When you modulate a
signal, you are varying one or more properties of a periodic waveform, called
the carrier signal, with another signal, the modulating signal, that contains
information to be transmitted. Typically the modulating signal has a lower
frequency than the carrier.
Modulation is
applicable for combining two or more signals. The lower frequency information
wave is said to modulate the higher frequency carrier wave. By this it is meant
that changes in amplitude of the lower frequency wave cause the higher
frequency energy to change in amplitude, frequency or phase.
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AM changes the amplitude of the carrier. FM changes the frequency of the
carrier.
Where there is
modulation, there must be demodulation. (A device in a single package that is
capable of performing both tasks is known as a modem, and it works well for
two-way communication.) Of course, the protocols for modulation and
demodulation must be agreed upon in advance by sender and receiver. This is one
of the dilemmas confronting earthbound researchers who are attempting to detect
extraterrestrial intelligence by means of radio signals coming our way. It may
be possible to detect intelligent patterns, but decoding their meaning is a
more difficult undertaking.
In AM, sidebands are centered between the carrier frequency and the
difference between the carrier and the maximum modulating frequency.
The first modulation
method was amplitude modulation (AM). Though first devised in the early part of
the twentieth century, its use is still quite common. Applications include AM
radio, portable two-way radio, aircraft communication and computer modems.
A continuous
radio-frequency carrier wave, fc, as modulated by an
information-bearing audio or video wave can be viewed in the time domain on an
oscilloscope of sufficient bandwidth such as the Tektronix MDO3104. The carrier
wave appears as an envelope of varying amplitude as it moves along the X axis.
Where AM really gets
interesting is when viewed in the frequency domain with a spectrum analyzer. A
spectrum display shows the frequencies present and the relative amount of
energy present in each one. The greatest amount of power appears at fc and
in the two sidebands on either side of fc. The two
sidebands are mirror images of one another. They both contain the same
information, so one of them would suffice for demodulation purposes.
When only one
sideband is transmitted, it is known as single-sideband transmission. In
single-sideband transmission, also known as single-sideband suppressed-carrier
modulation, transmitter power and bandwidth are conserved, and that is a plus.
However, instrumentation and tuning complexities have conspired to prevent this
protocol from becoming universal. (For one thing, the receiver must incorporate
a beat-frequency oscillator to reconstitute the carrier.)
In frequency
modulation (FM), the difference between the instantaneous and the
base-frequency of the carrier is directly proportional to the instantaneous
value of the input-signal amplitude. Thus the carrier wave amplitude is
unchanged but the carrier frequency varies as it is modulated by an audio or
video signal.
The spectrum of an FM signal consists of the carrier plus an infinite
number of harmonics extending on either side of the carrier at integral
multiples of the modulating frequency. The relative levels of the harmonics are
set by Bessel functions.
FM has the
advantage that naturally occurring RF noise is not an issue since it does not
impact the frequency of the carrier. Specifically, FM typically has a poorer
signal-to-noise ratio (SNR) below a certain signal amplitude called
the noise threshold. But above a higher level called the full quieting
threshold, the SNR is much improved over AM. The degree of improvement depends
on modulation level and deviation. FM broadcasting usually sees improvements
greater than about 15 db. Overall SNR in FM circuits can be further improved
through use of such methods as pre-emphasizing higher audio frequencies with
corresponding de-emphasis in the receiver. Because FM signals have
constant amplitude, FM receivers normally have limiters that remove AM noise,
further improving SNR.
Phase modulation is a
subcategory of frequency modulation, whereby the phase angle of the carrier
envelope is modified in response to changes in the signal. Phase modulation is
used in many transmission applications, notably Wi-Fi and satellite TV.
We have been
discussing analog modulation schemes. A future article will take a look at the
more complex topic of modulation as used in digital transmission.
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Source: testandmeasurementtips
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