Z. Ghassemlooy 1 AM Noise Analysis Professor Z Ghassemlooy Electronics and IT Division School of Engineering Sheffield Hallam University U.K.

Z. Ghassemlooy 2 Contents DSBC (AM) Receiver model Envelope detection Synchronous Detection Signal-to-noise ratio DSB-SC - Signal-to-noise ratio SSB-SC - Signal-to-noise ratio

Z. Ghassemlooy 3 AM Receiver - Envelope Detector Diode + LPF (B) Diode + LPF (B) BPF (B) BPF (B) DSB-C + + White noise w(t) Demodulator Receiver v i (t) = c r (t) + v n (t) v o (t) = m(t) + v n (t) SNR i SNR o Received modulated signal power Band-limited noise power Input signal-to-noise ratio Message signal + noise R = 1 

Z. Ghassemlooy 4 AM Receiver - Envelope Detector - cont. Recovered message signal power Recovered signal The vector diagram of AM + noise at the input of the demodulator is x(t)x(t) y(t)y(t) R(t)R(t) RT(t)RT(t) The envelope of AM + noise is Assuming SNR i >> 1, thus [….] 2 >> y 2 (t), therefore: DC blocked by a capacitor Output noise power

Z. Ghassemlooy 5 AM Receiver - Envelope Detector - cont. Thus the output signal-to-noise ratio Modulation noise improvement factor For M = 1 (i.e 100%) MNI = 1.75 dB The demodulator exhibits a threshold effect where below certain SNR i the SNR o deteriorate rapidly. SNR i SNR o SNR i Threshold

Z. Ghassemlooy 6 AM Receiver - Envelope Detector - cont. For the case where SNR i << 1 the vector diagram is The envelope of AM + noise is Dominant term R(t)R(t) RT(t)RT(t)  Note: output containes no term proportional to the information m(t) = E C Mcos w m t. The last term is the signal multiplied by time-varying noise, therefore is of no use in recovering m(t).

Z. Ghassemlooy 7 AM Receiver - Synchronous Detector X X cos  c t BPF (B) BPF (B) v o (t) = m(t) + v n (t) SNR o LPF (B) LPF (B) Message signal + noise v i (t) = c r (t) + v n (t) SNR i Demodulator Receiver z(t)z(t) DSB-C + + White noise w(t) Note that

Z. Ghassemlooy 8 AM Receiver - Synchronous Detector - cont. DC  High frequency  Information  Low frequency noise  High frequency  Output signals

Z. Ghassemlooy 9 AM Receiver - Synchronous Detector - cont. Recovered message signal power Output noise power SNR i is the same as in envelope detector For M =1, MNI = dB, i.e. degradation in SNR. SNR i SNR o SNR i Threshold Envelope detection Synch. detection

Z. Ghassemlooy 10 AM Receiver - Synchronous Detector - cont. DSB-C + + White noise w(t) cc c+mc+m c-mc-m EcEc 0.5ME c f Sw(f)Sw(f)  o /2 0.5P T cc c+mc+m c-mc-m cc -(  c +  m ) -c+m-c+m  o /2 B PTPT oo 0 mm -m-m B X X LO BPF (B) BPF (B) SNR o LPF (B) LPF (B) Message signal + noise SNR i Demodulator Receiver z(t)z(t) C

Z. Ghassemlooy 11 DSB-SC Noise Analysis X X cos  c t BPF (B) BPF (B) v o (t) = m(t) + v n (t) SNR o LPF (B) LPF (B) Message signal + noise v i (t) = c r (t) + v n (t) SNR i Demodulator Receiver z(t)z(t) DSB-SC + + White noise w(t) Substituting for

Z. Ghassemlooy 12 DSB-SC Noise Analysis - cont. High frequency  Information  Noise  Output signals Received modulated signal power Band-limited noise power R = 1  Power analysis

Z. Ghassemlooy 13 DSB-SC Noise Analysis - cont. Recovered message signal power Output noise power This improvement is due to presence of two sidebands in the received signal which is translated down to the baseband and added coherently. Noise power on the other hand does not add coherently (quadrature component is reject by the detector).

Z. Ghassemlooy 14 SSB-SC Noise Analysis X X cos  c t BPF (B) BPF (B) v o (t) = m(t) + v n (t) SNR o LPF (B) LPF (B) Message signal + noise v i (t) = c r (t) + v n (t) SNR i Demodulator Receiver z(t)z(t) SSB-SC + + White noise w(t)

Z. Ghassemlooy 15 SSB-SC Noise Analysis - cont. Input signal power Noise power R = 1  Power analysis Output signal power Output noise power SNR i SNR o