Electronics > RF, Microwave, Ham Radio

Colpitts oscillator biasing

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MrSlack:

--- Quote from: nugglix on April 20, 2016, 12:28:24 pm ---The capacity shouldn't be linear over voltage, it should follow a square function. This
will make the frequency change linear over the control voltage.
Which I think is the desired behaviour - at least for my VFO.

--- End quote ---

To confirm, this is linear with respect to the output frequency vs tuning voltage. Not the capacitance vs tuning voltage. If you plot the varactor's capacitance versus voltage on lin-log it's still curved slightly which gives a compressed region in the pot travel. It might actually be a better outcome with a fine/course tuning pot system yet :)

Opamp holy grail. One day we will reach this :)

nugglix:

--- Quote from: MrSlack on April 20, 2016, 12:34:56 pm ---To confirm, this is linear with respect to the output frequency vs tuning voltage. Not the capacitance vs tuning voltage. If you plot the varactor's capacitance versus voltage on lin-log it's still curved slightly which gives a compressed region in the pot travel. It might actually be a better outcome with a fine/course tuning pot system yet :)

--- End quote ---

Sorry, I thought you meant the capacity.

And this behaviour is the reason for the hand-tuned control voltage of my circuit.
I deliberately only use a portion of the possible tuning voltage and hope I found the
right range.

Now I need to wind the inductor. Didn't think of ordering some...  |O

And sorry for hijacking this thread.

MrSlack:
I might try that way of providing linearity - it's a good idea.

My design approach is to see what inductor cores I have floating around first. Fortunately I had two micrometals T50-6's with known Al then stick some kynar around them based on http://toroids.info/ . Then work out the rest based on what NP0 ceramics I have around and what trimmers I can steal out of the junk I have lying around. There is no science here :)

Apologies too.

uncle_bob:
Hi

Quick cookbook:

1) You need to bias the active device so it does not drift all over the place. That generally puts a volt or three on the emitter / source resistor.
2) You want enough current to give you transductance to provide 3 to 9 db of excess loop gain at turn on.
3) You want to bias the device so it clips (limits) in a "current cutoff" rather than a "voltage saturation" mode.

Do all that and it will work well, live long, pour your beer, and all sorts of wonderful stuff.

Bob 

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