[SOLVED] Help in Understanding Frequency of Analog PWM Generator

[hshah8970]

Hello everybody.

I've been using the analog PWM generator of which the schematic you can find here: **broken link removed**

While I understand the functioning of the circuit, I'm unclear as to how the frequency of the PWM waveform reduces to that mentioned on the page.

For people who'd prefer not to click on the link above, I'm attaching an image with the circuit schematic and the frequency equation.

Any help would be much appreciated.

Thanks.

Humza.

 

[crutschow]

U1A is an integrator with a time constant of RC (Vo = -Vin x t/RC).

U1B is configured as a comparator with hysteresis (difference between the high and low trip points) as determined by R1 and and the positive feedback from R2.

When the triangular wave from U1A reaches one of the trip points of U1B, U1B changes state and the integrator starts charging in the opposite direction until it reaches the other trip point. Since the Op Amp is a rail-rail type, the output voltage of the comparator will be a square-wave, oscillating between ground and VCC. These are the two input voltages to the integrator.

Using the above info you can calculate the frequency of the triangular wave oscillator consisting of U1A and U1B.

 

[KonstanU]

How to get this formula:
1. triangular-wave swing (difference between max and min. It is also comparator hysteresis) Vs= Vcc * R1/R2
2. integrated voltage Vint(t) = 1/RC*Vcc/2*t
3. If calculate time, that is necessary for Vint to reach Vs, it will be t= 2RC*R1/R2
4. FREQUENCY PERIOD T = 2*t = 4RC*R1/R2. Hence frequency F=1/T=R2/4RCR1