What Colpitts oscillator is this?

Status
Not open for further replies.
E

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

  • colpitts.png
    76.6 KB · Views: 407
  • cb.png
    19.5 KB · Views: 410
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.
 
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...
 
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.
 
Elerion said:
Any idea why that inductor in series with the xtal?
Click to expand...
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 said:
believe it or not, there is a [textbook] on the subject of crystal oscillators.
Click to expand...

Yes. "Practical Oscillator Handbook" is good one too, not only crystal related.

JimB said:
Adding the inductor increases to frequency pulling range.
Click to expand...
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.

JimB said:
Where did you find this circuit?
Click to expand...

It is part of an amateur CW transmitter.
 
JimB said:
In that case definitely a VXO.
Click to expand...

Yes.
You were absolutely right in that the inductor widens the capacitor tunning range a little bit.

unclejed613 said:
probably to insure the crystal oscillates at the fundamental rather than an overtone (harmonic).
Click to expand...

It doesn't seem so. At least not in this particular circuit. But I've seen other topologies where that seems to be the case.
 
RadioRon said:
You've mentioned one of my favorites...."Crystal Oscillator Circuits" by Robert Matthys (Kreiger Publishing 1992). Very practical and useful book.
Click to expand...

the more digging i do at archive.org, the more buried treasure i find.....
 
Status
Not open for further replies.

Similar threads

O
  • Question
Colpitts oscillator in multisim
Replies
19
Views
16K
danadak
O
  • Question
coupling and bypass capacitor value in Colpitts oscillator?
Replies
5
Views
11K
danadak
J
  • Question
What WiFi signal attenuation through concrete wall?
Replies
15
Views
5K
rjenkinsgb
O
  • Question
Is there any analog radio receiver that prevents fading of the SW reception?
Replies
6
Views
3K
Miles Prower
T
  • Question
Looking for the connection/pin-out diagram for HP 5086-7332 YIG oscillator.
Replies
2
Views
10K
Teeddd
T
Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…