One popular PLL unit is the 565, shown in Fig. 13.26 a. The 565 contains a phase detector,
an amplifier, and a voltage-controlled oscillator, which are only partially connected
internally. An external resistor and capacitor R 1 and C 1 , respectively, are used to set the
free-running or center frequency of the VCO. Another external capacitor, C 2 , is used to set
the low-pass filter passband, and the VCO output must be connected back as input to the
phase detector to close the PLL loop. The 565 typically uses two power supplies, V and V .