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Phase Shift Oscillator Distortion

D

David White

Jan 1, 1970
0
I have been studying phase shift oscillators, and built a few up
today. The circuit linked below has a slight problem I can't figure
out.

It is intended to output 10Hz, but the tops of the sinewaves are
slightly skewed to the left, and there is a "kink" as they approach 0
volts before progressing to the next cycle.

http://www.4shared.com/photo/mtkJplbI/Phase-Shift-Oscillator.html?

If I crank the trimmer back to just before the oscillation dies,
everything comes right, but the amplitude drops and wavers around
(unstable). There does not appear to be any in-between adjustment.

I have tried substituting components, to no avail. Can anyone please
explain what is happening, and if there is a fix?

Does it perhaps have to do with the relatively low frequency involved?

Thank you,

David White
 
M

miso

Jan 1, 1970
0
I have been studying phase shift oscillators, and built a few up
today. The circuit linked below has a slight problem I can't figure
out.

It is intended to output 10Hz, but the tops of the sinewaves are
slightly skewed to the left, and there is a "kink" as they approach 0
volts before progressing to the next cycle.

http://www.4shared.com/photo/mtkJplbI/Phase-Shift-Oscillator.html?

If I crank the trimmer back to just before the oscillation dies,
everything comes right, but the amplitude drops and wavers around
(unstable). There does not appear to be any in-between adjustment.

I have tried substituting components, to no avail. Can anyone please
explain what is happening, and if there is a fix?

Does it perhaps have to do with the relatively low frequency involved?

Thank you,

David White

Is this supposed to e a known working circuit. It looks to me like the
battery connects right to the load through a diode. I would be
suspicious of this circuit since the standard 1n4002 is drawn like a zener.

Note where you got the circuit and never go there again.
 
D

David White

Jan 1, 1970
0
1) Make sure that on the negative-going swing the collector of Q1 is not
dropping significantly below its base and forward biasing the
emitter-collector junction. That could account for the "kink" you see.

OK. Is this solved by adjusting the R3 and R4 ratio? What would you
suggest?
2) In general you're never going to get a great looking sine wave out of
a phase shift oscillator using a single active device. Even with
manually adjusting the gain and applying negative feedback via an
unbypassed emitter resistor so it's just barely clipping, a single
transistor just doesn't have enough loop gain to smooth its own
nonlinearities out.

Would this be particularly pronounced at low frequencies? The waveform
irregularity seems to even out as things go higher.
3) The DC coupling arrangement between Q1 and Q2 is kind of weird. If
you're going to use a zener for level shifting like that it would
probably be best to put the LED in Q2's cathode. R6 is also unnecessary.

The "1N4002" is not a zener, in spite of the symbol. Just diode
protection. Couldn't find a regular diode in the DipTrace library. At
least it's free.

BTW the "470R" is just a resistive load ... checking to see how much
the circuit will drive.

Thank you for looking at my circuit.

David White
 
P

Phil Allison

Jan 1, 1970
0
"David White = TROLL "

BTW the "470R" is just a resistive load .


** FFS - it is only loads the damn BATTERY !!!!

.. checking to see how much the circuit will drive.


** You are an utter imbecile.

Piss off.
 
D

David White

Jan 1, 1970
0
** You are an utter imbecile.

Piss off.


As you point out, I drew the circuit wrong. Sorry folks. The top of
the load resistor goes to emitter Q2.

Here is the corrected version:

http://www.4shared.com/photo/1Y6WR3dP/Phase-Shift-Oscillator.html?

My original questions still apply.

**********

I have been studying phase shift oscillators, and built a few up
today. The circuit linked below has a slight problem I can't figure
out.

It is intended to output 10Hz, but the tops of the sinewaves are
slightly skewed to the left, and there is a "kink" as they approach 0
volts before progressing to the next cycle.

If I crank the trimmer back to just before the oscillation dies,
everything comes right, but the amplitude drops and wavers around
(unstable). There does not appear to be any in-between adjustment.

I have tried substituting components, to no avail. Can anyone please
explain what is happening, and if there is a fix?

Does it perhaps have to do with the relatively low frequency involved?

**********

Any advice on these points would be appreciated.

David White
 
P

Phil Allison

Jan 1, 1970
0
"David White"
Can anyone please
explain what is happening, and if there is a fix?


** Replace D1 with a link and see what changes.

You are still an imbecile.
 
T

Tauno Voipio

Jan 1, 1970
0
As you point out, I drew the circuit wrong. Sorry folks. The top of
the load resistor goes to emitter Q2.

Here is the corrected version:

http://www.4shared.com/photo/1Y6WR3dP/Phase-Shift-Oscillator.html?

My original questions still apply.

**********

I have been studying phase shift oscillators, and built a few up
today. The circuit linked below has a slight problem I can't figure
out.

It is intended to output 10Hz, but the tops of the sinewaves are
slightly skewed to the left, and there is a "kink" as they approach 0
volts before progressing to the next cycle.

If I crank the trimmer back to just before the oscillation dies,
everything comes right, but the amplitude drops and wavers around
(unstable). There does not appear to be any in-between adjustment.

I have tried substituting components, to no avail. Can anyone please
explain what is happening, and if there is a fix?

Does it perhaps have to do with the relatively low frequency involved?

**********

Any advice on these points would be appreciated.

David White


To get pure signal from the oscillator, you need some kind
of low-distortion amplitude control, a kind of automatic
electronic version of you tuning the gain resistor. For
clean stable oscillation, you need a gain of exactly 1
around the amplifier-phase shift loop.

One source of distorsion is the resistor-zener-LED chain
in the output. Pick it off and re-check the signal.
 
P

Phil Allison

Jan 1, 1970
0
"Robert Bare Faced Liar"
Perhaps D1 is to give protection against accidental reverse polarity.


** No fooling ..........

( Apologies to FZ )




..... Phil
 
J

Jamie

Jan 1, 1970
0
David said:
I have been studying phase shift oscillators, and built a few up
today. The circuit linked below has a slight problem I can't figure
out.

It is intended to output 10Hz, but the tops of the sinewaves are
slightly skewed to the left, and there is a "kink" as they approach 0
volts before progressing to the next cycle.

http://www.4shared.com/photo/mtkJplbI/Phase-Shift-Oscillator.html?

If I crank the trimmer back to just before the oscillation dies,
everything comes right, but the amplitude drops and wavers around
(unstable). There does not appear to be any in-between adjustment.

I have tried substituting components, to no avail. Can anyone please
explain what is happening, and if there is a fix?

Does it perhaps have to do with the relatively low frequency involved?

Thank you,

David White
Another spam site... all garbage on the screen and nothing there that I
can see that has anything to do with what you are talking about.

Just another garbage dump site.

Jamie
 
J

Jamie

Jan 1, 1970
0
David said:
As you point out, I drew the circuit wrong. Sorry folks. The top of
the load resistor goes to emitter Q2.

Here is the corrected version:

http://www.4shared.com/photo/1Y6WR3dP/Phase-Shift-Oscillator.html?

My original questions still apply.

**********

I have been studying phase shift oscillators, and built a few up
today. The circuit linked below has a slight problem I can't figure
out.

It is intended to output 10Hz, but the tops of the sinewaves are
slightly skewed to the left, and there is a "kink" as they approach 0
volts before progressing to the next cycle.

If I crank the trimmer back to just before the oscillation dies,
everything comes right, but the amplitude drops and wavers around
(unstable). There does not appear to be any in-between adjustment.

I have tried substituting components, to no avail. Can anyone please
explain what is happening, and if there is a fix?

Does it perhaps have to do with the relatively low frequency involved?

**********

Any advice on these points would be appreciated.

David White
I can offer you a modification but you'll need to add an additional
stage on the end of all that if you need a large output swing..

If you intend to keep the selected frequency of design around 10hz
do the following.

Change R6 to 10k and place a cap of 1.5uf at the base of Q2 to
common. Also reduce R5 to 3.3k to help in the loss.

That network will give you a Q of 1 and supply a better symmetrical
wave to the base.. This offsets the base drive from what it was to ~
*0.707 now. You'll get a little less than 7v at the base and - one diode
drop at the emitter.

All of this breaks down to around 6V Pk-Pk DC on the final out come
and it'll give you a nice looking wave form.


jamie
 
D

David White

Jan 1, 1970
0
All of this breaks down to around 6V Pk-Pk DC on the final out come
and it'll give you a nice looking wave form.

And so it does. Yes, the gain was the problem. Thank you for taking
time to provide this learning experience.

David White
 
M

miso

Jan 1, 1970
0
David White
Another spam site... all garbage on the screen and nothing there that I
can see that has anything to do with what you are talking about.

Just another garbage dump site.

Jamie
There is a circuit there. Beware of a 4-way way. I had some trouble
seeing the feedback. I make it a rule to never make a 4-way connection
on a schematic, even if it makes the circuit look messy.
 
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