EE104: Lecture 20 Outline

Review of Last Lecture

Noise in AM Receivers

Single Sideband Modulation

Vestigial Sideband Modulation

AM 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

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 of interest

ProductModulator

m´(t)+ n´(t)

Accos(2fct+

s(t)=Accos(2fct+m(t)+

n(t): whiteLPF

1

Single Sideband

Only transmits upper or lower sideband of AM

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

USB LSB

Vestigial Sideband Transmits USB or LSB and vestige of other

sideband

Reduces bandwidth by roughly a factor of 2

Generated using standard AM or DSBSC modulation, then filtering

Standard AM or DSBSC demodulation

VSB used for image transmission in TV signals

USB

AM Radio and Superheterodyne

Receivers

Multiplexes AM radio signals in frequency10 KHz bandwidth, carrier in 530-1610 Khz

Receiver needs tight filtering to remove adjacent signals

LO can radiate out receiver front end Fix problems by IF processing

(Superheterodyne)

f1 f2 f3

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.

AM receivers (and others) downconvert to IF for demodulation to avoid filtering/reradiation problems.