Collpitts oscillator signal distorted - Page 1
Forum section: electronics / beginners

author: stafil
author_rank: Regular Contributor
timestamp: June 05, 2020, 08:52:29 pm
content: 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 aplitude of the input signal, but the distortion was still there.Do I have some secondary parasitic oscillation somewhere? Any idea how to fix it?PS: The crystal is 8Mhz not 8mhz.

author: orolo
author_rank: Frequent Contributor
timestamp: June 06, 2020, 10:29:00 am
content: Your waveform looks normal for a BJT Colpitts oscillator.In a Colpitts, the transistor in not at all in the linear region: you can see proof of that in your yellow emitter trace, where the waveform goes up from the valley first slowly, then stops a little, and then goes up quite fast. What you see there is the transistor turning on and then going into saturation. This can be seen better with a simulation (ltspice file attached):I've used a very fast transisor to illustrate to point better. BTW, the 47pF cap C4 in your circuit is not required: the crystal blocks DC, and C4 shifts the frequency up away from 8MHz. I also took away R3. And C2 could be smaller, perhaps 30-60p, but this seems to work with C2=100pF.In this simulation, the base (red) and emitter (green) waveforms are very similar to yours:To really see why you are not in the linear region, let's plot emitter voltage (red) versus collector current (green):There you have the transistor behaviour in a Colpitts oscillator: the BJT is almost always off, only turning on --quite hard-- for a brief part of the cycle. This is what causes the change in slope in the emitter voltage: initially the emitter follows the base via the capacitor, then the transistor kicks in and the emitter jumps up in slope.So, instead of a linear oscillator (except briefly at startup), you might think Colpitts as a crazy dwarf inside your crystal, hammering at it in synchrony with the resonant frequency. All the harmonics will be excited and you won't get a pure sine wave.

author: Vovk_Z
author_rank: Super Contributor
timestamp: June 06, 2020, 10:31:46 am
content: Try BJT of another type?

author: Manul
author_rank: Super Contributor
timestamp: June 06, 2020, 10:04:50 pm
content: As already mentioned, there are harmonics present because of non linearity. If you want a clean waveform you may consider an output filter, for eg. third-order low pass Pi butterworth works good if calculated properly. You need to consider (calculate or measure) input and output impedance. A nice and free software for calculation of filters is Elsie from Tonne Software.Also there are quite a lot of things involved in Collpitts oscillator and it is not so simple as it first looks, so tuning values may be necessary for reliable operation. For eg. it is not so hard to damage a crystal because of too high drive level. I attached a nice short pdf from Rakon about single transistor oscillators for your reference and ideas.

author: stafil
author_rank: Regular Contributor
timestamp: June 07, 2020, 03:04:58 am
content: Quote from: orolo on June 06, 2020, 10:29:00 amAh.. Thanks for the Crystal model. I used a different model for the 8Mhz crystal, that I found on the internet, and the output was fine on LTSpice, but distorted in real life. Thanks. I guess I can adjust the values of the component to try to stay within the linear region of the transistor.

author: stafil
author_rank: Regular Contributor
timestamp: June 07, 2020, 03:06:59 am
content: Quote from: Manul on June 06, 2020, 10:04:50 pmThanks for the PDF! Also this Elsie tool looks neat!I don't mind blowing up anything or damaging the crystal. I am only doing it as an experiment.