Electronic – Understanding a simple crystal oscillator transmitter circuit

crystaloscillatorresonancetransmitter

I've recently built this circuit: http://makerf.com/posts/mighty_simple_shortwave_transmitter
It works on the desired frequency as you can see from this video : https://www.youtube.com/watch?v=t6nFi7fdoK0 .
Now I'm trying to understand why and how it works, I would appreciate if you can guide me through the circuit in order to understand what each single components do.

This is what I think I've understood:

  1. The oscillation begins in the crystal (xtal) for the reason explained here What makes a crystal oscillation to begin
  2. There is a feedback loop which goes through the transistor Q1. Basically the RF is injected into the base and it gets amplified (and shifted by 180 deg?) by the transistor exiting from the collector.
  3. The L1 – variable capacitor is a tuned circuit but I don't understand why the collector is tapped there and in general what is its purpose.
  4. The L1 – L2 transformer is the way through the RF goest to the antenna and the transformer is required to adapt the impedance of the circuit to the 50 ohm required from the antenna

Finally, I'm not sure about the two resistor and the 0.05uF capacior.

Also, which oscillator circuit did inspired this design? Is it an Hartley Oscillator?
Thanks for your patience and if you have any good resource (book or website) which could help me in understanding oscillators, please let me know.

Best Answer

Gabric - here are my answers to the 4 questions:

1.) The oscillation starts as soon as you close the switch. This closing causes a sharp inrush transient which contains (among others) also the frequency component that fulfills Barkhausen`s oscillation condition (see point 2).

2.) The oscillation condition requires a loop gain (slighly) larger than unity and a loop phase of -360 deg (0 deg). The transistor in common emitter configuration contributes -180 deg phase shift between base and collector. The remaining phase shift is provided by the feedback loop.

This loop again consists of a third-order lowpass which is able to provide these -180 deg at one single frequency only. The lowpass consists of a second-order lowpass (L1-C) working upon a first-order lowpass (Lq-r,in). Here, Lq is the crystal working as an high-quality inductor and r,in is the finite input resistance of the transistor at the base node.

3.) The primary inductor is tapped to provide a certain kind of ac voltage division in order to limit the gain of the amplifier stage (because it should not be too large, just sufficient to allow self-excitement without severe non-linearities).

4.) The 10k resistor is necessary to provide the correct DC biasing of the transistor; the small emitter resistor provides dc feedback for stabilizing the dc bias point. The 0.05µF capacitor establishes signal ground potential at the pos. supply pin. This is necessary because the capacitor of the first L-C lowpass block is connected to the pos. supply but must be referenced to ac ground.

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