In amplitude modulation (AM) the amplitude of
a carrier wave whose frequency remains constant changes in response to the
modulating signal. In frequency modulation (FM), it is the frequency of the
carrier that varies with the amplitude of the modulating signal. The carrier
frequency deviates more when the modulating signal amplitude is higher. There
are two important consequences. Because noise is characterized by large
amplitude variations, it impacts AM transmission to a greater degree than FM
transmission, giving FM a higher signal-to-noise ratio. FM transmission,
however, requires greater bandwidth, which in today’s crowded FM spectrum, may
be seen as a liability.
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AM and FM are two
widely-used analog modulation methods. Others are phase modulation (PM),
quadrature amplitude modulation (QAM), space modulation (SM) and
single-sideband modulation (SSBM).
Phase modulation, like
frequency modulation, is a form of angle modulation. In FM the frequency of the
carrier varies to signify changes in amplitude of the modulating signal. In PM
it is the phase of the carrier wave that is varied. Here again, frequency and
amplitude of the carrier remain constant.
Forms of modulation are described by a
modulation index. For AM, the index can be defined as the extent of amplitude
variation about an unmodulated carrier. When expressed as a percentage it is
equal to M/A where M is the peak change in the RF amplitude from its
unmodulated value, and A is the carrier amplitude.
The phase of a
propagated wave with respect to another propagated wave refers to the relative
difference in their instantaneous values in time, as represented by positions
on the X-axis of an oscilloscope display when the instrument is operating in
the time domain. If successive waveforms are time-shifted in that way,
information can be conveyed.
Common applications
for phase modulation are Wi-Fi, the Global System for Mobile Communications
(GSM), satellite television, signal and waveform generation in digital
synthesizers, and phase distortion in sound synthesizers.
Quadrature
amplitude modulation appears as a scheme for information encoding in both
analog and digital modulation. In both modes, two analog signals or two digital
bit streams are conveyed by modulating the amplitudes of two simultaneous
carrier waves. In the digital domain, amplitude-shift keying is used while in
the analog domain it is amplitude modulation that is operative.
The trapezoid method of measuring amplitude
modulation uses a scope’s X-Y inputs. The modulating signal
goes on the X input while the modulated signal drives Y. For broadcast AM, a
radio tuned to the signal of interest can provide the modulating audio for a
microphone. The AM signal itself drives the Y axis. (Real measurements probably
would require a tunable RF amp to get the correct RF signal to the scope.) The
audio level is adjusted to produce a usable trapezoid display. The length of
the trapezoid’s left side shrinks with a rising modulation level. The
modulation index M = (A-B)/(A+B).
The two carrier
waves, which are of the same frequency, are in a quadrature relationship, or orthogonally,
which is to say that they are 90° out of phase. Accordingly, they can be
demodulated. QAM is used for Wi-Fi and for optical fiber signal transmission.
The sender and receiver must have accurate clock signals in common. If they do
not maintain synchronicity, the signals lose resolution and are subject to
crosstalk. To avoid this corruption, a burst subcarrier is typically included.
An example, in NTSC TV transmission, is the color burst.
Space Modulation (SM)
differs from the types discussed above in that its purpose is not to facilitate
transmission between transmitters and receivers, but rather to aid aircraft in
modeling surrounding spaces to help land safely. Demodulation takes place in
the space between an aircraft and its intended touchdown location rather than
within the instrumentation. Multiple antennas fed with diverse signals create
discrete depths of modulation, from which is derived the required positional
information.
In SM, carriers at
110 MHz and 330 MHz are modulated by 90 Hz and 150 Hz tones. These signals are
conveyed from runway to aircraft to facilitate accurate landing.
Single-sideband
(SSB) modulation, also known as single-sideband suppressed-carrier (SSB-SC)
modulation, has been used since the first decades of radio transmission to
convey information using reduced power and bandwidth. Conventional amplitude
modulation concerns itself with an output signal that is twice the bandwidth of
the modulating signal. Accordingly, one-half of an AM transmission is
eliminated in SSB modulation by suitable filtering with no loss of information;
dual sidebands are essentially redundant. The downside and reason SSB is not
used universally is the more complicated circuitry at the transmitter and
tuning problems at the receiver.
A conventional AM transmitter produces sidebands
on either side of the carrier. The spectrum of a single-side band-suppressed
carrier transmitter contains only one sideband and no carrier.
Despite its greater
efficiency and lower bandwidth requirements, SSB is not used for broadcasting.
Frequency stability and selectivity are beyond the capability of inexpensive
receivers. But SSB is justified in point-to-point communication where more
advanced receivers are the norm and can be modified as needed.
SSB was first
patented in 1915 and used successfully in a 1920s radio-telephone link between
New York and London. Telephone companies in the 1930s used SSB over
long-distance lines in conjunction with frequency-division multiplexing (FDM).
FDM is a basic form of
multiplexing which, as the name implies, consists of conveying two or more
signals simultaneously through a link. In its simplest form FDM frequencies
have non-overlapping bandwidths, so they can be selected at the receiver using
ordinary filtering techniques.
Multiplexing is a
generic term meaning that multiple signals are sent through a single conductor
without mutual interference. SSB lends itself to FDM because one sideband is
not part of the transmission, so the modulated carrier occupies only one-half
the conventional FM bandwidth. So twice as many multiplexed signal can be
transmitted.
Modulating a carrier with a square wave as in
FSK brings a sine X/X -type spectrum centered at the carrier frequency
Frequency modulation
is also used to convey digitized data. This is done by shifting the carrier
frequency among various frequencies that represent digits. In a typical
implementation, one specified frequency represents 0 and another represents 1.
This is frequency-shift keying (FSK) and it is used in fax and other modems,
for Morse Code and in radio teletype. Other varieties of modulation adapted for
digital communication are ASK, APSK, CPM, MFSK, MSK, OOK, PPM, PSK, SC-FDE, TCM
and WDM:
·
Amplitude-shift keying (ASK) is a
variety of AM that varies the amplitude of a carrier wave to denote 0 or 1.
·
Asymmetric phase-shift keying (APSK)
conveys digital information by modulating amplitude and phase of the carrier.
·
Continuous phase modulation (CPm0)
is used in wireless modems. Rather than the carrier phase resetting to zero at
the start of each symbol, the carrier phase is modulated continuously. CPM is
characterized by high spectral and power efficiency.
·
Multiple frequency-shift-keying
(MFSK) resembles FSK, but more than two frequencies are used.
·
Minimum-shift keying (MSK) rather
than using square pulses, consists of half sinusoids to encode each bit.
·
On-off keying (OOK) denotes digital
voltage levels, i.e. zeros and ones, by the presence or absence of the carrier
wave.
·
Pulse-position modulation (PPM)
denotes digital bits by transmitting single pulses in shifting positions.
·
Phase-shift keying (PSK) denotes
digital bits by modulating the phase of the carrier wave. It is used for LANs,
RFID and Bluetooth protocols.
·
Single-carrier FDMA is a
frequency-division multiple access format. It assigns multiple users to a
single communications channel.
·
Trellis coded modulation (TCM)
efficiently transmits information over narrow-band telephone lines.
·
Wavelet digital modulation (WDM)
uses wavelet transformations to denote digital values.
Pulse-width
modulation is used primarily to control industrial machinery including the
speed and torque of three-phase induction motors by means of variable frequency
drives (VFD).
Prior to the
introduction of VFD in the 1960s, the speed of an ac motor could not be
controlled practically. Reducing the voltage supplied to the motor would slow
it down, but this transformed it into a less powerful motor, slowed only
because it was now overloaded. The unfortunate result was immediate heating of
the motor windings. For this reason, the ac motor was unsuitable for many
applications, such as for elevators and ski lifts, where smooth speed control is
essential.
The VFD functions by
feeding into the motor windings, not the traditional sine wave as supplied by
the utility, but a square wave, whose duty cycle can be varied. The traditional
square wave has a 50% duty cycle, which means half the time the voltage is high
(on) and half the time it is low (off), with fast transitions. The VFD, in
response to a low-voltage control signal, can vary the duty cycle. Lowering the
duty cycle, meaning the power is off a greater portion of the time, slows the
motor because it reduces the average voltage. Under these conditions, however,
the motor never overheats because it is not actually powered by a lower
voltage.
Similarly, the duty
cycle can be raised above 50%, and the motor will run at higher-than-rated
speed with no adverse effects provided the bearings and cooling system are okay
with the increased RPM.
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CONTACT US
Dingshengwei Electronics Co., Ltd
Company Address: Buliding A,the first industry park of
Guanlong,Xili Town,Nanshan,Shenzhen,Guangdong,China
Tel : +86 0755 26909863
Fax : +86 0755 26984949
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Source：testandmeasurementtips
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