15934 am demodulation
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Transcript

  • 1. 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.
  • 2.
    • receiver antenna
    • Speaker
    RF Section Audio Section Bandpass Filter AM detector Bandpass Filter IF Section Bandpass Filter Mixer/Convertor Section Bandpass Filter
  • 3.
    • RF section : Receiver front end.
    • Detecting.
    • Bandlimiting
    • Amplifying 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.
  • 4. 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)
  • 5.
    • 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= B RF / B IF
    • NF improvement = 10log BI
  • 6.
    • 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.
  • 7.
    • 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.
  • 8.
    • 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.
  • 9.
    • 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.
  • 10.  
  • 11.
    • Equivalent Noise Temperature: Hypothetical value that cannot be directly measured. T e 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 T e range from 20º for cool receivers to 1000º for noisy receivers.
    • T e = T(F-1)
  • 12. AM Receivers
    • Basically there are two types of receivers
    • Synchronous Receivers: frequencies generated in receiver are synchronized to oscillator frequencies generated in the transmitter.
    • Asynchronous Receivers: also called envelope detection..
  • 13. Tuned Radio Frequency Receiver. Signal from antenna Detector RF Amplifier Speaker Audio Amplifier
  • 14. Super heterodyne Receivers Preselector BPF Mixer RF Amplifier Audio Detector IF Amplifier Audio Amplifier Local Oscilltor