Synchronization EE442 Lecture 17 All digital receivers must be synchronized to the incoming signal s(t). This means we must have a way to perform (1) Bit or symbol synchronization (2) Frame synchronization (3) Carrier and/ or clock synchronization http://mwrf.com/markets/regenerate-coherent-carriers-psk-signals 1
Pulse Restoration or Regeneration Goal Function of Amplification Initial Pulse into Channel Pulse as received Function of Regeneration After Amplification Initial Pulse into Channel Pulse as received As Regenerated http://www.rfwireless-world.com/terminology/optical-repeater-vs-optical-amplifier.html 2
Synchronization Overview Synchronization in a binary receiver: s(t) LPF Regenerator Output Message Incoming Signal Bit Sync Clock Frame Sync Frame Indicator We not only need to recover the data but also the clock (both phase & frequency). After Carlson & Crilly, Communication Systems, 5 th edition, 2010. 3
Polar NRZ to Unipolar Open-Loop Bit Synchronization cos 2Rt b s(t) () 2 s 2 (t) BPF Phase Adjust Feeds Clock BPF strips out the Harmonics. f o = R b Contains fundamental f b =1/T b s(t) s 2 (t) cos 2Rt b After Carlson & Crilly, Communication Systems, 5 th edition, 2010. 4
Another Open-Loop Bit Synchronization Technique s(t) LPF d/dt () 2 Feeds BPF Clock f o = R b ds/dt produces both + and - pulses; squaring the ds/dt output results in all pulses being +. Next we filter with a high-q BPF centered at the f clock. s(t) ds() t dt ds() t dt 2 After Carlson & Crilly, Communication Systems, 5 th edition, 2010. 5
Closed-Loop Bit Synchronization Early/Late Gate Sync (1) Late Gate s(t) T d dt VCO s 1 (t) Absolute Value Loop Filter e s 1 (t) + - + e = s 2 - s 1 T 0 d dt s 2 (t) Absolute Value s 2 (t) Early Gate Bit data synchronization by comparative measurements between the incoming sign s(t) and a locally generated clock signal. This can also be used for carrier tracking. 6
Closed-Loop Bit Synchronization Early/Late Gate Sync (2) +1 Data Signal +1 Data Signal -1 s 1 (t) 0 T T-d d Early Gate Integration -1 0 T s 1 (t) Requires a data state change before and after the channel bit. s 2 (t) d T-d Late Gate Integration s 2 (t) Error e = 0 No change! Error e = -2 This reduces the frequency of the VCO clock signal. 7
What is a Phase lock Loop? 1. A negative feedback control system whose operation is closely linked to frequency modulation (FM is our next topic) 2. Automatically adjusts the frequency and phase of a controlled oscillator to match a reference (or input) frequency 3. Commonly used for carrier synchronization and indirect frequency demodulation 4. A change in the input signal shows up as a change in phase of the input signal and the VCO frequency
Components in a Phase lock Loop 1. Phase detector (phase difference to voltage output) 2. Voltage-controlled oscillator (VCO) 3. Low-pass filter (filters out HF content of phase detector) A cos t ( t) C i Hs () 2B cos t ( t) C o e () o t Lathi & Ding Pages 212-213 9
Some Common Applications of a Phase lock Loop 1. Frequency multiplier which multiplies frequency of a reference oscillator 2. Modulator by adding modulating signal to phase error 3. Demodulator which tracks changes in modulation 4. Coherent receiver operating as a narrow band tunable filter to track the carrier frequency 5. Data synchronizer operating as narrow band tunable filter to recover the clock frequency (digital communication systems) 10
Phase Detector From a XOR Gate X 1 X 2 X out 0 0 0 0 1 1 1 0 1 1 1 0 The area under the pulses is proportional to the phase difference. 11
Low-Pass Filtering PD Output V pd 12
Low-Pass Filtering PD Output V pd cos( ) 1 2 1 2 13
Example: Costas Receiver Uses a Phase Lock Loop I-channel Input Output Phase Detector Costas carrier recovery PLL for BPSK (Binary Phase Shift Keyed) Q-channel Voltage-controlled Oscillator In the case of carrier-recovery in which coherent demodulation is to be performed on QAM-type signals, the Costas loop has found wide-spread use as an unbiased low-variance practical solution. 14
Costas Receiver Uses a Phase Lock Loop Coherent Demodulator Product Modulator cos 2 ft C Low-Pass Filter Local oscillator output Demodulated signal output v 0 (t) signal input Voltage-controlled oscillator Phase Discriminator A m( t) cos 2 f t C C -90 deg Phase Shifter Product Modulator sin 2 ft C Circuit for phase locking ( = 0) Low-Pass Filter 15
Questions? 16