Variable Frequency function generator ( HELP )

Thread Starter

ruwad

Joined May 11, 2012
8
In my college we were asked to make a variable frequency function generator that make ( sine , square , sawtooth ) waves ,
i know that if i had a sine wave i can convert it using a zero detecting circuit with an op-amp to a square wave then using an op-amp as an integrator i can change it into a sawtooth wave BUT my main problem is to build a simple sine wave oscillator with variable frequency ,
i tried many oscillators ; wein bridge , colpitt , crystal and yet i cant find anything that works , if anyone can help please ??
 

Audioguru

Joined Dec 20, 2007
11,248
A Wien bridge oscillator works perfectly and is very simple. Post your schematic so we can show you what was wrong.
Colpitts or crystal oscillators are also simple but are used for radio frequencies. Opamps do not work at radio frequencies.
 

Thread Starter

ruwad

Joined May 11, 2012
8
i have deleted most of my schematics as they arent working :(
and about Wien bridge oscillator , i have read that its hard to make a variable frequency with it because every component should be calibrated separetly in order to work properly , and personally i would prefer using a crystal oscillator and varying the frequency of the output using a varactor or something , i have read about it but i cant get it to work on my simulator , tried Multisim and Proteus and yet , not working ...
 

Thread Starter

ruwad

Joined May 11, 2012
8
here is a picture of what i m working on now , a colpitt based oscillator , but its't working , as u can see from the output its changing very fast and all in pico volts ...

 

Audioguru

Joined Dec 20, 2007
11,248
I have a Wien bridge oscillator made with an audio opamp and its frequency is tuned with a dual-gang variable capacitor from an old AM radio. It uses a Jfet for amplitude stabilization.

Why are you making a high frequency crystal oscillator that does not have a variable frequency? I think the value of its 1nF capacitors are too high and are shorting the crystal.
 

SgtWookie

Joined Jul 17, 2007
22,230
If you're wanting a variable frequency generator, you certainly don't want to use (a) crystal(s); as you'd only be able to adjust the frequency a few parts per million. Crystals are used when you want the frequency range to be very narrow, as in a system clock.
 

Thread Starter

ruwad

Joined May 11, 2012
8
after a little digging , i decided to make it with Wien bridge oscillator but yet again non of the simulations bring a result ....
 

SgtWookie

Joined Jul 17, 2007
22,230
Of course it's not working, as you didn't use any parts.
Oh, wait - you forgot to post your schematic and your parameters used in the simulation.
 

t06afre

Joined May 11, 2009
5,934
Simulating an analog oscillator may often be a problem. They have problems starting up then simulating. The simulator way to ideal in its approach
 
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#12

Joined Nov 30, 2010
18,224
If I may correct the spelling, "The simulator is way too ideal in its approach".

We need you to tell us what frequencies you want.
 

Thread Starter

ruwad

Joined May 11, 2012
8
well , i m just about to give up ......,i was working in the Lab and trying to get it to work and yet cant get to it , can anyone please post a working wien bridge oscillator circuit here ?
 

DickCappels

Joined Aug 21, 2008
10,152
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