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This is a 50% Square Wave VCO that can produce frequencies from as low as sub-Htz up to as high as upper VHF, (according to EveryCircuit)… And it achieves this using only 1 capacitor! This because this capacitor is being utilized as a coupling capacitor, rather than an RC delay. So basically you could say its acting as a High Pass filter with a variable delay; and the delay is controlled by a VCCS (Voltage Controlled Current Source).
This Square wave Voltage-Controlled-Oscillator is always a 50% duty cycle, and has a frequency range that can be altered simply by either using different capacitor values, or changing the resistance between the control voltage.
The Control voltage source must be able to sink current (Doesn’t need to be able to source anything though), and it should always have a resistor there… One super easy way to do this is by using a PNP follower, which as a huge benefit would also greatly increase the input impedance. The output of the the PNP follower, (if you decide to use; highly recommended) still needs the resistor, as it determines the frequency range and limits the current of the current mirrors, while the actual voltage sinking the resistor controls the oscillating frequency.
***Please Note***
-This design can perform really well, but it’s very sensitive to high base currents split across the current mirrors. If too high, it can be very easily cause an unequal distribution of current across the current mirror circuit and is extremely sensitive to temperature in this condition.
***This can easily be prevented by using a high enough base resistor between the control voltage. This will also affect the frequency range a bit, but can be compensated by simultaneously reducing the size of the capacitor. Although this increase in resistance ensures reliability, greater care must be taken not to use too high of a voltage source to power the oscillator, because the power dissipation will reach a critical point where it becomes too much for the current mirror to function properly. If your not careful to stay within these parameters, it can cause literally every single transistor to burn out and need complete replacement to restore functionality***
In my tests, too high of current causes even worse differences that completely spiral out of control till a component completely burns out… so be very mindful of this, the first sign will be the pulse width, which slowly becomes more and more unequal, and will change more rapidly as time goes by quickly affecting frequency just as much before everything stops working. Even just touching one of the transistors, in my tests that is, is enough to cause it to speed up this reaction and can result in completely smoking a transistor or more if the base current is not appropriately limited with a sufficiently high resistance (nothing crazy, just sufficient).
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