Author Topic: A School Project: A Function generator problems  (Read 3890 times)

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Offline vaualbus

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A School Project: A Function generator problems
« on: October 01, 2013, 08:27:07 pm »
I'm doing a school about design an analog  function generator but using a encoder and dac for the voltage and offset and amplitude controlling (It is too easy to use a dss and all that components are in smd).
Online I've found a lot of vco (voltage controlled oscillator) that given a square wave as output frequency and this is ok for me.
My function generator have also a sinusodial output, how I can generate a voltage controlled oscillator that give out a sine wave?
I've found nothigh about sine wave vco online but only some circuit but nothigh that work as I want, there were complex circuitS but there is no equation that describe the output frequency in term of the input voltage.
Anybody have some idea how to design  sine wave VCO?
For my project I have decide to have a frequency range between 0Hz - to 1MHz.
Best regards, Alberto Vaudagna
 

Offline c4757p

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Re: A School Project: A Function generator problems
« Reply #1 on: October 01, 2013, 08:44:49 pm »
Typically, analog function generators use a triangle VCO. This is easy: use two voltage-controlled current sources, one to charge, and the other to discharge, a capacitor. A comparator with hysteresis steers the current, with the help of a diode bridge, to select between charge and discharge, and the capacitor will then give a triangle waveform with a frequency that is a function of the capacitance and the current source control voltage.

This video by Alan Wolke is a good tutorial on this method:


Once you have a triangle wave, National Semiconductor AN-263 has two circuits to convert that to sine. Just Ctrl-F and search for "breakpoint shaper" and "logarithmic shaping".

The square wave can obviously be derived directly from the comparator output.

If you don't need a triangle, you can of course use a true sine VCO and use a comparator to get a square from that. I think that app note should have some things of interest to you if you want to follow that route as well.

Up near 1 MHz, the distortion on the triangle wave caused by the capacitance of the diode bridge does become a bit tricky to manage, so if you don't need a triangle output, you might consider the pure sine route. That might get you a lower-distortion sine wave, as well.
« Last Edit: October 01, 2013, 08:49:24 pm by c4757p »
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Offline vaualbus

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Re: A School Project: A Function generator problems
« Reply #2 on: October 01, 2013, 09:05:09 pm »
Thank you.
 

Offline c4757p

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Re: A School Project: A Function generator problems
« Reply #3 on: October 01, 2013, 09:06:23 pm »
My most important suggestion to you is to spend a lot of time with a breadboard. There are a lot of "traps for young players" building a wide-range VCO, even if it is simple in theory. Don't just bang on a SPICE simulator.

For my project I have decide to have a frequency range between 0Hz - to 1MHz.

Just caught this..... 0 Hz?? There's no such thing as 0 Hz. How low do you really want to go? 1 Hz? 0.0001 Hz? 1 cycle per week? It really does affect the types of oscillator you can use.
« Last Edit: October 01, 2013, 09:17:29 pm by c4757p »
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Offline vaualbus

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Re: A School Project: A Function generator problems
« Reply #4 on: October 01, 2013, 09:21:07 pm »
ya I suppose to say 1Hz.
 

Offline c4757p

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Re: A School Project: A Function generator problems
« Reply #5 on: October 01, 2013, 10:16:57 pm »
It would help if you could say how far along you are in school - what have you learned, and what are you expected to be able to do?
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Offline vaualbus

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Re: A School Project: A Function generator problems
« Reply #6 on: October 01, 2013, 10:41:50 pm »
I'm attending the last here of school here in italy. For now we have done transistors, amplifier, active and passive filter, integrator and derivator and something about the adc. We've also introduced the schmit trigger.
 

Offline vaualbus

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Re: A School Project: A Function generator problems
« Reply #7 on: October 01, 2013, 10:45:22 pm »
For the project as i said i want to be able to modify frequency and the amplitude of the waveform and adding an offset like in classical function gen. For input the data i'll have a keypad and and encoder. I'm not exceot to add modulation as i don't need it. The generator output triang, sine and square waves.
 

Offline c4757p

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Re: A School Project: A Function generator problems
« Reply #8 on: October 01, 2013, 10:46:46 pm »
OK, good, then I don't think you're out of your league here.
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