# How does this oscillator work?

Discussion in 'Electronic Basics' started by greg, Aug 11, 2009.

1. ### gregGuest

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

2. ### Michael BlackGuest

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

3. ### Bob EldGuest

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?

4. ### Michael BlackGuest

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

5. ### Paul E. SchoenGuest

This looks similar to an adjustable crystal oscillator:

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

6. ### gregGuest

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":

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