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What Colpitts oscillator is this?

Elerion

Member
Hi everyone.

I've come accross an oscillator circuit (first attached image, left), which is very similar to a common collector crystal Colpitts oscillator (right).
But it is not a common collector, as the load (hf transformmer) is placed in the collector node and output is taken from there.
Not sure what it is.

This is my guess.
It looks to me like a common base. (second attached image).
The feedback is then via collector-emitter capacitance.
The crystal is not in place of the inductor, but on the base, so it resonates near its series resonance frequency (effectively grounding the base).

What do you think?
Thanks!
 

Attachments

BobW

Active Member
It just looks like a regular Colpitts with emitter feedback, a fairly common configuration. With the output taken from the collector, neither emitter, base nor collector is really a common point, but from the point of view of oscillator function, I would call it a common collector.

Edit: Or, maybe phase splitter would be more accurate.
 

Elerion

Member
Yes, it makes sense.

Any idea why that inductor in series with the xtal?
It must be some kind of frequency compensation, but I've seen many many circuits like this, with the variable capacitor but no inductor...
 

unclejed613

Well-Known Member
Most Helpful Member
the feedback path is from a capacitive divider, that's what makes it a colpitts. using an inductive divider (usually a tapped inductor) would be a hartley.
 

JimB

Super Moderator
Most Helpful Member
Any idea why that inductor in series with the xtal?
The variable capacitor is in series with the crystal to adjust the frequency (slightly) so that it is exactly on its required frequency.

Adding the inductor increases to frequency pulling range.
If the inductor is selected to be resonant with the variable capacitor and circuit strays, as the capacitor is adjusted, the reactance in series with the crystal can be changed from capacitive to inductive.

Where did you find this circuit?
Sometimes this configuration with the inductor/capacitor in series with the crystal is referred to as a VXO (Variable Crystal Oscillator).

JimB
 

unclejed613

Well-Known Member
Most Helpful Member
Any idea why that inductor in series with the xtal?
probably to insure the crystal oscillates at the fundamental rather than an overtone (harmonic).
 

unclejed613

Well-Known Member
Most Helpful Member
believe it or not, there is a [textbook] on the subject of crystal oscillators.
 

Elerion

Member
believe it or not, there is a [textbook] on the subject of crystal oscillators.
Yes. "Practical Oscillator Handbook" is good one too, not only crystal related.

Adding the inductor increases to frequency pulling range.
With a 50 pF variable cap I can slow down the frequency a couple of kHz.
Strange... I thought that adding series capacitor increases crystal's resonance frequency.

Where did you find this circuit?
It is part of an amateur CW transmitter.
 

JimB

Super Moderator
Most Helpful Member

unclejed613

Well-Known Member
Most Helpful Member
You've mentioned one of my favorites...."Crystal Oscillator Circuits" by Robert Matthys (Kreiger Publishing 1992). Very practical and useful book.
the more digging i do at archive.org, the more buried treasure i find.....
 

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