Syllabus Sections:-

4j.1 Understand the term selectivity and 60 dB bandwidth.

Selectivity

The selectivity of a receiver is just like you trying to listen to a friend chatting to you in a crowded room with many other conversations going on at the same time. You have to focus you hearing on that one conversation.

Similarly the receiver has to be designed to receive just one signal and ignore the others on adjacent frequencies.

The selectivity is achieved by cutting out the signals of the adjacent frequencies and is usually quoted in dB reduction and a bandwidth.

Bandwidth

Bandwidth is the amount of the band that the receiver is detecting and producing audio in the speaker. If the selectivity is quoted as 2.5kHz at 6dB down then the signal heard on the edges of a band width of 2.5kHz will be only 1/4 of those in the centre. When it says 4.1kHz at 60dB then the reduction of the signals at that width will be very large ( you'll only get one millionth of the signal ).

4j.2 Recall that the dynamic range of a receiver is the difference between the minimum discernible signal and the maximum signal without overload. Recall the dynamic range is expressed in decibels.

Dynamic range

Dynamic range is a term used to describe the difference between the loudest possible undistorted sound to the quietest signal or to the noise level. The "dynamic range" and is express in decibels dB.

There will be some signals that you receiver will just not be able to hear because they are below the noise level of the receiver and those that are so strong that they overload the receiver. This is much like you can hear a conversation from across the room - until someone next to you starts shouting which overloads your ears and cuts out the weaker signal..

4j.3 Recall, in simple terms, the meaning of "signal to noise ratio" as applied to a receiver specification. Recall that the noise generated in the receiver will influence the minimum discernible signal.

Signal to noise ratio

Signal-to-noise ratio is an engineering term for the power ratio between a signal (the signal you want) and the background noise (any signal from within the receiver that you do not want to hear)

Signal

The amplitude of the wanted signal is that signal that you are wishing to receive and hear clearly and so that there is no ambiguity it should be stated as peak signal to peak noise.

Noise signal

Without any signal being received, eg when a dummy load is in the antenna socket, there will still be some "noise" coming out of the speaker. Most of this noise is generated in the RF amplifier. It is this noise level that a wanted signal has to be greater than so that the wanted signal can be heard in the speaker.

Hearing the weakest signal

So whether you can hear a weak signal all depends on the signal to noise ratio of your receiver. The stage that is responsible in all probability for noise is the RF amplifier stage so the better this stage is the less nose will be heard or rather the more "noise" generated in the receiver itself the less the ability to discern very low wanted radio signals.