Some problems about the frequency and the amplitude of the triangular wave

[rockman139]

I have tested my triangular wave generator on the bread board according to the schematic below, but I have some problems and I don't know what is happening.


The amplitude of the triangular wave should be calculated like this:
2×(R1/R2)×Vcc=2×50k/100k×12=12V

The frequency of the triangular wave should be calculated as below:
R2/(4×R1×R3×C)=100K/(4×50k×39.2k×10n)=1275Hz

However, I can't get these results.


I switched the observed amplitude to 1 and the time to be 0.5ms as the picture showed.


Vertically, the triangular wave just occupied a little more than one slot, hence the amplitude is 1.4V
horizontally, it occupied 2 slots, hence the time is 2×0.5=1ms, the frequency is 1000Hz.
Is it any problem with the selection of R1, R2 and R3? What values of the resistance should I choose?
And the results are so different to the theoretical calculations. Can anybody help me? Please!

 

[Nick_F]

Did you make sure your oscilloscope probe is not a 10:1 (attenuates signal 10 times)?
For the frequency, what is the tolerance of your resistors and capacitors? Capacitors can have a tolerance of 20%, while for resistors it is commonly 5% or 1%.
If your capacitor has a larger value, you can get lower frequency. Reduce the value of the capacitor or R3 to get frequency closer to what you want. Or use a semiadjustable resistor instead of R3.

Regards,
Nicolae

 

[BradtheRad]

I get 1.27 kHz in a simulation. I started with the triangle wave generator which comes with the program, then inserted your component values.
The result is similar to your theoretical value.

As Nick_F suggests, it is likely that your capacitor value is at the high end of the typical 20% tolerance.

By the way, I don't know how your simulator depicts a negative supply, but your V2 is labelled -12V, yet the icon shows a positive polarity going to the 741's negative supply terminal. This may or may not be a problem.

 

[Nick_F]

Hi Brad,

His circuit is ok, you can notice the value -12V in front of that power supply. If the minus were at the top, then the voltage should have been +12V.

Regards,
Nicolae

 

[rockman139]

Dear Nicolae

Thanks for your reply. I think it should be the problem of the oscilloscope probe because I don't know there is a specification of 10:1 and 1:1. I will go the lab and have a check. And thanks for told me the knowledge about the tolerance of the resistor and capacitor.

Regards

- - - Updated - - -

Hi BradtheRad,

Thanks for your simulation.
Now I confirm that my schematic is correct.
By the way, what is your simulation software? There is only PSpice in my university's lab and I don't think it is convenient to use.

Regards