Colpitts oscillator with heavily distorted output

Thread Starter

soterus

Joined Jul 26, 2021
3
Dear all:

I built the Colpitts oscillator given in the diagram below. The transistor model is 2N2222A, all
bypass capacitors are WIMA MKS4 (metalized Polyester), 100 nF. Capacitors of the tank circuit are ceramic.
When measuring the output under no load with the oscilloscope, the resulting sine function is heavily distorted
as seen below. I do not believe that this is due to the design on a protoboard and its parasitic capacitances.
What could I do to obtain a more beautiful sine output?
Thank you in advance.circuit.jpgoszi.jpg
 

Papabravo

Joined Feb 24, 2006
19,262
I believe the clipping is an indication of too much loop gain. Did you copy this design from some source, or did you just slap some component values into a template in the vain hope that it would give you what you wanted? What is with this beautiful sine wave output that seems to be trending lately on this forum?
Here is a simulation of a reasonably behaved, crystal controlled oscillator with a rich harmonic content of both even and odd components as you can see from the FFT display. Reducing the harmonic content would improve the "beauty" of the sinewave.

1664212592813.png
 
Last edited:

Papabravo

Joined Feb 24, 2006
19,262
Here is a nice JFET oscillator example for 1 MHz. with much better harmonic attenuation and correspondingly more "beautiful" sine wave output.
1664213395085.png
 

crutschow

Joined Mar 14, 2008
30,766
To get a "beautiful" sinewave, the feedback gain must be precisely set to just generate the oscillation but not more.
Since component variation with time and temperature precludes setting a precise fixed-gain, an AGC circuit is often used for this purpose.

In the first Hewlett Packard oscillator, this controlled feedback gain was provided by the variable resistance of a small incandescent bulb operating at a low voltage in the feedback loop.

Now it is usually done with active components, such as a FET, to provide the variable feedback.
 

Ian0

Joined Aug 7, 2020
6,286
How beautiful a sine wave does one need at 12MHz? It‘s not as if the distortion can be heard, and is it’s not for a radio transmitter where someone might object to the harmonics being broadcast, who cares?
 

Papabravo

Joined Feb 24, 2006
19,262
How beautiful a sine wave does one need at 12MHz? It‘s not as if the distortion can be heard, and is it’s not for a radio transmitter where someone might object to the harmonics being broadcast, who cares?
It can be really fugly without any problems at all as long as you can knock the spurs down by 60 dB or so.
 
Top