ENSC327 Communications Systems 4. Double Sideband Modulation. Jie Liang School of Engineering Science Simon Fraser University

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1 ENSC327 Communiations Systems 4. Double Sideband Modulation Jie Liang Shool of Engineering Siene Simon Fraser University 1

2 Outline DSB: Modulator Spetrum Coherent Demodulator: Three methods Quadrature-arrier Multiplexing 2

3 Double Sideband Modulation (DSB) AM: DSB: s( t) = A m( t)os(2πf t) [ 1+ k m( t) ] os(2πf ) s( t) = A t No arrier omponent in the DSB signal. Input: DSB Output: a Envelope is no longer the input, Cannot use envelope detetor as demodulation. 3

4 Spetrum of DSB The FT of DSB signal is Proof: A S f ) = + 2 [ M ( f f ) + M ( f f )] ( 4

5 Spetrum of DSB Upper Sideband and Lower Sideband Upper Sideband (USB): [f, f + W], [- f W, -f]. Lower Sideband (LSB): [f - W, f], [- f, -f + W]. USB LSB LSB USB -f Bandwidth: 2W (same as AM) No omponent at arrier frequeny, unless m(t) has a DC: DSB is also alled suppressed arrier DSB (DSB-SC) Better power effiieny The prie paid is more ompliated demodulation. f 2W f 5

6 Modulator & Demodulator of DSB Modulator: The generation of DSB signal is straightforward, Just multiply the message with the arrier. This is also alled Produt Modulator S(t) s( t) = A m( t)os(2πf t) Demodulator: The demodulation is more ompliated than the AM, beause the envelop detetor annot be used in DSB. The key idea of DSB demodulation: the trigonometri identity os 2 (2πf t) = 6

Coherent Detetion (Synhronous Demodulation) Assuming a oherent or synhronized arrier signal is available at the reeiver, the message an be reovered by multiplying the

7 Coherent Detetion (Synhronous Demodulation) Assuming a oherent or synhronized arrier signal is available at the reeiver, the message an be reovered by multiplying the reeived signal with the oherent arrier, followed by a lowpass filter. S(t) v(t) vo(t) A ' os(2πf After multiplying with oherent arrier: t) V(f) f After lowpass filter: 7

8 Example Modulation: m(t) s( t) = A m( t)os(2πf t) Demodulation: s(t) A os( 2πf t) A A ' v( t) = m( t) 1 t 2 [ + os(4πf )] A ' os(2πf LPF t) Freq is higher than in s(t). Sign of v(t) is same as m(t). A A ' m( t) / 2 8

9 Phase Error of the Demodulator So far we assume the demodulator has a arrier that has the same frequeny and phase as the transmitter If a phase error exists at the reeiver: After LPF: If the phase error is onstant: Otherwise: 9

10 DSB: Generation of Coherent Carrier Methods to generate a phase oherent arrier: Method 1. Costas reeiver Invented by John Costas at General Eletri in the 1950s. Also known as Costas Phase Loked Loop (PLL): A negative feedbak system that generates a signal, whose phase is loked to the phase of an input or "referene" signal. Aomplished by a voltage ontrolled osillator (VCO) Method 2. Squaring the reeived signal Method 3. Transmitting pilot signal Used in stereo FM 10

11 3.4 Costas Reeiver I-hannel Q-hannel Two loal osillator signals that are in quadrature: os( 2π f t+ φ) o with 90 phase differene sin( 2π f t+ φ) These loal arriers are used by two oherent detetors: In-phase oherent detetor (also known as I-hannel) Quadrature-phase oherent detetor (or Q-hannel) 11

12 3.4 Costas Reeiver The I-hannel output is The Q-hannel produt modulator output: The output after lowpass filter: 12

13 3.4 Costas Reeiver VCO When phase error is 0: I-hannel output: Q-hannel output: If the phase of the loal arrier drifts by a small value I-hannel output: Q-hannel output: φ: 13

14 α φ The phase disriminator onsists of a multiplier followed by a time-averaging unit: Multiplier output: Time averaging output (whih is input of VCO): The phase disriminator output is proportional to VCO is a negative feedbak system: The VCO will drive the loal arrier phase to 0. Synhronization with the input is thus ahieved. 14

15 DSB: Generation of Coherent Carrier Method 2. Squaring the reeived signal s( t) = A os(2πf t) m( t) A oherent arrier an be obtained by: 15

16 DSB: Generation of Coherent Carrier Method 3. Transmitting a pilot signal outside the passband of the modulated signal: The pilot is a low-power sinusoidal wave whose freq and phase are related to the arrier wave (e.g., divide by 2) Reeiver uses a narrowband bandpass filter to extrat the pilot signal, and then onverts it to the orret frequeny (e.g., double the freq) Example: Stereo FM (page 183) Single-sided spetrum of FM baseband signal. Carrier: 38 khz. Pilot: 19 khz 16

17 Outline DSB: Modulator Spetrum Coherent Demodulator: Three methods Quadrature-arrier Multiplexing 17

18 3.5 Quadrature-Carrier Multiplexing Also known as Quadrature-amplitude modulation (QAM): Transmit two DSB-SC signals in the same spetrum region. Use two modulators with quadrature phases. Transmitted signal: s(t) = 18

19 3.5 Quadrature-Carrier Multiplexing Demodulator: two oherent detetors with 90 degree phase shift Output of the top produt demodulator: After LPF: Similarly, m2(t) an be reovered by the 2 nd detetor. 19

20 3.5 Quadrature-Carrier Multiplexing It is important to maintain the synhronization of the osillators at the transmitter and at the reeiver. This an be ahieved using the Costas Reeiver or the pilot signal method. 20

ENSC327 Communications Systems 4. Double Sideband Modulation. School of Engineering Science Simon Fraser University

ENSC327 Communications Systems 4. Double Sideband Modulation. School of Engineering Science Simon Fraser University ENSC327 Communiations Systems 4. Double Sideband Modulation Shool of Engineering Siene Simon Fraser University 1 Outline Required Bakground DSB: Modulator Spetrum Coherent Demodulator: Three methods Quadrature-arrier