The oscillator generates a periodic signal, and the phase detector compares the phase of that signal with the phase of the input periodic signal, adjusting the oscillator to keep the phases matched. Keeping the a block diagram of television systems pdf and output phase in lock step also implies keeping the input and output frequencies the same.
Consequently, in addition to synchronizing signals, a phase-locked loop can track an input frequency, or it can generate a frequency that is a multiple of the input frequency. As an analogy of a PLL, consider an auto race with two cars. One represents the input frequency, the other the PLL’s output VCO frequency. Each lap corresponds to a complete cycle. During most of the race, each car is on its own and free to pass the other and lap the other. This is analogous to the PLL in an unlocked state. This means neither of the race cars is permitted to overtake and pass the other car.
The two race cars represent the input and output frequency of the PLL in a locked state. If the hind driver is too far away, he will increase his speed to close the gap. If he’s too close to the other car he will slow down. Since neither car is allowed to lap the other, the cars make the same number of laps in a given time period. Therefore the frequency of the two signals is the same.
Left on its own, each clock will mark time at slightly different rates. Over time, that time difference would become substantial. Left alone, the wall clock will continue to diverge from the reference clock at the same few seconds per hour rate. When the owner compared his wall clock’s time to the reference time, he noticed that his clock was too fast.
If things work out right, his clock will be more accurate than before. Vincent found that two electronic oscillators that had been tuned to oscillate at slightly different frequencies but that were coupled to a resonant circuit would soon oscillate at the same frequency. The resulting output signal included the original modulation information. The intent was to develop an alternative receiver circuit that required fewer tuned circuits than the superheterodyne receiver. Since the local oscillator would rapidly drift in frequency, an automatic correction signal was applied to the oscillator, maintaining it in the same phase and frequency of the desired signal. In analog television receivers since at least the late 1930s, phase-locked-loop horizontal and vertical sweep circuits are locked to synchronization pulses in the broadcast signal.