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I'm currently building a theremin from the circuits form here:LINK This uses Colpitts oscillators. However, when I've tried calculating the frequency, for example of the fixed-pitch oscillator (below), I've failed.

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I've always got wrong results. I've read on the wiki that this freq us determined by two capacitor values and the coil. The actual freq is calculated by the well-know formula, and C is the sum-capacitance of the 2 caps in series, eg do repluss.
According to the schematics on the wiki, I'd need to repluss the 680pF and the 33nF caps. But that makes no sense! I've got a wrong freq value, and there's even 2 variable circuit to change the freq! And those are caps that aren't on the wiki schematic! So wiki says that they don't affect the freq, but I know that they does!

Did I understand some(or every)thing wrong? How can I calculate those freq's?

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  • \$\begingroup\$ What does "repluss" mean? \$\endgroup\$
    – The Photon
    Jan 6, 2017 at 0:06
  • \$\begingroup\$ Capacitance of series connected caps is C1*C2/(C1+C2) Very similar to the formula of parallel resistors. \$\endgroup\$
    – Uncle Dino
    Jan 6, 2017 at 0:08
  • \$\begingroup\$ I don't really understand what that should mean. You use "repluss" as an action (something you can do); sadly, it's not an English word. Can you find another way to describe it, please? \$\endgroup\$
    – Marcus Müller
    Jan 6, 2017 at 1:00
  • \$\begingroup\$ en.wikipedia.org/wiki/Series_and_parallel_circuits#Capacitors Some math and you've got the formula I've described. I've seen that they just say "replus" for calculating that in electronics forums(at least on Hungarian ones) \$\endgroup\$
    – Uncle Dino
    Jan 6, 2017 at 1:06
  • \$\begingroup\$ it is far more critical that the two oscillators feeding the mixer have the same frequency (or nearly so). In any case, the L and C's work out to about 180 kHz. That's pretty close. \$\endgroup\$
    – glen_geek
    Jan 6, 2017 at 3:48

2 Answers 2

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schematic

simulate this circuit – Schematic created using CircuitLab

Draw the circulating currents.

There is a tight energy storage loop, at the collect/VDD.

There is a large circulating loop, being the 0.033 at VDD_to_GND, then the 0.033 across the emitter resistor, then then 680pF across emitter-collector. Thus the losses of these 3 capacitors helps set the QualityFactor of this oscillator.

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According to the schematics on the wiki, I'd need to repluss the 680pF and the 33nF caps. But that makes no sense!

No, the 33 nF has nothing to do with the oscillator frequency; it's just there to stabilize the power feed to the tuned circuit.

Did I understand some(or every)thing wrong? How can I calculate those freq's?

The oscillator frequency is dictated by the parallel sum of the three caps in parallel with the inductor and the 680 pF (the latter using product/sum calculations).

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  • \$\begingroup\$ I still don't understand it. There are two 33nF caps, one connects emitter to GND, one connects VCC to GND after the 1k resistor. The wikipedia article labels the first one as C2, and uses it in the formula. \$\endgroup\$
    – Uncle Dino
    Jan 6, 2017 at 17:13
  • \$\begingroup\$ @Sasszem What wiki article? \$\endgroup\$
    – Andy aka
    Jan 6, 2017 at 17:17
  • \$\begingroup\$ The article of the Colpitts oscillator. Fig. 1, common base voltage oscillator. The cap between the emitter and GND is labeled C2. \$\endgroup\$
    – Uncle Dino
    Jan 6, 2017 at 17:43

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