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AM Noise Analysis

AM Noise Analysis. Professor Z Ghassemlooy Electronics and IT Division School of Engineering Sheffield Hallam University U.K. Contents. DSBC (AM) Receiver model Envelope detection Synchronous Detection Signal-to-noise ratio DSB-SC - Signal-to-noise ratio

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AM Noise Analysis

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  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. SNRo SNRi Message signal + noise + DSB-C White noise w(t) vi(t) = cr(t) + vn(t) vo(t) = m(t) + vn(t) AM Receiver - Envelope Detector Receiver Demodulator Diode + LPF (B) BPF (B) Received modulated signal power R = 1 Band-limited noise power Input signal-to-noise ratio Z. Ghassemlooy

  4. Recovered signal AM Receiver - Envelope Detector - cont. The vector diagram of AM + noise at the input of the demodulator is RT(t) y(t) R(t) x(t) The envelope of AM + noise is Assuming SNRi >> 1, thus [….]2 >> y2(t), therefore: DC blocked by a capacitor Recovered message signal power Output noise power Z. Ghassemlooy

  5. SNRo Threshold SNRi SNRi 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 SNRi the SNRo deteriorate rapidly. Z. Ghassemlooy

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

  7. SNRi SNRo Message signal + noise LPF (B) BPF (B) + X DSB-C White noise w(t) cos ct vo(t) = m(t) + vn(t) AM Receiver - Synchronous Detector Receiver Demodulator z(t) vi(t) = cr(t) + vn(t) Note that Z. Ghassemlooy

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

  9. SNRo Threshold SNRi SNRi AM Receiver - Synchronous Detector - cont. SNRiis the same as in envelope detector Recovered message signal power Output noise power Synch. detection Envelope detection For M =1, MNI = -1.76 dB, i.e. degradation in SNR. Z. Ghassemlooy

  10. Receiver Demodulator SNRi SNRo z(t) Message signal + noise LPF (B) BPF (B) + X DSB-C C White noise w(t) LO Sw(f) Ec 0.5MEc o/2 f c c-m c+m o m -m 0 -c+m c-m -(c+m) c+m B c c AM Receiver - Synchronous Detector - cont. PT B 0.5PT o/2 Z. Ghassemlooy

  11. SNRi SNRo Message signal + noise LPF (B) BPF (B) X cos ct vo(t) = m(t) + vn(t) DSB-SC Noise Analysis Receiver Demodulator z(t) + DSB-SC White noise w(t) vi(t) = cr(t) + vn(t) Substituting for Z. Ghassemlooy

  12. DSB-SC Noise Analysis - cont. Information  Output signals Noise  High frequency  • Power analysis R = 1 Received modulated signal power Band-limited noise power 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. SNRi SNRo Message signal + noise LPF (B) BPF (B) X cos ct vo(t) = m(t) + vn(t) SSB-SC Noise Analysis Receiver Demodulator z(t) + SSB-SC White noise w(t) vi(t) = cr(t) + vn(t) Z. Ghassemlooy

  15. SNRo SNRi SSB-SC Noise Analysis - cont. • Power analysis R = 1 Input signal power Output signal power Output noise power Noise power Z. Ghassemlooy

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