Let us first consider the basic principle
of heterodyning action and its necessity.
Need
It is very difficult to design amplifiers
which give uniformly high gain over a wide range of
radio frequencies used in commercial broadcast stations.
However, it is possible to design amplifiers which can
provide high-gain uniform amplification over a narrow
band of comparatively lower frequencies called intermediate
frequencies (IF). Hence, it is necessary to convert
the modulated RF carrier into modulated IF carrier by
using a frequency converter.
This IF signal is then amplified by narrowband IF amplifiers
and passed on to the AM detectors.
Basic Principle
The frequency conversion can be achieved
by utilizing the heterodyne principle. For this purpose,
the modulated RF signal is mixed (in a mixer) with an
un-modulated RF signal produced by local oscillator
as shown in the following illustration:
FREQUENCY CONVERSION
The oscillator and the
mixer may be either two separate devices or combined
into one device called converter. The process of combining
two ac signals of different frequencies in order to
obtain a signal of new frequency is called heterodyning
action.
Amplification
Signal amplification is
frequently needed, both for increasing the signal level
of transducers such as thermocouples that have a low-magnitude
output, and also for compensating for attenuation of
signals during transmission from remote sensors.Amplification
by analogue means, is usually carried out by an operational
amplifier. This is normally required to have high-input
impedance so that its loading effect on the transducer
output signal is minimised. In some circumstances, such
as, when amplifying the output signal from accelerometers
and some optical detectors, the amplifier must also
have a high-frequency response, to avoid distortion
of the output reading.
Mixer
Mixer is a critical gateway
through which all signals-over a very wide frequency
range,must pass, but only one of which must be converted
to a new,fixed frequency range,the IF.All other competing
signals at the input must be tolerated, but eventually
ignored.Because of their highly non-linear nature, mixers
are not amendable to the familiar linear circuit analysis
methods which work quite well for most of the remaining
sections of a typical receiver.Partly, as a consequence
of this, they are often surrounded by an aura of mystery.The
internal behaviour of mixers is certainly complicated,but
it can be reduced to a few simple ideas.
Heterodyning
Action
Suppose the carrier signal
of frequency fsis heterodyned with
another signal of frequency fo,
then two additional signals are produced whose frequencies
are:
fo + fs-
the sum component.
fo - fs- the difference
component
Usually, the sum frequency
is removed by band-pass filtering. The difference frequency
(also called beat frequency) is retained and
forms the IF frequency in AM receivers.
Suppose, an AM carrier
signal of frequency 1500 KHz is mixed with an un-modulated
carrier (produced by local oscillator) of frequency
1955 KHz, then, following two frequencies are produced:
3455 kHz and
455 kHz
The higher frequency
is generally filtered out leaving behind the
difference frequency of 455 kHz which forms the IF frequency.
