A Phase-Locked Loop (PLL) demodulator is a circuit used to extract information from a modulated carrier signal, commonly used in communication systems and various other applications. It operates based on the principle of comparing the phase difference between the carrier signal and a reference signal and then adjusting the local oscillator to synchronize with the carrier signal. Here's how a PLL demodulator works:
Reference Signal Generation: The PLL starts by generating a stable and precise reference signal (often called the reference oscillator or reference clock). This signal serves as a reference against which the incoming carrier signal will be compared.
Phase Detector (PD): The phase detector compares the phase of the incoming modulated carrier signal with the phase of the reference signal. It generates an output voltage that is proportional to the phase difference between the two signals. The phase detector's goal is to keep this phase difference as close to zero as possible, indicating that the two signals are in phase.
Loop Filter: The output of the phase detector is a varying voltage, which contains information about the phase difference. However, this voltage usually contains high-frequency components that need to be filtered out. The loop filter's job is to smooth the output of the phase detector and eliminate unwanted high-frequency noise, ensuring that only the low-frequency variations remain.
Voltage-Controlled Oscillator (VCO): The filtered output from the loop filter is fed to the voltage-controlled oscillator (VCO). The VCO generates a periodic signal (usually a sinusoidal waveform) whose frequency is directly proportional to the input voltage. In other words, the VCO's frequency changes based on the output of the loop filter.
Local Oscillator (LO) and Mixer (Optional): In some cases, the output of the VCO may be directly used as the demodulated output. However, in many applications, the VCO output is mixed with the incoming modulated carrier signal to extract the baseband signal. This involves mixing the VCO signal and the carrier signal, which results in the sum and difference of their frequencies. The low-pass filtered difference frequency, which contains the baseband signal, is the demodulated output.
Feedback Loop: The demodulated output (baseband signal) is often fed back into the phase detector as a feedback signal. This allows the phase detector to continually compare the phase of the baseband signal with the reference signal and make necessary adjustments to the VCO's frequency, ensuring the PLL remains locked to the carrier signal's phase.
Locking Process: During the locking process, the PLL adjusts the VCO frequency to match the incoming carrier signal's frequency and phase. As the VCO frequency approaches the correct value, the phase difference between the reference signal and the carrier signal reduces. The feedback loop keeps refining the VCO frequency until the phase difference becomes very close to zero, indicating that the PLL has locked onto the carrier signal.
Demodulated Output: Once the PLL is locked, the VCO frequency accurately tracks the carrier signal's frequency and phase. The demodulated output is extracted from the low-pass filtered output of the mixer (if used), providing the original baseband signal with the modulation information.
Overall, the PLL demodulator provides an efficient and reliable way to recover the baseband signal from a modulated carrier signal, and it is widely used in various communication and signal processing applications.