Digital communications
The TSG4100A Series generators support two types of modulation: analog and
vector. Analog modulation refers to the modulation of a scalar parameter of
the carrier signal, such as amplitude, frequency, or phase. Vector modulation
refers to the modulation of the vector characteristics (amplitude and phase) of
the carrier signal. Vector modulation is implemented using In-phase/Quadrature
(I/Q) modulation techniques.
Introduction
Prior to the industrial revolution, communications over long distances took
long periods of time using written words or images. Today, long distance
communication is most frequently accomplished by encoding information onto an
electrical signal that can be transmitted over very long distances at close to the
speed of light. The electrical signal is usually an RF carrier and the information is
encoded by modulating or altering the carrier in some way. The modulations are
usually one of three types: amplitude, frequency, or phase.
In most cases, the information being transmitted is an analog signal. The
acoustic vibrations from a person’s voice, for instance, can be converted into an
electrical signal with the use of a microphone. The resulting electrical signal is an
analog signal, which may be easily converted back into voice with an ampli
fi
er
and a speaker. In traditional analog communications, the analog signal itself
is used to modulate the RF directly. In FM radio, for example, the ampli
fi
ed
analog voice signal from the microphone is used to modulate the frequency of
the RF carrier directly. The primary advantage of such a scheme is its simplicity
and affordability. Receivers were fairly easy to design and cheap to produce.
The disadvantage of analog communication is that it is wasteful of power and
bandwidth, and susceptible to degradation by noise.
Digital communications refers to the transmission of digital data or numbers,
instead of analog signals. Analog signals can be converted into digital data with
the use of an analog to digital converter (ADC). The ADC measures the analog
signal at an instant in time and assigns a number to it. Big signals are assigned big
numbers and small signals are assigned small numbers. The ADC samples the size
of the analog signal every few microseconds and assigns a number proportional to
the size of the signal at each instant. In this way, an analog signal is ultimately
converted into a sequence of numbers.
Digital data may then be converted back into analog signals with the use of a
digital to analog converter (DAC). A DAC takes a number and converts it into
a voltage proportional to the number—small numbers produce small voltages,
and large numbers produce large voltages. By updating the number in the DAC
every few microseconds with the sequence of numbers produced by the ADC, the
original analog signal may be reproduced.
TSG4100A Series RF Signal Generators User Manual
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