Multiplexing to refer to the combination of information streams from multiple sources for
transmission over a shared medium Demultiplexing to refer to the separation of a
combination back into separate information streams
There are four basic approaches to multiplexing that each have a set of variations and
– Frequency Division Multiplexing (FDM)
– Wavelength Division Multiplexing (WDM)
– Time Division Multiplexing (TDM)
– Code Division Multiplexing (CDM)
Frequency-division multiplexing (FDM) is a scheme in which numerous signals are
combined for transmission on a single communications line or channel. Each signal is
assigned a different frequency (sub channel) within the main channel.
In telecommunications, frequency-division multiplexing (FDM) is a technique by which the
total bandwidth available in a communication medium is divided into a series of nonoverlapping frequency sub-bands, each of which is used to carry a separate signal. These subbands can be used independently with completely different information streams, or used
dependently in the case of information sent in a parallel stream. This allows a single
transmission medium such as the radio spectrum, a cable or optical fiber to be shared by
multiple separate signals.
The most natural example of frequency-division multiplexing is radio and
television broadcasting, in which multiple radio signals at different frequencies pass through
the air at the same time. Another example is cable television, in which many television
channels are carried simultaneously on a single cable. FDM is also used by telephone systems
to transmit multiple telephone calls through high capacity trunk lines, communications
satellites to transmit multiple channels of data on uplink and downlink radio beams, and
broadband DSL modems to transmit large amounts of computer data through twisted
pair telephone lines, among many other uses.
An analogous technique called wavelength division multiplexing is used in fiber optic
communication, in which multiple channels of data are transmitted over a single optical
fiber using different wavelengths (frequencies) of light.
Useful bandwidth of medium exceeds required bandwidth of channel
Each signal is modulated to a different carrier frequency
Carrier frequencies separated so signals do not overlap (guard bands)
e.g. broadcast radio
Channel allocated even if no data
Amplitude Modulators Two basic amplitude modulation principles are discussed. They are
square law modulation and switching modulation.
Square law modulator
When the output of a device is not directly proportional to input throughout the operation, the
device is said to be non-linear. The Input-Output relation of a non-linear device can be
expressed as
V0 = a0 + a1Vin + a2Vin2 + a3 Vin3 + a 4Vin4 +
When the input is very small, the higher power terms can be neglected. Hence the output is
approximately given by
V0 = a0 + a1Vin + a2Vin2
When the output is considered up to square of the in put, the device is called a square law
device and the square law modulator is as shown in the figure
Diode detector basics
A number of methods can be used to demodulate AM, but the simplest is a diode detector.
It operates by detecting the envelope of the incoming signal which it does by rectifying the
signal. Current is allowed to flow through the diode in only one direction, giving either the
positive or negative half of the envelope at the output.
If the detector is to be used only for audio detection it does not matter which half of the
envelope is used, either will work equally well. Only when the detector is also used to supply
the automatic gain control (AGC) circuitry will the polarity of the diode matter.
The AM detector or demodulator includes a capacitor at the output. Its purpose is to remove
any radio frequency components of the signal at the output. The value is chosen so that it
does not affect the audio base-band signal. There is also a leakage path to enable the
capacitor to discharge, but this may be provided by the circuit into which the demodulator is
This type of detector or demodulator is called a linear envelope detector because the output is
proportional to the input envelope.
DC return required
In order for a diode detector to generate the required DC voltage, a DC return must be
available within the circuit. supplied.
This can be achieved by placing an RF choke across the input to the detector diode. This
appears like an open circuit to radio frequency signals, but acts as a DC return path for the
audio and other signals appearing from the detector.
Diode detector advantages & disadvantages
The diode detector is widely sued, but it has several advantages and disadvantages:
Diode detector advantages
Simplicity: The diode detector is very simple and is easy to construct. The circuit six
very straightforward, consisting of a very few components.
Low cost: Requiring so few components, and the fact that he components are not
specialised, this form of detector is very cheap. Accordingly it is widely used in AM
domestic radios.
Diode detector disadvantages
Distortion: Although the diode detector is able to operate in a reasonably linear
fashion over a reasonable range, outside this range high levels of distortion are
introduced, and even within the more linear range, distortion levels are not
particularly low. It is adequate for small low cost radios.
Selective fading: These detectors are susceptible to the effects of selective fading
experienced on short wave broadcast transmissions. Here the ionospheric propagation
may be such that certain small bands of the signal are removed. Under normal
circumstances signals received via the ionosphere reach the receiver via a number of
different paths. The overall signal is a combination of the signals received via each
path and as a result they will combine with each other, sometimes constructively to
increase the overall signal level and sometimes destructively to reduce it. It is found
that when the path lengths are considerably different this combination process can
mean that small portions of the signal are reduced in strength. An AM signal consists
of a carrier with two sidebands
Insensitive: Semiconductor diodes have a certain turn-on voltage. Accordingly the
voltage has to reach a certain level before the diode is able to operate reasonably
It utilizes the non-linear portion of the dynamic current-voltage characteristic of a diode. It
differs from the linear diode detector is that in this case the applied input carrier voltage is of
small magnitude and hence is restricted to the excessively non linear portion of the dynamic
characteristic, whereas in linear diode detector, a large amplitude modulated carrier voltage is
applied to the diode and most of the operation takes place over the linear region of the
The diode is biased positively to shift the zero-signal operating point to the small current non
linear region of the dynamic current-voltage characteristic. The capacitor-resistor
combination constitutes the load. To study the operation of this detector, we may consider
first only the resistor R to constitute the load impedance. Then the dynamic current-voltage
characteristic ofthe diode. Superposition of modulated carrier voltage on the dynamic
characteristic is also illustrated. This results in the output current waveform. Since the
operation takes place over the non linear region of the characteristic the current waveform has
its lower half compressed.
This average current consists of a steady or D C component I and a time varying
component at the modulation frequency. The shunt capacitor C bypasses all the radio
frequency components leaving only the average component to flow through the load resistor
R producing the desired detected output.