they are often used as frequency standards, against which other oscillators are cali-brated. The accuracy can be within a few Hertz at working frequencies of several mega-hertz.The voltage-controlled oscillatorThe frequency of a VFO can be adjusted via a varactor diode in the tuned LC circuit. Re-call that a varactor, also called a varicap, is a semiconductor diode that works as a vari-able capacitor when it is reverse-biased. The capacitance depends on the reverse-biasvoltage. The greater this voltage, the lower the value of the capacitance.The Hartley and Clapp oscillator circuits lend themselves well to varactor-diode fre-quency control. The varactor is placed in series or parallel with the tuning capacitor,and is isolated for dc by blocking capacitors. The schematic diagram of Fig. 25-7 showsan example of how a varactor can be connected in a tuned circuit. The resulting oscil-lator is called a voltage-controlled oscillator (VCO).The voltage-controlled oscillator46525-6A JFET Pierce oscillator.25-7Connection of a varactorin a tuned LC circuit.