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.5A c 2 P m Power in m(t) is.25A c 2 P m Power in n(t) is.5N 0 B SNR=.5A c 2 P m /(N 0 B) Power of s(t) over power of n(t) in BW of interest Product Modulator m´(t)+ n´(t) A c cos(2  f c t+  s(t)=A c cos(2  f c t+  m(t) + n(t): white LPF 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 frequency 10 KHz bandwidth, carrier in Khz Receiver needs tight filtering to remove adjacent signals LO can radiate out receiver front end Fix problems by IF processing (Superheterodyne) f1f1 f2f2 f3f3

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.