Electronic – Collpitts oscillator signal distorted

common-collectorcrystaloscillator

I am trying to build a Collpitts oscillator using an 8MHz crystal, but the signal I get, is completely distorted. Any idea why?

The yellow signal is probed at the output, while the pink is probing the base of the BJT.

To start with I thought the problem was that my BJT was not in linear region, so I increased the load resistor to 2k to reduce the amplitude of the input signal, but the distortion was still there.

Do I have some secondary oscillation somewhere? Any idea how to fix it?

PS: The crystal is 8MHz not 8mhz.

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Best Answer

Change R3 from 100k to 68k
Change R2 from 147k to 27k thus Rb=20k and Rin= 100k hFE*Re ~100 * 1k
Replace wire with 1k from emitter to C2//C3 to match R1.

Root Cause: The C loading Re at some harmonic of 8MHz is causing output distortion. When there is series C coupling on Emitter always make Rmin load >= Re. In this case equal.

Expect 2Vpp sine output. Use another buffer if you plan on driving any significant capacitance ( e.g 30 pf /ft)

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