The phase locked loop (PLL) is one of the interesting applications of the lock-in amplifier. Apart from FM stereo decoders, tracking filters, motor speed control, FM demodulators, etc. it has found wide applications in generation of local oscillator frequencies in house-hold TV and FM tuners as automatic frequency control (AFC). Indeed, PLL has emerged as one of the fundamental building blocks in electronics and it is commercially available as a single package. Basically, a PLL is a lock-in amplifier in which the reference signal is provided by its own output, converted to frequency by a voltage controlled oscillator (VCO). When locked to the input frequency the dc output is small but sufficient to drive the VCO to produce a frequency which is equal to that of the signal. In this tracking situation, the input signal and the VCO output are almost in phase quadrature and the lock-in amplifier produces a small dc voltage which is often referred to as error voltage. The moment input signal is fed, the VCO frequency starts changing and the PLL is said to be in the capture mode. The VCO continues to change its frequency until it equals that of the input and stays there ; the PLL is then in the phase-locked state. In this state, if there is any change in the input frequency, the loop automatically tracks it through its repetitive action.
A PLL is used for demodulation of which of the following signals ?
Frequency modulated signals
The PLL is the natural FM demodulator, and the reason is almost a restatement of what the loop does.
The argument in one step. When the loop is locked, the VCO frequency must follow the input frequency:
\(f_{VCO}=f_{in}\)
But the VCO's frequency is set by its control voltage,
\(f_{VCO}=f_{0}+K_{v}V_{c}\qquad\Rightarrow\qquad V_{c}=\dfrac{f_{in}-f_{0}}{K_{v}}\)
In an FM signal the instantaneous frequency is the message, so the control voltage is proportional to the message. The demodulated output is simply the voltage at the VCO input — no discriminator, no tuned circuits, no alignment. That is option 2.
Why the PLL displaced the older FM detectors. The Foster-Seeley discriminator and the ratio detector both rely on tuned transformers that must be aligned and that drift with temperature and age. A PLL demodulator has no tuned circuit at all, is available as a single IC, and gives markedly better linearity — hence lower distortion — because the loop forces the VCO to track rather than relying on the shape of a resonance curve.
| Modulation | Information in | PLL suitable? |
|---|---|---|
| AM | Amplitude | Not directly — the loop ignores amplitude (it can, however, regenerate a coherent carrier for a synchronous AM detector) |
| FM | Frequency | Yes — the control voltage is the output |
| PCM | Digital codes | No — needs a decoder, though a PLL recovers the clock |
| FSK | Two frequencies | Yes — the same principle, giving two output levels |
Option 4 deserves comment, because a PLL genuinely does demodulate FSK — the 565 PLL's classic application is exactly that. But FSK is the digital special case of frequency modulation, so FM is the general and intended answer; a key expecting one answer must take the broader category.
The passage lists the same family of uses — FM stereo decoders, FM demodulators, AFC in tuners — all of which rest on this one property: a locked loop's control voltage is a faithful copy of the input's frequency variation.
Hence, a PLL is used to demodulate frequency modulated signals.
Similar Questions
Mention the order of increasing circuit complexity of the below mentioned FM demodulators for the detection of modulating signal from the FM wave :
(a) Balanced Slope detector
(b) Slope detector
(c) Ratio detector
(d) Phase discriminator
Important Questions from FM Receivers
The function of de-emphasis circuit is -
The use of TRF receivers is limited because of their
In commercial FM broadcasting the maximum frequency deviation is normally:
Mention the order of increasing circuit complexity of the below mentioned FM demodulators for the detection of modulating signal from the FM wave :
(a) Balanced Slope detector
(b) Slope detector
(c) Ratio detector
(d) Phase discriminator
