In a frequency multiplier circuit, an inductance (L1) and a variable capacitor (C2) are connected in series between VCC+ and ground. The collector of a transistor is connected to a tap on L1. A fixed capacitor (C3) is connected between the VCC+ side of L1 and ground. C3 is a:

In this circuit, the fixed capacitor (C3) connects the DC power supply line (VCC+) directly to ground. Its primary function is to act as an RF by-pass capacitor, creating a low-impedance path for stray radio frequency energy to safely reach ground before it can enter and destabilize the DC power supply. Because a capacitor passes alternating current (like RF) but blocks direct current (like the VCC+ supply), it successfully prevents RF feedback without shorting out the power source. The other options serve different roles: a coupling or DC blocking capacitor is placed in series between stages to pass the AC signal while isolating DC biases, and a tuning capacitor is adjusted to set the resonant frequency of the tank circuit.