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Varactor Capacitance
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Tuning Frequency
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Capacitance Ratio
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Tuning Sensitivity
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A varactor diode is a variable capacitor that changes its capacitance based on the applied reverse bias voltage. It's used in voltage-controlled oscillators (VCOs) and tuning circuits for frequency control applications.
The M factor (grading coefficient) determines how capacitance varies with voltage. A value of 0.5 represents an abrupt junction diode, while 0.33 represents a linearly graded junction. Lower M factors provide better tuning linearity.
Varactor diodes are used in VCOs, phase-locked loops (PLLs), frequency synthesizers, TV tuners, FM radio receivers, and automatic frequency control (AFC) circuits where voltage-controlled tuning is required.
Varactors have different characteristics including temperature coefficients, junction conductance, and non-linearities. They also conduct when RF voltage exceeds the diode's forward voltage, requiring careful circuit design considerations.
The tuning range depends on the capacitance ratio (Cmax/Cmin) of the varactor. Higher voltage swings and appropriate M factors provide wider tuning ranges, but practical limits are set by breakdown voltage and minimum capacitance.
Varactor Q is affected by series resistance, junction conductance, and package parasitics. Higher Q values result in better frequency stability and lower phase noise in oscillator applications.