DSB-SC AM Tx signal  AM Tx signal spectrum Also called DSB-LC  Double Side Band - Large Carrier AM Tx signal spectrum Discrete delta functions represent sinusoidal carrier Delta functions provide DC term when s(t) is shifted to baseband DSB-SC is also an amplitude modulated signal but the carrier term present in AM is suppressed Double Side Band – Suppressed Carrier ECE 4710: Lecture #20

DSB-SC DSB-SC Tx signal  m(t) must have zero DC component for SC DSB-SC signal spectrum is identical to AM except the delta functions (LC) are removed Modulation Efficiency = 100% since no power used on carrier Best case AM efficiency was 50% Sideband power is four times that of AM signal for same peak level for Tx output signal  10 log (4) = 6 dB difference ECE 4710: Lecture #20

DSB-SC Power ECE 4710: Lecture #20 Let peak Tx output signal power be the same for AM and DSB-SC Same peak Tx output so For demonstration let MAM = 1 then MDSB-SC = 2 Sideband power   M 2 and ECE 4710: Lecture #20

Encoding, Pulse Shaping, DSB-SC Tx Antenna Mixer Gain = Ac Analog Input Baseband LPF Baseband Amplifier Power Amplifier cos(2pfct) DSP Digital Input ˜ fc Encoding, Pulse Shaping, Error Coding, DAC, etc. Carrier Oscillator ECE 4710: Lecture #20

Mixer = Product Detector DSB-SC Rx Antenna Mixer = Product Detector ~ Low Noise RF Amp LPF Baseband Amplifier DSP ˜ fc Local Oscillator Digital or Analog Output NOTE: SuperHeterodyne Rx often used but Zero-IF is shown for simplicity ECE 4710: Lecture #20

DSB-SC Spectrums f ECE 4710: Lecture #20 fc LPF 2fc BandpassSignal Desired Baseband Signal BandpassSignal Frequency Doubled Signal LPF NOTE: No LC present!! ECE 4710: Lecture #20

DSB-SC Product Detection Product Detector or Mixer: converts bandpass signal s(t) back into a baseband signal via another frequency translation Product Detector  multiplication of r (t) with cos(2p fc t) Product = multiplication & Detector = result is  original m(t) Envelope Detector  normally not possible Only possible if m(t) is always > 0  e.g. unipolar line code If m(t) is polar or bipolar then it will have + and - values 180° phase change occurs in carrier when m(t) transitions from + to - r (t) = A m(t) cos(2p fc t) so for m(t) > 0  cos(2p fc t) and for m(t) < 0  -cos(2p fc t) = cos(2p fc t + 180°) ECE 4710: Lecture #20

DSB-SC Product Detection +cos -cos +cos 1. Envelope of s(t) = | s(t) |  m(t)  cannot use envelope detector!! 2. For m(t) > 0  +cos(2p fc t) & for m(t) < 0  -cos(2p fc t) 3. 180° phase change between + m(t) & - m(t) 4. Sign of m(t) is stored in phase of carrier 5. Must have phase information from carrier to recover m(t)  coherent detection  mixer is a coherent detector ECE 4710: Lecture #20

DSB-SC Product Detection Desired Baseband Signal Frequency Doubled Signal After LPF then frequency doubled signal is eliminated and ECE 4710: Lecture #20

DSB-SC Product Detection What happens if there is a phase or frequency error in cosine supplied by local oscillator in Rx? After LPF term the frequency doubled term is gone and whereas for no errors we had !! ECE 4710: Lecture #20

DSB-SC Product Detection Phase Error  Non-linear distortion of information signal m(t) For Dq always < 20° then error is small since cos(20°) = 0.94 For Dq = ±90° then Rx signal is completely eliminated For random Dq the performance is not acceptable  channel propagation distance is unknown so received signal phase is random! ECE 4710: Lecture #20

DSB-SC Product Detection Frequency Error  m(t) is modulated by time-varying cos(2p Df t)!! Low frequency cosine (assuming Df <<< fc) distorts m(t) and periodically eliminates signal Completely unacceptable ** DSB-SC requires perfect knowledge of frequency and phase of Tx carrier must be present in the Rx ** ECE 4710: Lecture #20

DSB-SC Synchronization Frequency and phase synchronization required between Tx and Rx in DSB-SC  Coherent Detection Product Detector = Coherent Detector if oscillator is completely synchronized How do we synchronize Tx and Rx? Radar System  pass copy of Tx carrier to Rx Not possible for vast majority of communication systems Pilot Carrier  transmit copy of carrier outside spectrum for carrier recovery in Rx Carrier Recovery  PLL or Squaring Loop for carrier recovery in Rx ECE 4710: Lecture #20

Pilot Carrier f ECE 4710: Lecture #20 -0.5fc 0.5fc fc LPF 2fc Frequency Doubled Signal BPF to get 0.5 fc and then  2 multiply to recover carrier for local oscillator LPF 2fc 2fc Desired Baseband Signal ECE 4710: Lecture #20

Squaring Loop Recovery Square Law Device  Full wave diode rectifier ECE 4710: Lecture #20

Costas PLL Recovery ECE 4710: Lecture #20

DSB-SC Detection Complicated circuitry added to Rx for coherent detection of DSB-SC signals Good performance for low S/N at Rx input No distortion in recovered baseband signal spectrum Allows for data signals with non-zero power near DC in PSD Significant cost added to Rx design and manufacturing for coherent detection Squaring loop and Costas PLL have a 180° phase ambiguity At initial start up of loop either type can lock on wrong polarity for carrier phase  ambiguity between +m(t) and –m(t) ECE 4710: Lecture #20

DSB-SC Detection 180° phase ambiguity Ambiguity solutions: Not a problem for audio signal no auditory difference for +m(t) vs. –m(t)  tone not affected by sign!! Problem for polar data signal Ambiguity solutions: Send test signal with a priori known phase to lock phase of recovery Use differential encoding so that “1” and “0” stored in phase change rather than absolute phase value ECE 4710: Lecture #20

180° Carrier Phase Transitions: DSB-SC Data Signal If m(t) is digital data signal like polar NRZ we have Binary Phase Shift Keying = BPSK Special case of DSB-SC for a Polar NRZ m(t) Must have coherent detection for BPSK to measure absolute phase Can use non-coherent detection with DBPSK +1 -1 +1 -1 180° Carrier Phase Transitions: cos(2pfct) cos(2pfct) ECE 4710: Lecture #20

AM vs. DSB-SC ModulationType Advantages Disadvantages AM (DSB-LC) 1. Inefficient use of power 2. Poor performance for low S/N 3. High power & expensive Tx for good S/N @ Rx 4. Can’t use for most data signals AM (DSB-LC) 1. Envelope Detection 2. Simple & Cheap Rx’s 1. Good performance at low S/N 2. Can use for all data signals 3. Power efficient 1. Synchronization of f and q for coherent detection 2. Complicated and expensive Rx’s DSB-SC ECE 4710: Lecture #20