The BB173X is a single VHF varactor diode from NXP, designed for voltage-controlled tuning in RF front-ends. Its headline spec is a capacitance ratio of 15 measured between 1 V and 28 V reverse bias — that 15:1 swing gives an LC tank a wide frequency range without switching in extra capacitors. The junction capacitance sits at 2.754 pF when biased at 28 V and tested at 1 MHz. Peak reverse voltage is rated at 32 V, so the tuning supply rail should stay below that to avoid breakdown.
The wide range also gives headroom in hot RF power-amplifier stages or outdoor telecom enclosures where ambient temperatures push high.