# Homework Help: Phase shift oscillator and circuit

1. Feb 13, 2016

### suv79

1. The problem statement, all variables and given/known data

Gain & phase response of TL072 op-amp

2. Relevant equations
'Frequency of oscillation was observed to be about 15 % lower'

Why ???
3. The attempt at a solution

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

PSpice simulation

analysis

why is my frequency 22 Hz

2. Feb 13, 2016

### suv79

how can i find an 'ideal op-amp' in PSpice ?

3. Feb 13, 2016

### LvW

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. Feb 13, 2016

### suv79

but would the slew rate effect the frequency or only distort the waveform,
the phase shift oscillator should producesa sine wave output.

5. Feb 13, 2016

### LvW

Yes - the slew rate SR lowers the frequency because SR causes in addition a negative phase shift (delay corresponds with phase shift).

6. Feb 13, 2016

### suv79

65 kHz is my worked out frequency but PSpice is 22 Hz

7. Feb 14, 2016

### LvW

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.

Last edited: Feb 14, 2016
8. Feb 14, 2016

### suv79

you got 55 kHz that is what it should be about 15% lower.

9. Feb 14, 2016

### suv79

i think something is wrong with this iam still getting 22 Hz

10. Feb 14, 2016

### suv79

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. Feb 14, 2016

### suv79

could you email me your simulated circuit ?

12. Feb 14, 2016

### LvW

I have simulated exactly your circuit using a realistic model for TL072.
Why didnt you answer my question regarding simulation time amd time resolution??

Last edited: Feb 14, 2016
13. Feb 14, 2016

### suv79

the time i used was 1 Second

but time resolution i have no idea.

14. Feb 14, 2016

### suv79

the run time should not change the frequency

15. Feb 14, 2016

### LvW

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. Feb 14, 2016

### suv79

but i dont understand what is wrong with my circuit, why is it not getting 55 kHz

17. Feb 14, 2016

### suv79

i have chanced the run time to 1m the freq is now 21 kHz

but where can i change time resolution ?

18. Feb 14, 2016

### Staff: Mentor

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. Feb 15, 2016

### suv79

i have no idea how to work out the amplitude of the waveform

20. Feb 15, 2016

### suv79

why when the stop time is change dose it effect the frequency.

21. Feb 15, 2016

### suv79

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

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22. Feb 15, 2016

### Merlin3189

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?

23. Feb 15, 2016

### LvW

In my post#7 I gave you my simulation data (PSpice) with the result of app. 55kHz (clean sinewave).
I cannot understand why you didnt 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. Feb 15, 2016

### suv79

resolutiuon= .PRINT step ???

25. Feb 15, 2016

### suv79

i used, real time noise, which should help start the oscillation