Difference between Clock recovery vs carrier recovery
This page on Clock recovery vs carrier recovery covers difference between clock recovery and carrier recovery.
Both of clock and carrier recovery circuits help derive synchronized clock and carrier's phase/amplitude respectively. These modules help recover transmitted information at the receiver from received impaired modulated signal.
The difference between these techniques is that clock recovery tries to synchronize receiver clock with the transmitter clock (at baseband symbol rate) whereas carrier recovery helps align receiver LO (Local Oscillator) signal with the transmitted carrier signal.
Clock recovery
It is essential to have both transmitter and receiver synchronized. This provides receiver information about when to sample the incoming waveform. It provides reference which is used to compare any change in the phase of incoming waveform.
This module help recover synchronized clock which is needed as sampling clock for ADC and also used as DEMUX clock pulses.
Following different types of clock recovery systems are used. All the techniques make use of
properties of received waveform.
• Times-two clock recovery
• Early late clock recovery
• Zero crossing clock recovery
• Synchronizer clock recovery
Figure-1 depicts PAM clock recovery circuits for closed loop and open loop. As shown clock for demux is derived from the received PAM signal. Here PAM signal is first applied to amplifier-limiter circuit. This block amplifies input pulses to high level and later clips them at fixed constant level. This results into constant amplitude rectangular waveform. This is the frequency at which PAM pulses will occur. The rectangular pulses are applied to BPF which removes upper harmonics resulting into sinewave at transmitting clock frequency. This sinewave is applied as one input of Phase Detector. The other input is provided from VCO. The VCO frequency is set equal to frequency of PAM pulses. Refer rf synthesizer design for more information.
Carrier recovery
This module help extract or derive the carrier signal's frequency/phase. This carrier is used in demodulator to retrieve the transmitted baseband information. The carrier recovery is essential in coherent demodulation based systems.
Following different types of carrier recovery systems are used.
• Times-n carrier recovery
• Decision directed carrier recovery
Figure-2 depicts carrier recovery concept used in BPSK demodulator. As shown carrier is derived using BPSK input signal itself. As depicted BPSK signal is band pass filtered first. The signal is then squared or multiplied by itself using analog multiplier. Squaring operation removes all 180 degree phase shifts. The result is an output which is twice of input frequency. One more BPF is used to pass this frequency of 2f value.
This resulting frequency signal is applied to Phase Detector. The X2 frequency multiplier is used between VCO and Phase Detector. This ensures that VCO frequency is same as the carrier frequency.
The common problem of carrier recovery circuit is its inability to resolve phase ambiguities. This is overcome by differential coding concept. One example of such method is Differential PSK or DPSK. Refer DPSK modulator and demodulator for more information.
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