01-12-2012, 04:14 PM
AM Receiver
AM Receiver.ppt (Size: 4.82 MB / Downloads: 30)
RF section (Receiver front end)
used to detect, bandlimit and amplifying the received RF signal.
Mixer/converter
Down-converts the received RF frequencies to intermediate frequencies (IF).
Intermediate frequencies are the frequencies that fall somewhere between the RF and the information frequencies.
IF section
Used for amplification and selectivity.
AM detector
Demodulates the AM wave and converts it to the original information signal.
Audio section
Used to amplify the recovered signal
Receiver Parameters
Selectivity
Selectivity – parameter used to measure the ability of the receiver to accept a given band of frequencies and reject all others.
Ex : for the commercial AM broadcast band, each stations transmitter is allocated a 10 kHz bandwidth. For a receiver to select only those frequencies assigned in a single channel, the receiver must limit its bandwidth to 10 kHz.
A method to describe the selectivity of the receiver is to give the receiver a bandwidth at 2 levels of attenuation (e.g. -3 dB and -60 dB).
The ratio of these 2 bandwidths is called as shape factor (SF),
Bandwidth Improvement
Thermal noise is one form of noise occurs in communication system that is proportional to a bandwidth.
As signal propagates from the antenna through the RF section, mixer/converter section and IF section, the bandwidth of signal is reduced thus reducing the noise.
Noise reduction ratio achieved by reducing the bandwidth is called bandwidth improvement (BI) expressed as follow,
Sensitivity
Sensitivity of the receiver is defined as - the minimum RF signal level that can be detected at the input to the receiver and still produce a usable demodulated information signal.
Signal-to-noise ratio (SNR) and the power of signal at the output of the audio section are used to determine the quality of the received signal and whether it is usable.
Typical AM broadcast-band receivers, a 10 dB or more SNR with approximately 0.5W of signal power at audio section is considered usable.
Sensitivity of a receiver is expressed in microvolts of the received signal.
Typical sensitivity for commercial broadcast-band AM receiver is 50 μV.
Sensitivity of the receiver depends on :
Noise power present at the input to the receiver
Receiver noise figure
Sensitivity of the AM detector
Bandwidth improvement factor of the receiver
The best way to improve the sensitivity is to reduce the noise level