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Oscillator design
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JeanF:
Hi,

not too long ago I took apart an old OCXO to see what was inside, some of you may remember I posted a thread in Beginners section about the heater circuit.

Here I would like to talk about the oscillator circuit; I mostly understand what's going on but I have a few specific questions, maybe some of you guys can point me in the right direction!

I tried my best (and spent a lot of time) to avoid mistakes when tracing the circuit but obviously it's maybe not perfect, so if you find something suspect I can check once more.


So this seems to be a Colpitts quartz oscillator. I cropped the scanned image in order to leave the heater circuit out of the frame, it's just a basic comparator circuit with an opamp and a thermistor.

At first I was intrigued by L1 and C12 and then I read in "Design of crystal oscillator circuits" by B. Neubig (here, page 5) that it was a resonant circuit, which is capacitive at nominal frequency, to be used in place of the usual capacitor to prevent the oscillation at the third or higher overtone of the crystal, or other unwanted resonances. So that's sorted out.

My questions are:
- why in that case the bottom of L1 / C12 is not tied to ground but to the little R24/ C11 filter instead ; and do these filters have a more specific name than "parallel RC filter" so I can read up on them (I think R12 / C9 seems very similar, same question for them)
- what is Q1 doing ? I guess it is some kind of feedback. I think it is injecting current into R7 and into the filter mentioned previously but I don't trust my measurement enough. I would have been tempted to think it could be positive feedback to help sustain the oscillations, but when I desolder R7 (just to see what could happen) then the amplitude at the base of Q3 goes through the roof instead, so that's more like negative feedback? but why? I also tried to change the value of R7 a bit (between 1k and 2k) but I couldn't see any difference anywhere.
- in any case, what could be the point of the R14 / R15 voltage divider which does not divide by much? What would be the drawback of tying the left pin of C8 to the emitter of Q4 instead?
- (this last question is also related to the fact that I don't get what Q1 is doing) - would it have been possible for the designer to reduce BOM by 1 transistor by removing Q1 and tying R7 to the output of Q5 instead?

Thank you very much. This is not for work, not for school, I'm just curious :)
ArthurDent:
The feedback (as you found out) is like automatic gain control. The oscillator will shift frequency somewhat if the level of oscillation doesn't remain constant. The voltage on C11/R24 comes from C7 (4u7) which is D.C. This is similar to keeping the frequency stable by keeping the temperature constant. There are a number of variables that affect the frequency and they try to compensate for these with additional circuitry.
moffy:
Exactly as ArthurDent said. The gm/gain of Q2 is dependent on the amount of current flowing through it. Less current then lower gm. Q1 and R7/C7 provide that feedback. As the amplitude increases R7 steals current from Q2's emitter lowering its gm until a stable amplitude is found. It would also tend to make the output more sinusoidal with fewer distortion products.
pardo-bsso:

--- Quote from: JeanF on May 06, 2020, 08:54:11 pm ---
My questions are:
- why in that case the bottom of L1 / C12 is not tied to ground but to the little R24/ C11 filter instead ; and do these filters have a more specific name than "parallel RC filter" so I can read up on them (I think R12 / C9 seems very similar, same question for them)


--- End quote ---

They are there to give more gain at AC. When you calculate the bias point the transistor 'sees' only the resistor. However, for the ac analysis the capacitors are considered as a short.
JeanF:
Thank you very much everyone  :-+ I really appreciate it !
this circuit is a good opportunity for me to learn more about transistor theory, which I've been neglecting a bit until now :)
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