phase shift oscillator distortion

Thread Starter

Fluxor1964

Joined Jun 11, 2015
182
Hi Guys....
I am in need of a sinewave oscillator for playing and experimenting with transistors and small signal amplifiers
so I thought i'd use a phase shift oscillator since I don't need to vary the frequency, just a stable 1Khz signal is all I
need, I built the circuit that I found online and have included which gives me 578 Hz or near as damn it, thing is
the distortion shows up really good and I would like something more pure, where is the first place to go to try
to correct this distortion?....i'm not looking for perfection 100% but surely a better result is achievable somehow?
 
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Thread Starter

Fluxor1964

Joined Jun 11, 2015
182
Hi Guys....
I am in need of a sinewave oscillator for playing and experimenting with transistors and small signal amplifiers
so I thought i'd use a phase shift oscillator since I don't need to vary the frequency, just a stable 1Khz signal is all I
need, I built the circuit that I found online and have included which gives me 578 Hz or near as damn it, thing is
the distortion shows up really good and I would like something more pure, where is the first place to go to try
to correct this distortion?....i'm not looking for perfection 100% but surely a better result is achievable somehow?
 

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danadak

Joined Mar 10, 2018
3,569
Whats the collector DC bias point ? Should be ~ 1/2 Vcc, 4.5V. You
also have no negative feedback, re C5 is killing any negative feedback.
Maybe drop R6 by 50 ohms, and put 50 ohms in series with emitter and the
parallel combo of R6 C5 to produce some - feedback. This will also help
raise the Zin presented by transistor base a little and reduce loading
on phase shift network.

You also have the problem there is no AGC in the circuit.

Do you have a specturm analyzer, if not see below.


Regards, Dana.

You can start with a PC sound card based scope for free. Will give you basically
audio range scope, spectrum analyzer, and function generator all using your
PC sound card.

https://www.zeitnitz.eu/scope_en

http://www.zelscope.com/

http://www.ledametrix.com/oscope/

http://www.virtins.com/downloads.shtml


But first build a simple circuit to protect sound card inputs so you do not
ruin from transients, overvoltage. Google "protect sound card input".


For example http://makezine.com/projects/sound-card-oscilloscope/
 
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crutschow

Joined Mar 14, 2008
23,291
What are you using for TR1?

That simple circuit will tend to have distortion.
It needs to be more complex if you want a better sinewave.
 
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Thread Starter

Fluxor1964

Joined Jun 11, 2015
182
Maybe someone can point me in the direction of a phase shift oscillator that uses an op amp and actually works?..... I have built a couple found on the web but they didn't work....they must be tricky little beasts.
 

crutschow

Joined Mar 14, 2008
23,291
Here's a Bubba phase-shift oscillator and its LTspice simulation.
It uses four RC, 45° phase-shift op amp stages (one quad opamp) to give the required 180° phase shift, which with the 180° phase-shift from U1, gives the 360° phase-shift for oscillation.

The signal is softly clipped by D1 and D2 to stabilize the amplitude (yellow trace).

This is filtered by the following three phase-shift stages which act as a 3-pole low-pass filter to reduce the signal distortion to a low level at the output (blue trace).

The output can be adjusted by a pot U5 to any voltage level you want up to about 5Vpp.

If you have trouble getting it to oscillate, try reducing the value of R1 to increase the loop gain.

upload_2018-11-21_20-37-29.png
 
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