Receiver Characteristics

There are several important characteristics of a receiver that affect its performance and suitability for a particular application. Here are some common receiver characteristics:

1. Sensitivity: Sensitivity refers to the ability of a receiver to detect weak signals. A receiver with high sensitivity can detect and process weak signals, while a receiver with low sensitivity may miss weak signals.

2. Selectivity: Selectivity refers to the ability of a receiver to reject unwanted signals, such as interference from other sources. A receiver with high selectivity can reject unwanted signals, while a receiver with low selectivity may be susceptible to interference.

3. Bandwidth: Bandwidth refers to the range of frequencies that a receiver can receive and process. A receiver with a wider bandwidth can process a wider range of signals, while a receiver with a narrow bandwidth is limited to processing signals within a narrower range of frequencies.

4. Noise Figure: Noise figure refers to the level of noise introduced by a receiver into the received signal. A receiver with a low noise figure introduces less noise into the received signal, which can improve the overall quality of the received signal.

5. Dynamic Range: Dynamic range refers to the range of signal amplitudes that a receiver can handle without distortion. A receiver with a high dynamic range can handle a wide range of signal amplitudes without distortion, while a receiver with a low dynamic range may experience distortion at higher signal amplitudes.

6. Linearity: Linearity refers to the ability of a receiver to accurately process signals at varying amplitudes. A receiver with high linearity can accurately process signals at varying amplitudes, while a receiver with low linearity may introduce distortion or other inaccuracies.

7. Power Consumption: Power consumption refers to the amount of power required to operate the receiver. A receiver with low power consumption is more energy-efficient and may be better suited for portable or battery-powered applications.

Overall, the specific receiver characteristics required will depend on the application and the type of signals that need to be received and processed.

Selectivity

The ability of the receiver to select the wanted signals among the various incoming
signals is termed as Selectivity. It rejects the other signals at closely lying frequencies.
Selectivity of a receiver changes with incoming signal frequency and are poorer at high
frequencies.
Selectivity in a receiver is obtained by using tuned circuits. These are LC circuits tuned to resonate at a desired signal frequency. The Q of these tuned circuits determines the selectivity. Selectivity shows the attenuation that the receiver offers to signals at frequencies near to the one to which it is tuned. A good receiver isolates the desired signal in the RF spectrum and eliminates all other signals. 

Sensitivity

The sensitivity of a radio receiver is its ability to amplify weak signals. It is often defined in terms of the voltage that must be applied to the receiver input terminals to give a standard output power, measured at the output terminals.The most important factors determining the sensitivity of a superheterodyne receiver are the gain of the IF amplifier(s) and that of the RF amplifier .The more gain that a receiver has, the smaller the input signal necessary to produce the desired output power. Therefore sensitivity is a primary function of the overall receiver gain. Good communication receiver has a sensitivity of 0.2 to 1 µV

Fidelity

Fidelity refers to the ability of the receiver to reproduce all the modulating frequencies
equally. Figure shows the typical fidelity curve for radio receiver.

Typical Fidelity curve

The fidelity at the lower modulating frequencies is determined by the low frequency response of the IF amplifier and the fidelity at the higher modulating frequencies is determined by the high frequency response of the IF amplifier. Fidelity is difficult to obtain in AM receiver because good fidelity requires more bandwidth of IF amplifier resulting in poor selectivity.

Image frequency and its Rejection

In a standard broadcast receiver the local oscillator frequency is made higher than the
incoming signal frequency for reasons that will become apparent .It is made equal at all times to the signal frequency plus the intermediate frequency.Thus f0=fs+fi or f0=fs−fi, no matter what the signal frequency may be. When f0 and fs are mixed, the difference frequency, which is one of the by-products, equal to fi is passed and amplified by the IF stage.If a frequency fsi manages to reach the mixer, such that fsi=fo+fi, that is , fsi=fs+2fi then this frequency will also produce fi when mixed with f0.
Unfortunately, this spurious intermediate-frequency signal will also be amplified by the
IF stage and will therefore provide interference.This has the effect of two stations being received simultaneously and is naturally undesirable.The term fsi is called image frequency and is defined as the signal frequency plus twice the intermediate frequency.
The rejection of an image frequency by a single –tuned circuit, i.e., the ratio of the gain at the signal frequency to the gain at the image frequency, is given by
α= √1+Q2ρ2
where
Q=loaded Qoftuned circuit
If the receiver has an RF stage, then there are two tuned circuits , both tuned to fs .The rejection of each will be calculated by the same formula , and the total rejection will be product of the two.
Image rejection depends on the front-end selectivity of the receiver and must be achieved before the IF stage.Once the spurious frequency enters the first IF amplifier, it becomes impossible to remove it from the wanted signal.

Double spotting

This is well-known phenomenon, which manifests itself by the picking up of the same
shortwave station at two nearby points on the receiver dial.It is caused by poor front-end selectivity, i.e., inadequate image-frequency rejection .That is to say, the front end of the receiver does not select different adjacent signals very well , but the IF stage takes care of eliminating almost all of them .
As a matter of interest, double spotting may be used to calculate the intermediate
frequency of an unknown receiver, since the spurious point on the dial is precisely 2f, below the correct frequency .An improvement in image-frequency rejection will produce a corresponding reduction in double spotting.