2-11
above the tuned frequency. As you approach the tuned frequency, the input level required to maintain a
given output level will fall. As you pass the tuned frequency, the required input level will rise. Input
voltage levels are then compared with frequency. They can be plotted on paper or you might view them
on an oscilloscope. They would appear in the form of a response curve. The steepness of the response
curve at the tuned frequency indicates the selectivity of the receiver.
Fidelity
The fidelity of a receiver is its ability to accurately reproduce, in its output, the signal that appears at
its input. You will usually find the broader the band passed by frequency selection circuits, the greater
your fidelity. You may measure fidelity by modulating an input frequency with a series of audio
frequencies; you then plot the output measurements at each step against the audio input frequencies. The
resulting curve will show the limits of reproduction.
You should remember that good selectivity requires that a receiver pass a narrow frequency band.
Good fidelity requires that the receiver pass a broader band to amplify the outermost frequencies of the
sidebands. Receivers you find in general use are a compromise between good selectivity and high fidelity.
Q11. What four basic functions must a receiver perform?
Q12. What are the four basic receiver characteristics?
SUPERHETERODYNE RECEIVER
The superheterodyne is the type receiver most familiar to you. You probably see one daily in your
home in the form of an AM and/or fm radio. We will discuss the basic workings of both AM and fm types
and their differences.
Amplitude Modulation Receiver
Figure 2-9 shows a block diagram with waveforms of a typical AM superheterodyne receiver
developed to overcome the disadvantages of earlier type receivers. Let’s assume you are tuning the
receiver. When doing this you are actually changing the frequency to which the rf amplifier is tuned. The
rf carrier comes in from the antenna and is applied to the rf amplifier. The output of the amplifier is an
amplified carrier and is sent to the mixer. The mixer also receives an input from the local oscillator. These
two signals are beat together to obtain the IF through the process of heterodyning. (Heterodyning will be
further discussed later in this chapter and was covered in NEETS, Module 12, Modulation Principles.) At
this time you should note the dotted lines connecting the local oscillator, rf amplifier, and the mixer. This
is used on block diagrams and schematics to indicate GANGED TUNING. Ganged tuning is the process
used to tune two or more circuits with a single control. In our example, when you change the frequency of
the receiver all three stages change by the same amount. There is a fixed difference in frequency between
the local oscillator and the rf amplifier at all times. This difference in frequency is the IF. This fixed
difference and ganged tuning ensures a constant IF over the frequency range of the receiver.
