kit from AliExpress: https://www.aliexpress.com/item/1005004620944315.html. The design looked very promising and for a price that was impossible to resist. I've since put it together and I'm really impressed with its stability and audio quality. Fun little project!


I wonder how the oscillator around Q2 works though. It resembles a Clapp oscillator with an extra cap in parallel to the coil. Is this a well-known topology?

C12 and L4 will produce a damped sinewave, which C8 couples into the base of Q2. Q2 amplifies it and must feed some of it back to keep things going, but not too much; how is the gain being stabilized? The inner loop of C12 and L4 must get a well-timed push from the outer loop with C8, C11 and C14, but how? I find it hard to reason about LC oscillators like this with several things happening concurrently.

[dmowziz]

Hi Hackdog...

I do not know much but I think I have an idea why the parallel C12 and L4..

Looking at the image... The arrow (to the left of that base point). The only way to make this circuit oscillate is if that point (looking to the left) sees an inductor.

Due to the parallel C12 and L4, after a particular frequency, this combination becomes capacitive with increasing frequency.


So, the parallel C12 and L4 will be to suppress harmonics.

This is my reasoning, but not much I know about these things... Another beautiful thing will be the purpose of L1 ?

[BigBoss]

This is a standard Clapp-Gouriet oscillator and nothing special.
C11, C14 and R9 form a feedback circuit and creates a negative resistance that is a must for an oscillator.
C8 is a simple DC blocking capacitor and prevents the base to GND short circuit.
C12 and L4 form a tank circuit that defines the oscillator frequency together with C11, C14, and C8 and internal intrinsic and extrinsic capacitors of the transistor.
Audio signal changes the OP of the oscillator and the Osc. frequency shifts somehow.(FM Modulation)
Q3 is a tuned Power Amplifier. L1 is a radio Frequency Choke Coil for FM frequencies.

The circuit is so simple and it won't be very stable against temperature variation because there is neither a compensation circuit nor OP stabilisation circuit.
It's OK for fun.

[danmc]

The OP asked also about how gain gets stabilized.  If you're not just getting into saturation then the mechanism is the DC point at the emitter goes up as oscillation amplitude increases and that lowers gain.  The reason that voltage goes up is  there is enough nonlinearity that on negative going parts of the signal you source more current into the capacitors and on positive going parts, sink less and this moves that voltage up.

[hackdog]

This is a standard Clapp-Gouriet oscillator and nothing special.

C12 is not part of a standard Clapp oscillator as far as I know, which led me wondering if this particular configuration had its own eponym. Similar to how Clapp's oscillator itself is basically just a Colpitts with an extra cap.

Q3 is a tuned Power Amplifier. L1 is a radio Frequency Choke Coil for FM frequencies.

Isn't Q3 just an untuned amplifier/buffer? Both L1 and L2 are RF chokes and C4 and C1 further tie their top ends to DC.

the mechanism is the DC point at the emitter goes up as oscillation amplitude increases and that lowers gain.

Aha, makes sense.

Another thing I found remarkable of this kit is its humble power-on LED. It has a 10K series resistor, yet it still lights up when powered by 3V. That's a mere 0.2 mA or so.

[dmowziz]

I do not think L1 is a choke. If yes, then surely we can say L3 is a choke as well? No much difference between 5 and 8 turns
How will the signal get to the antenna?

L2 and C7 forms a high pass filter. L3 and C10 forms a low pass filter. C9 is the coupling between them making a bandpass filter.

Just my reasoning, no proof, no simulation.



My last $0.02

[Damianos]

I recently got this FM transmitter kit from AliExpress: https://www.aliexpress.com/item/1005004620944315.html. The design looked very promising and for a price that was impossible to resist. I've since put it together and I'm really impressed with its stability and audio quality. Fun little project!
(Attachment Link)

I wonder how the oscillator around Q2 works though. It resembles a Clapp oscillator with an extra cap in parallel to the coil. Is this a well-known topology?

C12 and L4 will produce a damped sinewave, which C8 couples into the base of Q2. Q2 amplifies it and must feed some of it back to keep things going, but not too much; how is the gain being stabilized? The inner loop of C12 and L4 must get a well-timed push from the outer loop with C8, C11 and C14, but how? I find it hard to reason about LC oscillators like this with several things happening concurrently.

It seems more as a Colpitts oscillator, not Clapp. The tuned circuit is in parallel resonance, not in series. It is consisted of the L4, C12 and the combination of C8, C11 and C14. The base is connected at the node between C8 and C11, similarly to the connection between base and emitter, to adapt the impedances. This way the tuning around L4 is less impacted from the rest of the circuit. Calculating the capacitances, we can see that about half of the current of the L4 is passing through the C12 and half to the rest of the circuit...