Communication Receiver

- equipment capable of receiving RF signal , bandlimitng and tuning in the desired signal and performing demodulation.

Demodulation

- process of recovering the information signal from the composite modulated signal.

Basic Types of Receiver1. Coherent of Synchronous - The frequency generated in the receiver used for demodulation used is synchronized to oscillator frequency in the transmitter.2. Non-coherent or Asynchronous - demodulation is completely independent from the transmitter carrier frequency.

Standard AM Receiver Types 1. Tuned Radio Frequency Receiver (TRF) - consists of a number of amplifier stages which are tuned to resonance at the carrier frequency of the desired signal by a gaged capacitor. The amplifier signal at the original carrier frequency are fed directly into the detector for demodulation and the resulting signals are amplifier and reproduce by a loudspeaker. Parts1. RF Amplifier - amplifies the weak signal. It has a variable ________ controls RF gain and sensitivity. 2. Detector - provides rectification and detection for the modulated signal recovers the F signal. 3. AF Amplifier - raises the power level of the audio signal to drive the loudspeaker.

Advantages of TRF Receivers1. Simple to design and align at lower frequency application.2. High sensitivity at single frequency applications. Disadvantages:1. Selectivity varies when it is tuned over a wide range of input frequencies. 2. Instability due to large numbers of RF amplifiers all tuned to the same center frequency. 3. Non-uniform gain over a very wide frequency range.4. Requires multistage tuning.

Sensitivity- ability to pick up weak signalsSelectivity - ability to select the desired signal in the presence of other signals.

2. Super heterodyne Receiver- a receiver in which all incoming modulated RF carrier signals are converted as a common IF carrier value for additional amplification and selectivity prior to demodulation, using heterodyne action.

Heterodyne - to mix two frequencies in a non-linear device and translate one frequency into another frequency by use of non-linear mixing. Parts:1. RF section - RF amplifiers increases the level of weak RF signal received from the antenna.2. Local oscillator – “a stable crystal oscillator” – whose frequency “beats” with the incoming signal to reproduce the correct intermediate frequency.3. Mixer - it combines the incoming RF signal and the LO signal and produces at its output. The sum and difference signals and various harmonious. 4. 1 st and 2 nd IF amplifier - tuned to 455 KHz amplifies the 455 KHz output of the mixer and rejects the remaining output frequencies; provides further selectivity and amplifications.5. 2 nd Detector - demodulates the 455 KHz signal and extracts the audio signal.

Advantages of Using RF Amplifier1. Provides higher gain and sensitivity.2. Improve-image frequency rejection.3. Enhanced S/N.4. Better selectivity

Tracking - process of tuning the local oscillator to a pre-determined frequency for each station. Throughout the AM band.a). High tracking of hide side injection .f0 = fs + IFb). Low tracking or low side injectionf0 = fs – IFwhere: f0 = local oscillator frequencyfi = input signal frequencyIF = intermediate frequency (455 KHz)

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* Note: high tracking is preferred because it is easier to filter the undesired frequency.

Image frequency, fs: - any frequency other than the selected RF carrier, that

if allowed the receiver and mix with the LO signal will produce a cross-product frequency. That is equal to IF.

fsi = f0 + Iffsi = fs + 2IF

since; fo = fs + IF (for high tracking)

rejection of the image frequencyIFFR:

IFFR = √1+Q2ρ2where:

ρ= fsifs

− fsfsi

in dB;

IFFR (dB) = 20logIFRR

fsi = 20 MHz mixer 21 – 20 = 1 f52 = 22MHz 22 – 21 = 1

f0= 21 Mhz

Note: Image and frequency rejection depends on the front and selectivity of the receiver and must be achieved before the IF stage. Ex: In a broadcast superheterodynereceiving having an RF amplifier. The loaded Q of the antenna coupling circuit is 100. If IF is 455 Khz. Calculate:a.) Image and frequency and its rejection at 100 KHz.b.) Image frequency and its rejection at 25 MHz

Example Problem:Given:Q = 100IF = 455 MHz

Superheteroby dyne Tracking The signal is received by the first stage RF amplifier (which is wideband class A amplifier) whose resonant frequency response curve can be tuned from 540 KHz to 1650 KHz (The standard Broadcast Band). The modulated signal is amplified and fed to the mixer stage (a class C circuit capable of producing the sum, difference and original frequencies), which is receiving signals from two sources RF amplifier and the local oscillator). The unmodulated signal from the local oscillator is fed to the mixer simultaneously with the modulated signal from the RF amplifier. (These two circuits are mechanically linked). The local oscillator (LO) is a tunable circuit with a tuning range that extends from 995 KHz to 2,105 KHz. The output from the mixer circuit is connected to the intermediate frequency amplifier (IF amp) with amplifies a narrowband of select frequencies (455 KHz ± 3 KHz). In some receivers this class A circuit acts not only as an amplifier but also as a filter for unwanted frequencies which would interfere with the selected one. This new IF frequency contains the same modulated information as that transmitted from the source but a frequency range lower than the standard broadcast band. This conversion process helps reduce unwanted interference from outside sources. The signals is rectified and filtered to eliminate one sideband and the carrier (conversion from RF to AF) and is finally amplified for listening.

Choice of IF1. If the intermediate frequency is too high poor selectivity and poor adjacent – channel rejection unless sharp cut-off filters are used in the IF stages.2. A high value of intermediate frequency increased tracking difficulties.3. As the intermediate frequency is lowered, image frequency rejection become poorer.4. A very low intermediate frequency can make the selectivity too sharp cutting off the sidebands.5. If the IF is very low, the frequency stability of the local oscillator must be made corresponding higher because any frequency drift is now a large proportion of the low IF than of a high IF.6. The intermediate frequency must not fall within the tuning range of the receiver or else instability will occur and heterodyne whistles will be heard, making it is impossible to tune the frequency bond immediately adjacent to the intermediate frequency.

1 MHz 1 F