How does this oscillator work?

Question

I have come across a design for an oscillator that, boiled down to its essentials, has a configuration like this:

,--- VCC | .-. | | 330 | | '-' | | .----UUU----o | | | | --- | --- | | o--- Output | | | 22k | | ___ | o---|___|---o | | | | | |/ '---------| |>

| | '--- GND

But I can't see how it's supposed to work. The series LC network from collector to base looks like it should be providing negative feedback rather than positive!

(The full circuit is shown below, in case it makes any difference. The frequency-determining capacitor is actually a pair of varicap diodes used for frequency control.)

,--- VCC | | C C C | | | .-. | | 330 | | 390p '-' | || | ,-----o----||-----UUU----o | | || | | V | | - | 47k | - | ___ | | o--- Output Control ---|___|----)-----o | | | 22k | | - ___ | | - ---|___|---o | ^ | | .-. | | | | | | | |/ 47k | | '------o---------| '-' |>

| | | | '------------------------o--- GND

Michael Black · Accepted Answer

It looks like a Pierce oscillator I can't recall ever seeing one with an LC circuit, but it sure is common as a crystal oscillator (the crystal has the capacitance and inductance in series). The crystal provies the 180degree phase shift.

Michael

Bob Eld · Answer

I don't think it is a Pierce circuit because in a Pierce, the resonator (xtal) is between the collector and base providing the required addional 180 deg of phase shift. This circit has nothing but capacitance in that position. So, where does the required phase shift come from? It looks dubious to me, there must be more to it. Maybe the thing marked "UUU" is supposed to be an xtal?

Michael Black · Answer

He said the UUU was a coil, which is where I got the idea that it's an inductor.

But after I posted, I wondered if it was a real circuit, or to represent a circuit with a crystal. Like I said, I don't recall seeing a Pierce oscillator with an LC network, but if someone was trying to explain the circuit, they might represent the crystal as a coil and capacitor in series.

Michael

Paul E. Schoen · Answer

This looks similar to an adjustable crystal oscillator:

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But perhaps the component in series with the collector, marked CCC, is also a choke, and coupled to the one shown as UUU, which would make it something like an Armstrong oscillator.

Paul

greg · Answer

It's a real circuit, and there's definitely no crystal in it anywhere.

The circuit is from a Mitel Semiconductor application note titled "A Low Cost 1.5 to 2.2GHz Voltage Controlled Oscillator":

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My current theory is that it's actually a Colpitts, and the feedback is provided by the collector-emitter and base-emitter capacitances of the transistor (and maybe any other stray capacitances that happen to be floating around).

At those frequencies, it wouldn't be surprising if such parasitic elements formed a vital part of the circuit. The "coil" is actually specified as 0.5cm of component lead!

How does this oscillator work?

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