文档介绍：Supplementary Reading for 27 - Oscillators Ron Bertrand VK2DQ
THE BASICS OF PLL FREQUENCY SYNTHESIS
The phase locked loop (PLL) method of frequency synthesis is now the monly
used method of producing high frequency oscillations in munications
equipment. There would not be an amateur mercial transceiver of any worth today
that does not employ at least one if not several, phase locked loop systems, to generate
stable high frequency oscillations.
PLL circuits are now frequently being used to demodulate FM signals, making obsolete the
Foster-Seerly and ratio detectors of early years. Other applications for PLL circuits include
AM demodulators, FSK decoders, two-tone decoders and motor speed controls.
The PLL technique has, surprisingly, been around for a long time. In the 1930s the
superheterodyne receiver was in its heyday (and it's still going strong today), however
attempts were made to simplify the number of tuned stages in the superheterodyne.
THE HOMODYNE RECEIVER
Around 1932, British radio engineers developed a new type of receiver to challenge the
'superhet' - it was called the homodyne or synchrodyne.
The idea was simple: the receiver consisted of a mixer and a local oscillator, followed
immediately by an audio amplifier!
When the input signal and local oscillator are mixed at the same phase and frequency, the
output is an exac