Counting and Correcting 213 For example, if the frequency of 10.480468 MHz is divided by 5366, the result is 1953.125 Hz, which is identical to the frequency from the crystal-controlled oscillator. If the VCO frequency changes slightly, the frequencies into the PLL will no longer be equal, and a voltage will be generated from the PLL and used to correct the VCO. If you want to change channels, you press a switch that alters the divider ratio, causing a change in the PLL output that will then alter the frequency of the VCO until the new channel is perfectly tuned. This is all a long way from the variable-capacitor tuning of the early radios. Summary Frequency synthesizers are another application of counter circuits, and this type of action is now extensively used in radio and television receivers to produce the correct oscillator frequency for a superhet receiver circuit. This type of tuning is better able to use remote control and push- button selection actions, and has completely replaced the old-style rotating-knob tuning for television receivers, and FM or digital radio receivers. Registers