How does this radio transmitter circuit oscillate?

Question

I am trying to understand how this circuit operates.

I understand how the circuit works on the right side of the transistor, but the oscillation stage with the crystal confuses me.

It appears that the crystal has no feedback from the output of the oscillator. I researched this and found out that the collector-base capacitance of the transistor provides a feedback path, but wouldn’t that only give a 90° phase shift instead of the 180° phase shift required for positive feedback?

I have seen similar circuits where a variable capacitor is included with the crystal to adjust frequency. Would that give the phase shift for the remaining 90°?

Answer 1

Yes, it could oscillate, but in a SPICE simulator, it didn't. Not quite. A few component changes did start oscillations. The 7MHz crystal equivalent circuit is a guess (C1, L2, R5, C2):
The base-to-emitter capacitance of the 2N2222 is large enough that this is a Colpitts-type oscillator.

Answer 2

This question has quite interesting answer history - at least for +10k rep members who can see the whole history. But there's done some reductions => I think that now here's also room for my answer:

At first: The crystal can be any reactive impedance from nearly zero ohms to very high number of ohms. The reactance can be as well inductive as capacitive and the losses are extremely low when compared to practical LC-circuits. And all those reactance values are found from very narrow frequency band around the stamped frequency of the crystal.

=> It's well possible that at some frequency the CB capacitance of the transistor and the crystal form together a phase inverting voltage divider which attenuates less than the amplifier amplifies => oscillation.

In practice also the input impedance of the transistor must be taken into the account => exact full 180 degrees phase shift in the feedback route doesn't happen. But the amp also doesn't cause exact 180 degrees phase shift, because the loading is partially reactive => It's still well possible that oscillation happens.

There's no need to try to classify this oscillator "is it hartley or colpitts or or clapp or some other well known type". Those well-known LC oscillators were designed to make oscillations possible and controllable with low gain triode electron tubes. We have here a high gain transistor and the crystal. But if someone forced me to name one old electron tube oscillator that can be considered to be the grandma of this circuit, I would write TGTP (=tuned grid, tuned plate).

ADD: Radio circuit engineers do amplifier stability calculations. It's not uncommon to find that amplifier is unstable due the reactances of input signal source, load reactance and the internal feedback of the transistor. Microwave oscillators are often constructed as unstable amplifiers. In place of the crystal there's a high-Q microwave resonator.

Answer 3

Draw the circuit like this. The inverting amplifier inverts between base and collector.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 4

The missing knowledge is that: A cap's current leads it's voltage by 90 degrees. An inductor's current lags it's voltage by 90 degrees.

When they are in series, the current is the same for both, so the junction voltage is 180 degrees at resonance.Thats also why a series resonant circuit appears as a short.

Now reason out the parallel resonance circuit, where the two elements both have the same voltage.

As mentioned above a crystal is a series or parallel resonance circuit.

Yes the transistor's collector-base capacitance provides driving energy.

BTW: Many FETs oscillate due to gate inductance and drain to gate capacitance. Often at a frequency so high, it is Only noticed as a DC shift when you wave your hand over it.

Answer 5

If you temporarily remove the crystal, you should see that the circuit will oscillate at a frequency primarily determined by RFC1 and C1. The only thing the crystal does, is stabilize the oscillation frequency!

Answer 6

If you put a pulse voltage source in the circuit, it will work on spice too.

In real world there are a lot of signals that can cause the oscillator to start up.

But in spice you have to add a source of signal to let the oscillator to start up.

This can be done for example by changing the oscillator input voltage for a short duration of time like 1 uS.