Author Topic: Making huge change in frequency of oscillator (schmitt trigger, op-amp)  (Read 1896 times)

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Offline ThijssjihT

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I am designing an oscillator based on the op-amp relaxation oscillator:

It's not exactly my circuit: I use varicaps with powerrail to change capicitance. Also, I use some potentiometers to tweak. But it comes down to the above image.

I would like to make it voltage controllable with a varicap, so the capacitor in the circuit above will have a typical value of around 60pF. I don't have much bigger voltage controlled capacitors available.
Also, I would like to have the frequency of the oscillator output somewhere between 100-5000Hz.

For this to work with these specs and values, you would need very, very high or low resistors (high as in 100Mohms to 1Gohms, and low as in 1ohm or .1 ohms). It works perfectly when I simulate my design, but of course, using such extreme resistances won't work in real life.

I know you should keep your resistors in this circuit between 1k and 100k. Bigger resistors don't work because you will pick up signals from powerlines, radio or other sources. Too small resistors don't work, but I don't know why. Does it have to do with too high current flow? What if I have a 1 ohm resistor from my output to my non-inverting input, and have a 100k resistor + 100k potentiometer to ground? The big resistors aren't too big, and limit the current, so the 1 ohm resistor is no problem. Or am I missing something here? Should I continue from this design, or take another design to start with, that better suits my needs?
 

Offline seriouscoinage

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A varactor is not really practical at these frequencies, because it has too small a value of capacitance. You could still make a voltage-controlled relaxation oscillator with a circuit like shown here :http://sound.westhost.com/project162.htm (see figure 2.) The 74HC4046 also includes a voltage-controlled oscillator for the frequency range you're looking at. Might I ask what the oscillator is for? Is there a particular reason it needs to be an analog voltage-controlled oscillator, or could a digitally/numerically controlled oscillator work too?
« Last Edit: June 06, 2016, 02:02:06 pm by seriouscoinage »
 

Offline ThijssjihT

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My main goal is learning analog electronics. To reach this goal I would like to make an analog synthesizer, because I think it is fun, it will cost me a lot of time, and I learn a lot from it. The idea is in my head for about 3 years now, with a lot of easy, small, sometimes digital projects in between. I've done a lot of research, and I got a lot of designs for filters and effects. Problem is the vco. I could use a NE565, but it isn't much fun, and I won't learn very much from it, so I'm trying different designs.
 

Offline seriouscoinage

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That's a good idea. In this case, the varactor isn't a good choice, though - for the reasons you pointed out in your opening post. An RC relaxation oscillator is still appropriate, but we need to find a different way to change the frequency. Obviously varying the capacitance in the RC timer will change the frequency, but that's not practical (as you found out yourself.) Instead, we should try and change the rate at which the capacitor is charged. The circuit I linked to in my last post achieves this - a capacitor is charged with a current that varies depending on the input voltage. A more thorough explanation is given in the article.
 

Offline ThijssjihT

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Ok, I will not look further into the varactor design then.

Changing the frequency in a relaxation oscillator is easy. Just change the resistance or the capacitance. I couldn't easily find voltage dependable resistors, so I chose the varactor. Got the idea from this:

I will try to find a way to emulate a voltage dependable resistance then. I'll first try it myself, that's the best way of learning. If I can't, I hope you could help me again :)

Thank you very much for your help. I'll try and study some of the designs in the article you've send. I think it'll be very helpfull.
 

Offline danadak

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« Last Edit: June 06, 2016, 09:58:24 pm by danadak »
Love Cypress PSOC, ATTiny, Bit Slice, OpAmps, Oscilloscopes, and Analog Gurus like Pease, Miller, Widlar, Dobkin, obsessed with being an engineer
 


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