Mumbai University > Electronics > Sem 4 > Priniciples of Communication Engineering

(i) Selectively

Fig1: Selectivity

• The selectivity of an AM receiver is defined as its ability to accept or select the desired band of frequency and reject all other unwanted frequencies which can be interfering signals.
• Adjacent channel rejection of the receiver can be obtained from the selectivity parameter. Response of IF section, mixer and RF section considerably contribute towards selectivity.
• The signal bandwidth should be narrow for better selectivity.
• Graphically selectivity can be represented as a curve shown in Fig1. below, which depicts the attenuation offered to the unwanted signals around the tuned frequency.

(ii) Sensitivity

Fig2: Sensitivity curve

• Sensitivity of a receiver is its ability to identify and amplify weak signals at the receiver output.
• It is often defined in terms of voltage that must be applied to the input terminals of the receiver to produce a standard output power which is measured at the output terminals.
• The higher value of receiver gain ensures smaller input signal necessary to produce the desired output power.Thus a receiver with good sensitivity will detect minimum RF signal at the input and still produce utilizable demodulated signal.
• Sensitivity is also known as receiver threshold. It is expressed in microvolts or decibels.
• Sensitivity of the receiver mostly depends on the gain of IF amplifier. It can be improved by reducing the noise level and bandwidth of the receiver.
• Sensitivity can be graphically represented as a curve shown in Fig2. Below, which depicts that sensitivity varies over the tuning band.

(iii) Fidelity

• Fidelity of a receiver is its ability to reproduce the exact replica of the transmitted signals at the receiver output.
• For better fidelity, the amplifier must pass high bandwidth signals to amplify the frequencies of the outermost sidebands, while for better selectivity the signal should have narrow bandwidth. Thus a trade off is made between selectivity and fidelity.
• Low frequency response of IF amplifier determines fidelity at the lower modulating frequencies while high frequency response of the IF amplifier determines fidelity at the higher modulating frequencies.

(iv) AGC

• Automatic gain control (AGC) is a mechanism wherein the overall gain of the radio receiver is automatically varied according to the changing strength of the received signal. This is done to maintain the output at a constant level.
• AGC facilitates tuning to varying signal strength stations providing a constant output. AGC smoothens the amplitude variations of the input signal and the gain control does not have to be recalibrated every time the receiver is tuned from station to station.
• An AGC which is not designed correctly can lead to considerable distortion to a smooth signal.
• There are two types of AGC circuits:

i. Simple AGC: the gain control mechanism is active for high as well as low value of carrier voltage.

ii. Delayed AGC: AGC bias is not applied to the amplifiers until signal strength crosses a predetermined level, after which AGC bias is applied.

(v) Double spotting

• Double spotting is a condition where the same desired signal is detected at two nearby points on the receiver tuning dial.
• One point is the desired point while the other is called the spurious or image point. It can be used to determine the IF of an unknown receiver.
• Poor front-end selectivity and inadequate image frequency rejection leads to double spotting.
• Double spotting is undesirable since the strong signal might mask and overpower the weak signal at the spurious point in the frequency spectrum.
• Double spotting can be counter acted by improving the selectivity of RF amplifier and increasing the value of IF.
• Consider an incoming strong signal of 1000 kHz and local oscillator tuned at 1455 kHz. Thus a signal of 455 kHz is produced at the output of the mixer which is the IF frequency.
• Now consider the same signal but with 545kHz tuned local oscillator. Again we get 455 kHz signal at the output.
• Therefore the same 1000 kHz signal will appear at 1455 kHz as well as 545 kHz on the receiver dial and the image will not get rejected. This is known as Double spotting phenomenon.
• It is also known as Adjacent channel selectivity.