2019-11-27 “Oscilator Colpitts” Doesn’t!

I found this schematic on a website and it’s recognizable because the author misspelled it oscilator – he left out one of the Ls (I corrected it on my copy). So I built it as shown but it wouldn’t oscillate. The values of base bias resistors R1 and R2 gave a saturated transistor. I had to increase R1 to 41k to get the base bias correct. But it still wouldn’t oscillate. I got it to oscillate by putting a 0.1 uF bypass capacitor across the emitter resistor R4. I increased it to 0.56 uF which was in my spare parts box.

It’s now putting out a decent sine wave at 456 kHz. The problem now is to modulate it with an audio tone of about 400 Hz. I injected that into the base of the transistor, and it can fully modulate the RF. But the modulation is distorted and asymmetrical. I need to try another way, maybe by modulating the supply voltage. I can also add another stage and modulate that. More experimenting is needed.

Update 11-28 – I changed the modulation to the emitter as shown in the schematic below (I forgot to remove the resistor and 400 Hz sine wave at the bottom – ignore it). The 400 Hz sine wave modulation is still asymmetrical, the bottom is higher than the top. And the top and bottom are slightly shifted like with the base modulation. I’m going to try adding a separate RF amp and modulation stage. Check out my blog for a continuation of this.

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