Electronics > RF, Microwave, Ham Radio

Colpitts selective xtal oscillator

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Teknow:
Considering the following circuit atttached

 I am confused and can't understand how the tank circuit, which is open at resonance, and the transistor pass the 2nd or any other harmonic of a crystal of a fundamental frequency of, say, 48 MHz.
What frequency is at the output if the tank is open at resonance 48MHz for example?

1. How does the tank circuit contribute to or govern the choice of the desired Harmonic at the output if a parallel LC is "open circuit" at resonance?

2. How can I calculate L1 and C4 to have the desired frequency at the output?
Do I have to include the transistors' output capacitance in calculating the tank circuit? What is formula for that?
Many thanks in advance!

vk3yedotcom:
Although the circuit will likely work without it, there should be a 10nF (or similar) bypass capacitor between the positive rail and earth. 

Now visualise this part of the circuit as being ground (as far as RF is concerned).

Note that in the method of coupling chosen C5 needs to be small relative to the value of the capacitor in the tank circuit so it doesn't detune it. 

Alternative methods of coupling (depending on the input impedance of the following stage) include a tap partway along the coil or a secondary winding.

Teknow:
Thanks for the tips for the next stage + VCC rail coupling. But still the parallel LC is open at resonance and the series is short gets me nowhere in relation to the ciruit.
They short what, and they block what?
Why does this tank circuit decide what frequency is at the output, how?

MagicSmoker:

--- Quote from: Teknow on April 30, 2017, 10:03:48 pm ---Thanks for the tips for the next stage + VCC rail coupling. But still the parallel LC is open at resonance and the series is short gets me nowhere in relation to the ciruit.
They short what, and they block what?
Why does this tank circuit decide what frequency is at the output, how?

--- End quote ---

The tank circuit doesn't decide the frequency of the output - the crystal does. However, if the resonant frequency of the LC tank is close to that of the crystal's, then it will present a high (not infinite!) impedance to the collector of the common emitter amplifier which is prerequisite for high voltage gain. A bit of positive feedback from the emitter (via capacitive voltage divider C2/C3) and you have yourself an oscillator.

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