Lecture 20

EE104: Lecture 20 Outline  Review  Noise

of Last Lecture

in AM Receivers

 Single

Sideband Modulation

 Vestigial  AM

Sideband Modulation

Radio and Superheterodyne Receivers

Review of Last Lecture  Generation

of AM Waves

 Square

Law and Envelope Detection of AM

 Double

Side Band Suppressed

Carrier  Product

Modulators for DSBSC

 Coherent

Detection for DSBSC: Costas Loop

Noise in AM Receivers n(t): white s(t)=Accos(2πfct+φ)m(t) Product + Modulator

LPF 1

m´(t)+ n´(t)

Accos(2πfct+φ)

 Power

in s(t) is .5Ac2Pm

 Power

in m′(t) is .25Ac2Pm

 Power

in n′(t) is .5N0B

 SNR=.5Ac2Pm/(N0B)  Power

of s(t) over power of n(t) in BW

Single Sideband  Only

transmits upper or lower sideband of AM LSB

USB

 Reduces

bandwidth by factor of 2

 Transmitted

signal can be written in terms of Hilbert transform of m(t)

 SSB

can introduce distortion at DC

Vestigial Sideband  Transmits

USB or LSB and vestige of other sideband USB

 Reduces

bandwidth by roughly a factor

of 2  Generated

using standard AM or DSBSC modulation, then filtering

 Standard

AM or DSBSC demodulation

AM Radio and Superheterodyne Receivers  Multiplexes

AM radio signals in

frequency  10

KHz bandwidth, carrier in 530-1610 Khz f1

f2

f3

 Receiver

needs tight filtering to remove adjacent signals

 LO

can radiate out receiver front end

Main Points  SNR

in DSBSC is power of transmit signal over power of noise in the bandwidth of interest.

 SSB

is a spectrally efficient AM technique with half the BW requirements of standard AM and DSBSC.

 VSB

similar to SSB, uses slightly more BW for a lower DC distortion.