7/31/2019 Intermediate Frequency
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In communications and electronic engineering , an intermediate
frequency (IF ) is a frequency to which acarrier frequency is
shifted as an intermediate step in transmission or reception. [1]
The intermediate frequencyis created by mixing the carrier signal
with a local oscillator signal in a process called heterodyning ,
resulting ina signal at the difference or beat frequency .
Intermediate frequencies are used in superheterodyne radioreceivers
, in which an incoming signal is shifted to an IF for amplification
before final detection is done.
Conversion to an intermediate frequency is useful for several
reasons. When several stages of filters are used,they can all be
set to a fixed frequency, which makes them easier to build and to
tune. Lower frequencytransistors generally have higher gains so
fewer stages are required. It's easier to make sharply selective
filters atlower fixed frequencies.
There may be several such stages of intermediate frequency in a
superheterodyne receiver; two or three stagesare called double or
triple conversion .
Overview
The intermediate frequency (IF) is generated by mixing the
received radio frequency (RF) signal with a localoscillator (LO)
frequency.
Intermediate frequencies are used for three general reasons. The
first reason to use an IF, in receivers that canbe tuned to
different frequencies, is to convert the various different
frequencies of the stations to a commonfrequency for processing. It
is difficult to build amplifiers , filters , and detectors that can
be tuned to differentfrequencies, but easy to build tunable
oscillators . Superheterodyne receivers tune in different
frequencies byadjusting the frequency of the local oscillator on
the input stage, and all processing after that is done at the
samefixed frequency, the IF. Without using an IF, all the
complicated filters and detectors in a radio or televisionwould
have to be tuned in unison each time the frequency was changed, as
was necessary in the early tunedradio frequency receivers .
Converting to a lower intermediate frequency may offer an
advantage where the available amplifier deviceshave upper frequency
gain limits comparable to the frequency in use. A lower IF may
offer higher gain perstage than is possible at the RF. [1][2]
The main reason for using an intermediate frequency is to
improve frequency selectivity . [1] In communicationcircuits, a
very common task is to separate out or extract signals or
components of a signal that are closetogether in frequency. This is
called filtering . Some examples are, picking up a radio station
among several thatare close in frequency, or extracting the
chrominance subcarrier from a TV signal. With all known
filteringtechniques the filter's bandwidth increases
proportionately with the frequency. So a narrower bandwidth andmore
selectivity can be achieved by converting the signal to a lower IF
and performing the filtering at thatfrequency.
Perhaps the most commonly used intermediate frequencies for
broadcast receivers are around 455 kHz for AM
receivers and 10.7 MHz for FM receivers. In special purpose
receivers other frequencies can be used. A dual-conversion receiver
may have two intermediate frequencies, one higher one to improve
image rejection and asecond, lower one, for desired selectivity. A
first intermediate frequency may even be higher than the
inputsignal, so that all undesired responses can be easily filtered
out by a fixed-tuned RF stage. [3]
In a digital receiver, the ADC operates at low sampling rates,
so input RF must be mixed down to IF to beprocessed. Intermediate
frequency tends to be lower frequency range compared to the
transmitted RF frequency.However, the choices for the IF are most
dependent on the available components such as mixer,
filters,amplifiers and others that can operate at lower frequency.
There are other factors involved in deciding the IFfrequency,
because lower IF is susceptible to noise and higher IF can cause
clock jitters.
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