A Phase Locked Loop (or PLL as it’s commonly called) is one of the most important electronic circuits ever developed. The PLL functions as a key building block for the many RF systems all around us.
However, despite its popularity, many people don’t know how the loop functions. Join us as we discuss its working in order to establish its importance.
Construction:
In order to understand the importance of phase locked loop, we must take a closer look at its construction and working. Generally speaking, a phase locked loop has three basic building blocks, as seen from the block diagram below
1. Phase Detector:
Phase detector is one of the most important elements of the phase locked loop. The phase detector takes two input signals (1 from the reference signal and 1 from the Voltage Controlled Oscillators) and then generates a voltage directly proportional to the phase difference.
Phase detectors can be of many types, the two most important being the phase only sensitive detector and the phase frequency sensitive detector. For the PLL, we’ll be looking at the phase only sensitive detector.
2. Voltage Controlled Oscillator (VCO):
The second part of the circuit is the Voltage Controlled Oscillator. The VCO is a simple oscillator that has two varactor diodes connected back to back in the electrical circuit. The two diodes are connected via a resistor to the control terminal where voltage is applied.
As the applied control voltage is varied, the capacitance of the varactor diodes changes. This, in turn, changes the frequency of the VCO. The output frequency of the VCO is directly proportional to the control voltage.
It’s important to understand that any varactor diodes can be used for the VCO. The only condition is that they must have a positive Voltage-Frequency curve, as non-monotonic V-F curves can lead to stability. So for the illustration above, the diode with the V-F curve ‘d’ will destabilize the system, while diodes with curves ‘a’, ‘b’, and ‘c’ are perfectly fine.
3. Loop Filter:
Now we turn our attention to another key element of the phase locked loop i.e. the loop filter. This element governs many loop characteristics and also has a major say in the stability of the system, which is interesting, especially considering the fact that the loop filter is nothing but a resistor and a capacitor connected as illustrated below:
The purpose of the phase locked loop is to attenuate the level of the reference frequency appearing across the output terminal and to provide a near DC control voltage to the control input of the VCO.
Working:
Now that we understand the purpose of each individual component, it’s time we learned how the configuration works on a whole. Simply put, the phase locked loop works on the phase difference of the signals that it is fed. To understand the concept of phase, look at the two waveforms below:
It is clear that the two waveforms don’t perfectly coincide with each other (nor will they, given the fact that they represent sine and cosine waves). That being said, it can be seen from the above waveform that the two waves have a constant phase difference (i.e. they are coherent).
Now if two signals have a constant phase difference, it is understood that they must have the same frequency. However, if the phase difference continuously varies, the two operate at varying frequencies. This concept is the basic principle of operation of the phase locked loop.
When we apply a reference voltage and the VCO signal to the phase detector, a voltage proportional to the phase difference of the two signals is generated. This generated voltage passes through the loop filter which removes high frequency components and feeds it back to the VCO to limit its frequency.
This leads to the generation of an error voltage from the phase detector such that it automatically tries to reduce the phase difference between the reference signal and the VCO. The phase detector draws VCO frequency to that of the reference until a steady state phase difference is established between the two.
As the phase difference is now the same, the frequency of the two signals (the VCO and the reference) are the same and the circuit is considered to be locked!
Applications:
Owing to its usefulness, the applications of the Phase Locked Loop are widespread. The PLL is used in tuners, radars, spectrum analyzers, FM demodulators, radios and televisions, etc. But perhaps the most important application is that of the frequency synthesizer (which allows digital lines from a circuit, e.g. a microprocessor, to control the frequency of the system)
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