Optimized Create 1kHz Square Wave Oscillator in Multisim

[luxi2]

Homework Statement

The assignment is to create a square wave oscillator using an op-amp in Multisim. The wave's frequency should be 1 kHz and the duty cycle 60%.

Homework Equations

f = 1 / (R*C)
R1 < R
R1 < Rf

I guess.

The Attempt at a Solution

Here's what I've done:

http://img156.imageshack.us/img156/8494/sqwavegenaj6.png

(It's such a basic textbook example.) I don't know why the wave form looks like that. The problem is those unnecessary "bumps" that should be flat:

http://img403.imageshack.us/img403/4951/osczh4.png

Am I doing something wrong (very probable) or what?

 

[trevie1970]

just had a look, you may want to think about where you are placing the Cap in your circuit, what purpose does in play on the output from the signal Gen. Also, Placing a 10 Volt signal injection into a 5v Op amp will drive the output to Saturation? think about what is the appropiate signal input.

 

[luxi2]

So the input signal should be 5V? And I even tried removing the capacitor, but it didn't seem to have any effect in the output signal.

I would really appreciate it if someone could even show me the right direction to go from here.

 

[trevie1970]

Well at the moment, The cap where it is now, looks like its acting as a DC filter, Redesign the circuit so the cap works as a reservoir so it store an amount of charge to effect the output, this hopefully will give you your 40:60 Duty cycle. It does work, although its very limited on how far you can push the duty cycle limits. I used a version of crocclips to simulate the circuit.

 

[luxi2]

I just don't get it. Every website that tells about square wave generators have these kind of pictures:

Where's the input signal? I even tried this but:
http://img117.imageshack.us/img117/4063/oscqp1.png

 

[trevie1970]

well its almost there, Have a look at the Op Amp inputs, the polarity is different on your design, and the input is in the wrong place. Try in parallel with the cap.

 

[luxi2]

I finally got it! I removed the AC power source since this is an oscillator, not a square wave inverter. I changed the DC voltage to 15 V and added a -15 V source to the pin 4. Works perfectly. Thanks.

 

[trevie1970]

cool. good to hear.