Complex sentence: In the 1970s, receivers began to be designed using frequency synthesizer circuits, which synthesized the receiver's input frequency from a crystal oscillator using the vibrations of a precision quartz crystal, which had a stable enough frequency that AFC was not needed.
Split into simpler sentences: In the 1970s, receivers began to be designed using frequency synthesizer circuits, which synthesized the receiver's input frequency from a crystal oscillator using the vibrations of a ultra-stable quartz crystal. These had a stable enough frequency that AFC was not needed.