Tuning varactor for VCO and PLL loop filters
The KVX3901A-23-4 is a Microchip Technology silicon tuning varactor diode, configured as a 1 pair common-cathode in a single SOT-23-3 package. It is designed for voltage-controlled oscillator (VCO) tuning, phase-locked loop (PLL) loop filters, and RF filter tuning applications where a high Q and stable capacitance over temperature are required. With a peak reverse voltage of 27 V, the tuning voltage can swing up to that rail before breakdown — useful for wideband VCOs that need a large capacitance ratio. The Q factor of 500 at 4 V and 50 MHz indicates low series resistance, keeping phase noise low in the oscillator tank.
Capacitance and tuning range at working bias
At 3 V reverse bias and 1 MHz, the capacitance measures 32 pF. This value, combined with the Q and the capacitance ratio from zero bias to 27 V, defines the tuning bandwidth of the LC tank. For a typical 2.4 GHz VCO, this varactor pair can cover a several-hundred-MHz tuning range with a reasonable inductance. The common-cathode configuration allows two varactors to share a single cathode connection, simplifying the tank layout when using a differential oscillator topology. Both diodes track together, reducing imbalance in the tuned circuit.
Package, temperature grade, and compliance
Housed in a surface-mount SOT-23-3 (TO-236-3, SC-59 equivalent), the package footprint is compact — suitable for handheld RF modules where board space is tight.