Wired up a phase shift oscillator but I dont think im getting a sinusoidal wave - Page 2 - Electronic Circuits Projects Diagrams Free

Qaiphyx · 21st July 2008, 03:59 AM

Quote:

Originally Posted by Roff

If you want to use Hero999's circuit, I would modify it as below.

Alright.

I just realized that im using all the same resistors for everything. but in the schematics its showing r1 greater than r2 whats the explination of this?

as you can tell im fairly new to electronic circuits.

Roff · 21st July 2008, 04:26 AM

Quote:

Originally Posted by Qaiphyx

Alright.

I just realized that im using all the same resistors for everything. but in the schematics its showing r1 greater than r2 whats the explination of this?

as you can tell im fairly new to electronic circuits.

You have it backwards. The schematic shows R2 greater than 29 times R1. I added the note in red, which means R1 should be much greater than R. I would try R=3.3k, R1=33k, R2=1Meg. The first circuit I posted has fewer constraints.

Qaiphyx · 21st July 2008, 04:36 AM

Quote:

Originally Posted by Roff

You have it backwards. The schematic shows R2 greater than 29 times R1. I added the note in red, which means R1 should be much greater than R. I would try R=3.3k, R1=33k, R2=1Meg. The first circuit I posted has fewer constraints.

Ok thanks. Yah I had a bunch of 1 k resistors and that was it ill have to run to radioshack tomorrow lol. Would this mistake prevent oscillation?

Also, what should I use for a frequency around 3400hz?

Roff · 21st July 2008, 05:10 AM

Quote:

Originally Posted by Qaiphyx

Ok thanks. Yah I had a bunch of 1 k resistors and that was it ill have to run to radioshack tomorrow lol. Would this mistake prevent oscillation?

Also, what should I use for a frequency around 3400hz?

You can use the 1k resistors for R. Use 10k for R1, and 300k for R2. To get around 3400Hz, see the original link:

f=1/(2*pi*R*C*√6)
R*C=1/(2*pi*f*√6)
R*C=1.911*10^-5
If R=1k=1000,
C=19.11nF
You can use either 18nF or 20nF to get close to 3400Hz.

Roff · 21st July 2008, 05:44 AM

You should change R2 to 330k. In a simulation, it would not run with 300k, but with 330k, and 18nF caps, it gave me 3.5kHz with very low harmonic distortion (-50dB). You might want to use a 270k or 300k resistor in series with a 50k or 100k pot, and crank the pot value down to where it just breaks into oscillation. This will give you the least distortion.

Hero999 · 21st July 2008, 02:37 PM

It seems like you don't understand how this circuit works.

In order for an amplifier to oscillate the gain needs to be >1 and have positive feedback and a particular frequency.

The RC ladder network has a total phase shift of 180° at the required frequency.

The amplifier has a phase shift of 180° at broad band ofl frequencies which must include the one your oscilator to work at.

Unfortunately the RC ladder also has a loss of 29 so the smplifier must have a gain of exactly 29 to compensate for this.

More about his can be found on Wikipedia:
http://en.wikipedia.org/wiki/Phase_shift_oscillator

Roff's circuit solves the following problems:

The amplifier has to have a gain of exactly 29.

If it's slightly lower than 29, it won't oscillate, the most likely scenario is it will produce a few oscillations and die away.

If it's higher than 29 you'll get distortion and clipping.

Roff's circuit solves this problems by adding diodes which feed some of the output signal back into the input when the output signal becomes too high, thus introducing more negative feedback and reducing the gain.

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21st July 2008, 05:44 AM	(permalink)
Roff	You should change R2 to 330k. In a simulation, it would not run with 300k, but with 330k, and 18nF caps, it gave me 3.5kHz with very low harmonic distortion (-50dB). You might want to use a 270k or 300k resistor in series with a 50k or 100k pot, and crank the pot value down to where it just breaks into oscillation. This will give you the least distortion. __________________ Ron

21st July 2008, 02:37 PM	(permalink)
Hero999	It seems like you don't understand how this circuit works. In order for an amplifier to oscillate the gain needs to be >1 and have positive feedback and a particular frequency. The RC ladder network has a total phase shift of 180° at the required frequency. The amplifier has a phase shift of 180° at broad band ofl frequencies which must include the one your oscilator to work at. Unfortunately the RC ladder also has a loss of 29 so the smplifier must have a gain of exactly 29 to compensate for this. More about his can be found on Wikipedia: http://en.wikipedia.org/wiki/Phase_shift_oscillator Roff's circuit solves the following problems: The amplifier has to have a gain of exactly 29. If it's slightly lower than 29, it won't oscillate, the most likely scenario is it will produce a few oscillations and die away. If it's higher than 29 you'll get distortion and clipping. Roff's circuit solves this problems by adding diodes which feed some of the output signal back into the input when the output signal becomes too high, thus introducing more negative feedback and reducing the gain. __________________ I also post at the following sites: http://www.stop-microsoft.org http://www.heated-debates.com Screen name: Aloone_Jonez