Explain the following in relation of radio receiver. I) selectivity II) Sensitivity III) Double spotting.

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Selectivity

The selectivity of a receiver is its ability to reject unwanted signals.
It is expressed as a curve, which shows the attenuation that the receiver offers to signals at frequencies near to the one to which it is tuned.
Selectivity is measured at the end of a sensitivity test with the conditions the same as for sensitivity , except that now the frequency of the generator is varied to either side of the frequency to which the receiver is tuned.
The output of the receiver naturally falls, since the input frequency is now incorrect. The input voltage must be increased until the output is same as it was originally.
The ratio of the voltage required of resonance to the voltage required when the generator is tuned to the receiver’s frequency is calculated at a number points and then plotted in decibels to give a curve , of which is shown in figure . is representative .Looking at the curve, we see that at 20 kHz below the receiver tuned frequency , an interfering signal would have to be 60dB greater than the wanted signal to come out with the same amplitude .
Selectivity varies with receiving frequency if ordinary tuned circuits are used in the IF section, and becomes somewhat worse when the receiving frequency is raised.
In general , it is determined by the response of the IF section , with the mixer and RF amplifier input circuits playing a small but significant part .It should be noted that it is selectivity that determines the adjacent-channel rejection of a receiver.

Sensitivity:

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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.
For AM broadcast receivers, several of the relevant quantities have been standardized. Thus 30 percent modulation by a 400-Hz sine wave is used, and the signal is applied to the receiver through a standard coupling network known as dummy antenna.
The standard output is 50 milli watts (50mW), and for all types of receivers the loudspeaker is replaced by a load resistance of equal value.
The most important factors determining the sensitivity of a super heterodyne receiver are the gain of the IF amplifier(s) and that of the RF amplifier.
It is obvious that the noise figure plays an important part.

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.
This being the case, it is obvious that the precise tuning of the local oscillator is what determines which signal will be amplified by the IF stage.
Within broad limits, the setting of the tuned circuit at the input of the mixer is far less important (it being assumed that there is no RF amplifier in a receiver which badly suffers from double spotting).
Lack of selectivity is harmful because a weak station may be masked by the reception of a nearby strong station at the spurious point on the dial.
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 .)

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