Signal that has to be transmitted is impressed onto a carrier
wave in any of the modulation methods, and it is then amplified
and applied to the transmitting antenna. The modulated signal
is radiated, propagated and collected by the receiving antenna
.The signal at this point is generally quite weak, therefore
the receiver must first amplify the received signals. Since
the signal is quite likely to be complied by lots of other
unwanted signals, probably at neighboring frequencies, it
must be selected and the other rejected .Finally, since modulation
took place in transmitter, the reverse process of this, demodulation,
must be performed in the receiver to recover the original
modulating voltages.
It is understood that a receiver has the function of selecting
the desired signal from all other unwanted signals, amplifying
and demodulating it, and displaying it in the desired manner.
This outline of functions that must be performed shows that
the major difference between receivers of various types is
likely to be in the way in which they demodulate the received
signal. This will depend on the type of modulation employed.
Of the various forms of receiver proposed at one time or another,
only two have any real practical or commercial significance.
They are the Tuned Radio Frequency receiver and the Superheterodyne
receiver. Only the second of these is used to a large extent
today, but it is easy to explain the operation of the tuned
radio frequency receiver since it is simpler of two. The best
way of justifying the existence and overwhelming popularity
of the Superheterodyne receiver is by showing the shortcomings
of the tuned radio frequency type.
Until shortly before World War II, most radio receivers were
of Tuned Radio Frequency type. It is simple logical receiver;
a person with little knowledge of communication would probably
expect all radio receivers to have this form. The virtues
of this type, which is now not used expect as a fixed frequency
receiver in the special applications, are its simplicity and
high sensitivity. It must also be mentioned that the tuned
radio frequency receiver was first introduced; it was a great
improvement on the types used previously mainly crystal, regenerative
and super regenerative receivers.
In the superheterodyne receiver the incoming signal voltage
is combined with a signal generated in the receiver. The local
oscillator voltage is normally converted into a signal of
lower fixed frequency. The signal at this intermediate frequency
contains the same modulations of the original carrier, and
it is now amplified and detected to reproduce the original
information. The superhet has the same essential components
as the receiver in addition to the mixer, local oscillator
and intermediate frequency amplifier.
A constant frequency difference is maintained between the
local oscillator and the radio frequency tuned circuits, normally
through capacitance tuning in which all the capacitors are
ganged together and operated in unison by one control knob.
The intermediate frequency amplifier generally uses two or
three transformers, each consisting of a pair of mutually
coupled tuned circuits. With this large number of double tuned
circuits operating at a constant specially chosen frequency,
the intermediate frequency amplifier provides most of the
gain and bandwidth requirements of the receiver. Since the
characteristics of the intermediate frequency amplifier are
independent of the frequency to which the receiver is tuned,
the selectivity and sensitivity of the superhet are usually
fairly uniform throughout its tuning range and not subject
to the variations that affect the Tuned Radio Frequency receiver.
The radio frequency circuits are now used mainly to select
the wanted frequency to reject interference such as the image
frequency and to reduce the noise figure of the receiver.
Tymon Hytem has worked in the electronics feild for the past
15 years. He enjoys helping people decide on electronic gadgets
from telephones to
XM Radio and choosing the perfect XM
Satellite Radio system for their needs.
