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First VCO - Help needed
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author First VCO - Help needed
Hi all,

So I'm doing some elementary circuit building and settled on making a very classic triangle-core VCO. I made a Falstad circuit online so you guys can see exactly what I made.

Link to the circuit simulation.

What's not shown in the simulation is that I buffered both the outputs of the two OPAMPS with another set of TL072's used as a buffer. After that I put trimmers to adjust the output voltage and thus the volume (so I could listen to it). All hooked up to two thonkiconn connectors in which I can plug my oscilloscope or headset.

First of all, when plugging in my headphones, no signal at all. I'm thinking the headset is just too low-impedance that it pulls it to 0V right away. Didn't double check that with a multi-meter. That's strange since the signals have been buffered. Got me puzzled.

When checking the signal from the supposed TRIANGLE I come across a strange square-like pattern with an odd duty-cycle. Here's a picture of that.

The SQUARE wave is based off of that and of course shares the same duty-cycle, but inverted. Here's the picture of that shape. I think that fixing the triangle will fix the square wave to have a even duty cycle.

I started from this and made my own adjustments.

I also noticed that changing the input voltage (which would normally come from the 1V/OCT linear/exponential converter) doesn't change the frequency as dramatically as I expected it would at first. I arranged a trimmer potentiometer attached to +12V and GND, and the wiper would then be the input voltage. Trimming this barely changes the frequency. I'm really puzzled as to why this is. I would expect a small change to really zoom in/out the entire wave on the scope, but instead it barely moves and even glitches at some points. I checked the resistance on the trimmer and it effectively goes full range (22k).

I could share you a picture of the breadboard, but I don't think that will help a lot finding the problem. But I can post one if any of you wants to have a look. Basically I quadruple-checked the circuit to resemble the simulation I made.

Right now I'm thinking the capacitor is charging way faster than I thought, and there's a delay out of nowhere before it saturates and the square wave switches the transistor to unload it again, making the odd-duty cycle happen in the form of a square-like pattern where it should be triangular. It's also annoying that I can't test it at its full supposed bandwidth.

I really hope I can figure this one out...
So I just measured the bandwidth and seems like the lowest possible frequency starts at 7 kHz, and goes all the way to above 80 kHz, my previous claim that the frequency barely changes is false. Because this is not an exponential trimmer pot, there's just an area where it doesn't seem to have a lot of effect, and an area in which is starts changing rapidly.

I don't know what would cause this higher-than-expected bandwidth. I selected the resistors and capacitor according to the best fit in the simulation. I'm kind of puzzled...
1) In the original schematic look at what the left end of R13 is connected too.
2) Look at the ratio of your R13/R14 compared to the original.
3) Simulations are useful but unless you really know how to simulate things and how to interpret the results you may not be getting real world results.
the opamps wont be able to drive headphones. if you put 1k resistors at the outputs, you might be able to hear some sounds without pulling the outputs down too much.

both scope shots you posted were of the same thing. is the scale right on that? 0.2V/div? thats a really small signal.

this is a linear VCO, so the sweep will be pretty small, maybe 1:100, and most of that change will happen at the low end.

a picture of the breadboard might help.
also, on that scope shot, the horiontal scale makes it look like your frequency is up at 50kHz, which would be inaudible. maybe put a bigger capacitor in.
Thanks for the replies.

As for the case of R13 in the original schematic. Please take note that the LM358 OPAMP has its rails set to GND & Vcc, not Vcc- & Vcc+. So the Vref in that schematic is set to be half the voltage, and it thus oscillates with a DC-component. This isn't the case in my schematic, and is therefore connected to GND instead. Obviously the simulation seems to reflect this.

As I thought, the headphone was pulling the output down cry. I might want to calculate the charging time of that capacitor/integrator. As for the scope shots and the signal scale, I've scaled them down from 12V. Normally they go 24V pp. So that's really a rail-to-rail signal. The horizontal scale is another problem. I should try using another cap. Will post any other findings if make progress!
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