AM Demodulation• Reverse process of AM modulation.• Converts received AM wave back to the

original source information.• Receive, Amplify and Demodulate an AM

wave.• Tuning the receiver: bandlimiting total RF

spectrum to a specific desired band of frequencies.

receiver antenna

Speaker

RF Section

Audio SectionBandpass FilterAM detector

Bandpass FilterIF SectionBandpass Filter

Mixer/Convertor SectionBandpass Filter

• RF section: Receiver front end. Detecting.BandlimitingAmplifying the received RF signal.• Mixer/Convertor : downconverts the received

RF frequencies to intermediate frequencies.• IF section: amplification and selectivity.• AM detector: demodulates AM wave.• Audio section: amplifies the recovered

information.

Receiver Parameters• Selectivity: measure the ability of the

receiver to accept a given band of frequencies and reject all other.

One common way to describe selectivity of receiver is to give bandwidth of receiver at -3dB points.

Shape factor= ratio between two bandwidths.SF= B(-60dB)/B(-3dB)

• Bandwidth Improvement: thermal noise is directly proportional to bandwidth.

Circuit bandwidth must exceed bandwidth of the information signal.

Input SNR is calculated at receiver input using the RF bandwidth for noise power measurement.

Reducing the BW effectively improves the noise figure of the receiver.

BI= BRF/ BIF

NFimprovement = 10log BI

• Sensitivity: minimum RF signal level that can be detected at the input to the receiver and still produce a usable demodulated information signal.

Also called receiver threshold.Expressed in µV.Depends on the noise power present at input to

the receiver, receiver’s noise figure, sensitivity of AM detector, and BW improvement factor of the receiver.

Sensitivity can be improved by reducing noise level of the receiver.

• Dynamic Range: difference in dB between the minimum input level necessary to discern a signal and input level that will overdrive the receiver and produce distortion.

Minimum receive level is function of frond end noise, noise figure and desired signal quality.

Signal producing overload distortion is function of net gain of the receiver.

1db compression point is generally used for upper limit of usefulness.

• Fidelity: ability of communication system to produce at o/p , an exact replica of original source information.

• Three distortions that can deteriorate the fidelity of communication system: amplitude , frequency and phase.

• Phase distortion is not important to voice transmission but can be devastating to data transmission.

• Filtering is the predominant cause of phase distortion.

• Absolute phase shift and differential phase shift.

• Amplitude distortion is due to non uniform gain in amplifiers.

• Frequency distortion occurs when frequencies present in received signal were not present in original signal.

• It is due to harmonic and intermodulation distortion.

• Equivalent Noise Temperature: Hypothetical value that cannot be directly measured. Te is an indication of the reduction in the SNR ratio as signal propagates through a receiver.

Lower is the equivalent noise temp, better is the receiver.

Typical values for Te range from 20º for cool receivers to 1000º for noisy receivers.

Te = T(F-1)

AM Receivers

• Basically there are two types of receivers1.Synchronous Receivers: frequencies

generated in receiver are synchronized to oscillator frequencies generated in the transmitter.

2.Asynchronous Receivers: also called envelope detection..

Tuned Radio Frequency Receiver.

Signal from antenna DetectorRF Amplifier

Speaker Audio Amplifier

Super heterodyne Receivers

Preselector

BPF

Mixer RF Amplifier

Audio DetectorIF Amplifier Audio

Amplifier

Local Oscilltor