programmable logic VCO

Posted on Dec 10, 2012

A VCO (voltage-controlled oscillator) is an analog circuit, so you cannot find it in the libraries for the design of digital programmable chips. When you need such a circuit for synchronization or clock multiplication, you need to find a circuit that works with the standard digital functions, such as AND and NAND. Several possibilities exist for building variable-frequency oscillators. For example, you can change the frequency using a varactor diode. Unfortunately, these diodes have a small change of frequency per volt.

So, the standard Pierce oscillator with one inverter and capacitors is not useful for these applications. Another idea is to use a Schmitt-trigger inverter and to vary the charging resistor. This method can work, but the hysteresis of the IC usually has a

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