Hello all,
As part of a first-year electronics project, I decided to do a voltage controlled oscillator. After looking at a few schematics online for ideas, my lab partner and I settled on two designs, the first of which didn't work. After building the second design, it worked perfectly...only one problem - we don't know why!
Here is the circuit as we constructed it:
It was based on a drawing we found on the internet that was similar, but there was a 10nF capacitor in parallel with resistor R1 in that design, also wired up to the ground. In terms of our circuit, it would have looked like this:
Unfortunately, the lowest capacitance capacitor we had access to was a 100nF, which didn't work (it just outputted a constant DC voltage of around 15mV), so we took it out to get the first circuit shown, and lo and behold, we got a lovely oscillating waveform.
The interesting thing about this oscillator is that there's no DC input into the OP amp. The way my lab partner described it was that it must "feedback off of itself", possibly using the power supply for the OP amp.
I did a bit of research to try and figure out how exactly the circuit worked, and came across a type of oscillator called a Wien Bridge oscillator that looked very similar to our circuit, except they all had capacitors going into the OP amp +ve input (like the original schematic, which we ignored because it didn't work). I think I understand the basic concept of how the Wien Bridge oscillator works - the oscillating waveform is formed by the charge and discharge of a capacitor? What I don't understand though, is, in our circuit, the only capacitor (C2) is one that's not part of the feedback loop - it's only part of the circuit that goes to the output. So how does it know when to charge/discharge? It's really weird - the capacitor that's meant to cause the oscillation in theory, is the one that stopped the oscillation occurring in practice!
I tried plotting the output in LTSPICE, which came up with even more weird results:
...and a close up of the signal:
The waveform we got looked like this:
So. in summary, I'm pretty confused. One possible explanation is that the wires themselves caused a capacitance to occur, providing the 10nF from the original schematic. However, I don't really like this explanation, as it's making a pretty large assumption. Another thought that occurred to me was that the comparator that's meant to cause the capacitor to discharge when it reaches the correct voltage isn't able to keep up with the speed of the charge discharge, and so there's periods of time when the voltage is at a fixed level, before the comparator "catches up", and causes the capacitor to discharge. This would explain the waveform on the oscilloscope, and go part way to explain the results in LTSPICE. Is there something I'm missing?
Thanks very much guys.
As part of a first-year electronics project, I decided to do a voltage controlled oscillator. After looking at a few schematics online for ideas, my lab partner and I settled on two designs, the first of which didn't work. After building the second design, it worked perfectly...only one problem - we don't know why!
Here is the circuit as we constructed it:
It was based on a drawing we found on the internet that was similar, but there was a 10nF capacitor in parallel with resistor R1 in that design, also wired up to the ground. In terms of our circuit, it would have looked like this:
Unfortunately, the lowest capacitance capacitor we had access to was a 100nF, which didn't work (it just outputted a constant DC voltage of around 15mV), so we took it out to get the first circuit shown, and lo and behold, we got a lovely oscillating waveform.
The interesting thing about this oscillator is that there's no DC input into the OP amp. The way my lab partner described it was that it must "feedback off of itself", possibly using the power supply for the OP amp.
I did a bit of research to try and figure out how exactly the circuit worked, and came across a type of oscillator called a Wien Bridge oscillator that looked very similar to our circuit, except they all had capacitors going into the OP amp +ve input (like the original schematic, which we ignored because it didn't work). I think I understand the basic concept of how the Wien Bridge oscillator works - the oscillating waveform is formed by the charge and discharge of a capacitor? What I don't understand though, is, in our circuit, the only capacitor (C2) is one that's not part of the feedback loop - it's only part of the circuit that goes to the output. So how does it know when to charge/discharge? It's really weird - the capacitor that's meant to cause the oscillation in theory, is the one that stopped the oscillation occurring in practice!
I tried plotting the output in LTSPICE, which came up with even more weird results:
...and a close up of the signal:
The waveform we got looked like this:
So. in summary, I'm pretty confused. One possible explanation is that the wires themselves caused a capacitance to occur, providing the 10nF from the original schematic. However, I don't really like this explanation, as it's making a pretty large assumption. Another thought that occurred to me was that the comparator that's meant to cause the capacitor to discharge when it reaches the correct voltage isn't able to keep up with the speed of the charge discharge, and so there's periods of time when the voltage is at a fixed level, before the comparator "catches up", and causes the capacitor to discharge. This would explain the waveform on the oscilloscope, and go part way to explain the results in LTSPICE. Is there something I'm missing?
Thanks very much guys.
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