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 Post subject: Please explain how this works
PostPosted: Aug Sat 29, 2020 1:29 am 
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Here's a VCO circuit that I modeled in LTSpice. Output is the OUT pin of U2.

It works and I'm stumped as to how the input voltage influences the oscillation. As for the oscillation, what exactly is the feedback path necessary for it to oscillate? What does each diode do? Anyone care to explain?


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 Post subject: Re: Please explain how this works
PostPosted: Aug Sat 29, 2020 2:02 pm 
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This is an RC oscillator. The frequency is inversely proportional to the product of R3 and C1. The principle of operation is similar to the 555.

U2 compares the voltage across C1 to a threshold voltage applied to its non-inverting input. The threshold voltage has two values. One value determines the upper voltage across C1 and the other the lower limit. They are alternatively selected by U1 and D1 based on the state of the output of U2. The upper threshold voltage is provided by the input voltage V1. The lower threshold voltage is -V1*(R6/R1). The ratio R6/R1 determines the duty cycle of the square wave. When R6 = R1, the square wave is symmetrical.

When U2 output is positive (close to Vcc), D2 conducts. The non-inverting input of U1 is higher than V1 at the inverting input. This causes U1 output to go positive and reversely bias D1. Since D1 is off, the threshold voltage is V1. C1 is allowed to charge up to V1. The higher V1 is, the longer C1 charges and the lower the resulting frequency. When C1 voltage is slightly above V1, U2 output flips negative.

When U2 output is negative, D2 is off. The non-inverting input of U1 is zero. D1 conducts and the threshold voltage is negative. C1 discharges until the negative threshold is reached at which point U2 output flips positive.

Note that the oscillator may become unstable when V1 is close to positive output voltage. The frequency and duty cycle are not very precise when V1 is close 0. Pratical control range may be in the range from 1V to 8V for 12V Vcc.

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 Post subject: Re: Please explain how this works
PostPosted: Aug Sat 29, 2020 6:00 pm 
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Excellent. Perfectly clear now. THANKS.

In LTSpice I set V1 to be a slow sine wave. As you predict, V1 is less useful above 10.25 volts.

In the original schematic R6 = R1 = 22K. I set R6 to 1K to be able to have a wide range of V1. I didn't notice that R1=R6 results in 50% duty cycle. Thanks for pointing that out. The second graph shows that duty cycle is closer 50%, but V1 is limited to below 4 volts.


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