Phase shift oscillator and circuit

In summary, the conversation is about the simulation of a circuit involving a TL072 op-amp. The circuit is a phase shift oscillator and the goal is to determine the frequency of oscillation using PSpice simulation. The conversation includes discussions about the impact of the op-amp's phase shift on the frequency and how to find an ideal op-amp in PSpice. The circuit is found to have a frequency of 55 kHz, but the time steps and resolution of the simulation need to be adjusted for better results. The conversation also suggests using an initial voltage for one of the capacitors and implementing a smooth amplitude limiting device for better waveform results.
  • #1
suv79
65
0

Homework Statement


upload_2016-2-13_18-16-57.png

upload_2016-2-13_18-17-20.png

Gain & phase response of TL072 op-amp
upload_2016-2-13_18-19-1.png


Homework Equations


'Frequency of oscillation was observed to be about 15 % lower'[/B]
Why ?

The Attempt at a Solution


upload_2016-2-13_18-24-39.png

so at this frequency the op-amp has 45 phase shift, but why will this impact the oscillator and lower the frequency.

PSpice simulation
upload_2016-2-13_18-32-10.png

analysis
upload_2016-2-13_18-33-34.png

why is my frequency 22 Hz
 
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  • #2
how can i find an 'ideal op-amp' in PSpice ?
 
  • #3
An ideal opamp model in PSpice is simply called up with "OPAMP".
Regarding your result - I suppose the critical parameter is the limited slew rate which drastically lowers the frequency (and causes a triangle form).
 
  • #4
but would the slew rate effect the frequency or only distort the waveform,
the phase shift oscillator should producesa sine wave output.
 
  • #5
suv79 said:
but would the slew rate effect the frequency or only distort the waveform,
the phase shift oscillator should producesa sine wave output.
Yes - the slew rate SR lowers the frequency because SR causes in addition a negative phase shift (delay corresponds with phase shift).
 
  • #6
65 kHz is my worked out frequency but PSpice is 22 Hz
 
  • #7
I have simulated your circuit - and the result is at 55 kHz.
What are the time steps you have chosen?
My choice: End of simulation 1ms and resolution 1µs.
Why such a large simulation time for 55 kHz(1s)?

Comment: A much better waveform can be achieved if you split the feedback resistor in two (33k=23k +10k) and connect two diodes (antiparallel) across the 10k resistor.
 
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  • #8
you got 55 kHz that is what it should be about 15% lower.
 
  • #9
upload_2016-2-14_17-36-57.png

i think something is wrong with this iam still getting 22 Hz
 
  • #10
i have to write a report about why the circuit is 15% lower from designed frequency, when the TL072 op-amp was used.

have simulated your circuit - and the result is at 55 kHz.
What are the time steps you have chosen?
My choice: End of simulation 1ms and resolution 1µs.
Why such a large simulation time for 55 kHz(1s)?

Comment: A much better waveform can be achieved if you split the feedback resistor in two (33k=23k +10k) and connect two diodes (antiparallel) across the 10k resistor.

did you use PSpice to simulated the circuit? can you see any differences between your circuit and my one ?
 
  • #11
could you email me your simulated circuit ?
 
  • #12
suv79 said:
could you email me your simulated circuit ?

I have simulated exactly your circuit using a realistic model for TL072.
Why didn`t you answer my question regarding simulation time amd time resolution??
 
Last edited:
  • #13
the time i used was 1 Second
upload_2016-2-14_21-4-47.png

but time resolution i have no idea.
upload_2016-2-14_21-5-42.png
 
  • #14
the run time should not change the frequency
 
  • #15
suv79 said:
the run time should not change the frequency
OK - if you know better I have no problems.
It seems you are very familiar with simulation programs.

I have again removed my explanation for the frequency variation in my answer#12.
I see no necessity to give you hints if you know better.
 
  • #16
but i don't understand what is wrong with my circuit, why is it not getting 55 kHz
 
  • #17
i have chanced the run time to 1m the freq is now 21 kHz
upload_2016-2-14_21-35-13.png

but where can i change time resolution ?
 
  • #18
What is the DC value of your circuit's output, and what value were you expecting it to be?

What pk-pk amplitude are the oscillations you are seeing?
 
  • #19
i have no idea how to work out the amplitude of the waveform
 
  • #20
upload_2016-2-15_14-55-14.png

why when the stop time is change dose it effect the frequency.
 
  • #21
upload_2016-2-15_14-58-48.png

red is 50 u
green is 100u
blue is 200u
 

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  • #22
I can't help because I don't use (or know about) Pspice (For a simple cct like this, I'd just build it!)

But going back to your first post, why do you say that the phase shift of the op amp is 45o at 55kHz? AFAI can see, it would still be close to 90o. It would appear to be 45o around 12Hz (and maybe 50MHz.) (Both taken from your graph - not checked by me.)

Looking at your Pspice output from an innocent perspective I wonder;
why your output amplitude is so small?
whether you would be able to see 50-60kHz oscillations?
what the waveform you are seeing really is?
what sort of mathematical accuracy Pspice is capable of? Does it give "digital noise" like rounding errors?

If I were you I'd listen to people like LvW, who know about this stuff.
 
  • #23
suv79 said:
why when the stop time is change dose it effect the frequency.

In my post#7 I gave you my simulation data (PSpice) with the result of app. 55kHz (clean sinewave).
I cannot understand why you didn`t try the same end time (1ms) and resolutiuon (1µs).

Are you aware that for an end time of 1s you expect to see on the screen 55000 oscillations? Does this make sense?
More than that, you should know that the simulator automatically selects a resolution (a certain percentage of the end time) which in many cases (as in yours) is bad because you had specified an end time which is much too large (1s).

That is the reason you see no sine wave - and the frequency is incorrect (too small).
Therefore you must override this automatic and specify a maximum time step as I did for my simulation.(1µs).
It should be possible for you to find out how you can command such an upper limit for the time step.

Furthermore, for a quick and save start of oscillations it is to be recommended to give one of the capacitors an "initial voltage".
Otherwise it takes a long time to see oscillations (if any).

EDIT: Which simulator are you using? Try ".print step" or "advanced options" for specifying the maximum time step.
 
  • #24
resolutiuon= .PRINT step ?
upload_2016-2-15_15-54-54.png

upload_2016-2-15_15-57-39.png
 
  • #25
i used, real time noise, which should help start the oscillation
upload_2016-2-15_16-5-42.png
 
  • #26
upload_2016-2-15_16-13-9.png

ok with the initial voltage on the capacitors this is much better :)

thank you LvW
sorry iam a bit slow
 

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  • #27
OK - I hope you have learned now how to simulater an oscillator:
* Use a simulation time of app. 20-50 periods (maximum)
* Use a resolution (time step) of app 1/1000 of the max. simulation time
* Provide a starting aid by using an initial voltage for a capacitor.
* For a better waveform (low distortions, lowTHD) use a smooth amplitude limiting device (diodes, for example).
 

What is a phase shift oscillator?

A phase shift oscillator is an electronic circuit that produces a sine wave output without the use of an external input signal. It uses a combination of resistors, capacitors, and inductors to create a feedback loop that generates the oscillating signal.

What is the purpose of a phase shift oscillator?

A phase shift oscillator is commonly used in electronic devices such as audio equipment, radio transmitters, and digital clocks. It can also be used in scientific research and experimentation.

What are the components of a phase shift oscillator circuit?

A phase shift oscillator circuit typically consists of resistors, capacitors, and inductors connected in a specific arrangement. The number of components and their values may vary depending on the specific design of the oscillator.

What is the principle behind a phase shift oscillator?

The principle behind a phase shift oscillator is based on the principle of positive feedback. The output signal is fed back to the input with a phase shift, which causes the signal to oscillate at a specific frequency determined by the circuit components.

What are the advantages of a phase shift oscillator?

One of the main advantages of a phase shift oscillator is its simplicity and cost-effectiveness. It also has a wide frequency range and can produce a stable and reliable output signal. Additionally, it can be easily adjusted to produce different frequencies by changing the values of the circuit components.

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