CHAPTER 3 Noise In Amplitude Modulation Systems

CHAPTER 3Noise in Amplitude ModulationSystems

NOISEReview: Typesof Noise External(Atmospheric(sky),Solar(Cosmic),Hotspot) Internal(Shot, Thermal) Parameters of Noiseoooo Signal to Noise ratioNoise Figure or Noise FactorEffective Noise temperatureNoise BandwidthNarrow Band noise and its Components

Representation of AM Modulatedsignal

Noisy Receiver Model wherethe receiver noise is included in N0 givenby:the bandwidth and center frequency of idealband-pass channel filter are identical to thetransmission bandwidth BT and the centerfrequencyofmodulatedwaveform,respectively.

filtered noisy received signal x(t) availablefor demodulation is defined by: TheNoise n(t) is the band-pass filtered versionof w(t) Note:

Power spectral density (PSD) ofband-pass filtered noiseaverage noise power may be calculatedfrom the power spectral density. The average power N of filtered Gaussian whitenoise is: The

Signal to Noise Ratio (SNR) Ameasure of the degree to which asignal is contaminated with additivenoise is the signal-to-noise ratio(SNR)

FigureofMeritModulation SchemesOfCW Signal-to-noiseratio (SNR) is a measure of thedegree to which a signal is contaminated bynoise. Assumethat the only source of degradation inmessage signal quality is the additive noisew(t). Noisyreceiver model:

The signal-to-noise ratio at the demodulator input: The signal-to-noise ratio at the demodulator output:is well defined only if the recovered messagesignal and noise appear additively at demodulatoroutput. This condition is: (SNR)O Always valid for coherent demodulators But is valid for non-coherent demodulators only if the inputsignal to- noise ratio (SNR)I is high enough Outputsignal-to-noise ratio (SNR)O depends on: Modulation scheme Type of demodulator

Conditions of comparison To get a fair comparison of CW modulationschemes and receiver configurations, it mustbe made on an equal basis. Modulated signal s(t) transmitted by each modulationscheme has the same average power Channel and receiver noise w(t) has the same averagepower measured in the message bandwidth W Accordingto the equal basis, the channelsignal-to-noise ratio is defined as:

Noiseperformance of a given CW modulationscheme and a given type of demodulator ischaracterized by the figure of merit. Bydefinition, the figure of merit is: Thehigher the value of the figure of merit, thebetter the noise performance

Noise in AM DSB-FC Receivers

Threshold effectthreshold is a value of carrierto-noise ratio below which the noiseperformanceofademodulatordeteriorates much more rapidly thanproportionately to the carrier-to-noiseratio. The Everynoncoherent detector exhibitsa threshold effect, below the thresholdthe restored message signal becomespractically useless.

Threshold effectPhysical explanation: Ifthe carrier-to-noise ratio is highenough then the signal dominates and thenoise causes only a small unwanted AM andPM. However,if the carrier-to-noise ratio issmall then the noise dominates whichresults in a complete loss of information. Asa result, the demodulator output doesnot contain the message signal at all.

Threshold Effect : loss of message in an envelope detector that operates at a low CNR.

Figureof merit for DSB modulation:where P denotes the average power ofmessage signal m(t) and ka is theamplitude sensitivity of AM modulator. Thebest figure of merit is achieved if themodulation factor is µ kaAm 1 DSBsystem using envelope detectionmust transmit three times as muchaverage power as a suppressed-carriersystem

Noise in AM DSB-SC Receivers

Finding (SNR)O

Noise performance of AMreceiversNote:For high value of (SNR)C, the noiseperformance of coherent and noncoherent DSBare identical. But noncoherent DSB has athreshold effect. Coherent AM detectors have nothreshold effect!

Comparison of noise performanceof AM modulation schemesRemarks CurveI: DSBmodulation andenvelope detector withmodulation factor µ 1 CurveII: DSB–SC andSSB with coherentdemodulator Notethe thresholdeffect that appearsat about 10 dB

Figure of merit for DSB modulation: where P denotes the average power of message signal m(t) and ka is the amplitude sensitivity of AM modulator. The best figure of merit is achieved if the modulation factor is µ kaAm 1 DSB system using envelope detection must transmit three times as much