09.pdf
(
7226 KB
)
Pobierz
Chapter 9
Modes and
Modulation Sources
The various modes we use partly reflect
the different types of information we
might wish to transmit. These could in-
clude text, graphics, speech (but not mu-
sic, which is prohibited in Amateur Radio
communications) and control messages—
that might be used to steer a model plane
or turn on a remote piece of equipment.
Various levels of quality or fidelity might
be desired for each of the above informa-
tion types depending on the use for which
the information was intended.
The communications
Channel
is as im-
portant in determining what mode to use as
is the message content. With radio commu-
nication, we are limited to what exists in
nature. The spectrum is a limited resource,
putting a restriction on the bandwidth we
can occupy. Attenuation is also present in
any radio link and this dictates factors such
as power and receiver sensitivity. Varia-
tions in the propagation path produce fad-
ing, delay, frequency shift (Doppler) and
distortion. Finally, noise of various types
may be present. While all of the above prob-
lems affect all our available frequency
assignments to some extent, there are wide
variations. Some of the bands used by ama-
teurs are wide and well behaved, such as
short VHF links. Others may be narrow,
unstable and hostile to our signals, such as
a long HF path through the auroral zones.
Such conditions dictate which mode will
be most successful.
modes, such as CW and PSK31, and wide-
band modes such as TV and spread spec-
trum. Not all modes are legal on all
amateur bands. Wideband modes can only
be used where the total width of the ama-
teur assignment is sufficient to contain the
wide signal. In addition, gentlemen’s
agreements and legal restrictions keep
some wideband modes out of certain bands
or subbands so that one person’s signal
does not preclude operation by a large
number of others using the narrower
modes. All users of the radio spectrum
must comply with FCC
occupied band-
width
rules. The occupied bandwidth is
determined not only by the mode being
used, but by proper operation of that mode.
Many of the legal modes can become too
wide when improperly adjusted. Perhaps
the greatest source of conflict between
ham operators is the “splatter” caused by
over-modulated or otherwise improperly
operated equipment, regardless of the
mode being used. An amateur signal must
be no wider than is necessary for good
communication, and as clean as the “state
of the art” will allow. Section 97.3 (a)(8)
of the FCC rules defines occupied band-
width as the point where spurious energy
drops to 26 dB below the total power.
Sensitivity
refers to the relative ability of
a mode to decode weak signals. Some
modes are favored by DXers in that they
have a greater ability to “get through” when
the signals are very weak. For local com-
munications, sensitivity may not be the
major concern.
Fidelity
is not often a major
issue for amateurs, although many hams
rightly take pride in the clear sound of their
transmissions.
Intelligibility
is related to
fidelity in a complex way and, sometimes,
voice signals are modified in such a way as
to make them more understandable, per-
haps under difficult conditions, even
though not as natural as they might other-
wise be.
Quality
is the corresponding term
for images, and
accuracy
describes the
degree to which a text mode reproduces the
original message.
Robustness
or
Reliabil-
ity
refers to the ability of a mode to main-
tain continuous communication under
changing conditions. For example, a very
robust signal is desired when controlling a
model airplane. DXers are not overly con-
cerned with reliability in that continuous
contact is not needed. However, they do
want a signal that gets through when needed
to work a rare station.
Efficiency
is the ability of a mode to
get the signal through with minimum
energy expended. Within the legal power
limit, energy cost is not a major concern
for most home stations. Thus, efficiency
is a concern mainly to those on battery
power—using handheld or portable sta-
tions. Emergency operators also need to
consider using efficient modes. For radio
services which use high power, such as
shortwave broadcasters, efficiency is
very important. QRP is a popular activ-
ity, where operators take pride in making
contact with a very small amount of trans-
mitter power (maximum miles per milli-
watt!)
Stability
is the ability to maintain
the frequency of the transmission very
precisely. Some modes require precise
frequency control. Most modern equip-
ment is very stable, but some vintage or
ISSUES COMMON TO ALL
TRANSMISSION MODES
Bandwidth
is the amount of space that a
signal occupies. There are narrowband
Modes and Modulation Sources
9.1
homemade gear may be limited in fre-
quency stability. Higher frequency work
can put tight limits on frequency stabil-
ity. Channel stability refers to both fre-
quency, amplitude and phase variations
of the transmission medium itself. The
inherent instability of a radio channel
may permit some modes but preclude
others.
Noise immunity
is the ability of a
radio system to reject noise of various
types that could otherwise destroy the
meaning or impair the quality of the mes-
sage. This is all important in HF mobile
operations, and for those living in
densely-populated areas. Man-made
electrical noise is an increasingly serious
threat to ham operations and requires
both legal and technical solutions.
Voice Modes
AMPLITUDE MODULATION (AM)
AM is as old as radio. The first wireless
signals transmitted by Marconi across his
garden consisted of trains of pulses that
were excited by a spark which recurred at a
more or less audio rate. This excited radio
waves in a band of frequencies determined
by the broad resonance between a coil
and the capacity of the antenna. The rough
buzz of early spark transmitters could be
received with diodes made of various sub-
stances that provided envelope detection,
which recovers audio that follows the aver-
aged peak amplitude of the wave.
Spark transmitters have passed into his-
tory, but amplitude modulation of radio
waves has not. Though likely to be replaced
by digital speech modes in the not too dis-
tant future, classical full carrier AM is, at
present, still used by the world’s most pow-
erful radio stations which broadcast on
long, medium and shortwave frequencies
to huge audiences. This material was writ-
ten by John O. Stanley, K4ERO.
The first AM broadcast of speech and
music occurred nearly a century ago, when
on Christmas eve of 1906, Fessenden, using
a modulated high frequency alternator, sur-
prised ship operators with a program of
music, Bible readings and poetry. The
development of a continuous wave trans-
mitter, one that produced a constant sine
wave output, rather than the rough spark sig-
nal, made AM practical. Thus, CW, as this
pure wave was called, not only greatly en-
hanced Morse communications, but allowed
voice transmissions as well. By changing the
strength or amplitude of this smooth con-
tinuous wave, a voice could be superim-
posed on or “carried” by the radio frequency.
The decade of the 1920s saw not only the
rapid development of the broadcast indus-
try, but also enabled many hams to try the
new voice mode. Indeed, in those early
years, there was sometimes little difference
between a ham who used voice and a broad-
caster. The legal situation was a mess, and
QRM was king! By 1929 it was legally and
technically possible to use AM voice in lim-
ited portions of our amateur spectrum and,
on some bands, only the most qualified lic-
ensees had the privilege. Users of AM had to
learn that an RF wave could only have a
certain amount of audio imposed upon it
before overmodulation occurred. Trying to
go above 100% modulation produced
severe distortion and QRM. AM remained
the dominant voice mode for ham opera-
tions well into the second half of the 20th
century, when it was gradually eclipsed by
SSB (SSB is actually a form of AM) and
FM. We can still hear AM on the ham
bands today, mostly coming from stations
using vintage gear. AM’ers usually choose
operating times when the bands are less
crowded, and often take pride in a clean
and clear signal.
The great advantage of AM, and one
reason for its long history, is the ease with
which a full carrier AM signal can be re-
ceived. This was all important in broad-
casting where, for every transmitter, there
were thousands or millions of receivers.
With modern integrated circuits, complex
detectors now cost very little. Therefore,
the biggest reason for keeping AM broad-
casting, at present, is to avoid obsoleting
the billions of existing receivers. These
will gradually have to be replaced when
digital broadcasts begin in the AM and
shortwave bands.
There are many ways to produce an AM
signal, but all of them involve multiplying
the amplitude of the information to be
transmitted by the amplitude of the radio
wave which will carry it. When multi-
plication of two signals takes place, as
opposed to their simple addition, mixing
is involved. The result is multiple signals,
including the sum and difference of the
AF and RF frequencies. These two “prod-
ucts” will appear as sidebands alongside
what was the original RF frequency. Mix-
ing, modulation, detection, demodulation,
and heterodyning all refer to this multipli-
cation process and can all be analyzed by
the same mathematical treatment. See the
Mixers, Modulators and Demodulators
(A)
(B)
Fig 9.1 — Electronic displays of AM signals in the frequency and time domains. A
shows an unmodulated carrier or single-tone SSB signal. B shows a full-carrier
AM signal modulated 20% with a sine wave.
9.2
Chapter 9
chapter of this
Handbook
for a more de-
tailed discussion of this process.
If an RF signal is modulated by a single
audio tone, and observed on an oscillo-
scope, it will appear as shown on the right
in
Fig 9.1B
. Observing the same signal on
a spectrum analyzer will show that the
composite signal observed on the scope is
composed of three discrete parts as shown
on the left in Fig 9.1B. The center peak,
which is identical with the original
unmodulated wave shown in Fig 9.1A, is
usually called the carrier, although this
terminology is deceiving and imprecise. It
is the composite RF signal, as seen on the
oscilloscope, which actually carries the
audio in the form of variations in its am-
plitude, so we might well have referred to
the center frequency as a “reference” or
some other such term.
As a reference signal, the carrier con-
tains important, though not indispensable,
information. For a signal with both side-
bands present, it provides a very important
frequency and phase reference which
allows simple and undistorted detection,
using nothing more than a diode. The car-
rier also provides an amplitude reference,
which is used by AM receivers to set the
gain of the receiver, using AGC or auto-
matic gain control. The carrier also con-
tains most of the power of the transmitted
signal, while most of the important infor-
mation is in the sidebands. See the
Mixers,
Modulators and Demodulators
chapter
in this
Handbook
which gives details of
power distribution in an AM signal.
Telephone engineers developed a sys-
tem of using only one of the two sidebands
which, being mirror images of each other,
contain the same information. SSB sys-
tems attracted the attention of hams soon
after WWII and gradually became the
voice mode of choice for the HF bands.
SSB is considered a form of AM, in that it
is identical to an AM signal with one side-
band, and all or part of the carrier re-
moved. The complexity of generating a
SSB signal, plus the difficulty of tuning
the generally unstable receivers common
in the 1950s, slowed the changeover to the
new mode, but its adoption was inevitable.
SSB became popular because of its greater
power efficiency which allowed each watt
of RF to go further. The fact that it occu-
pied less bandwidth was a plus also and
very welcome on the most crowded bands.
See the sidebar
SSB on 20 and 75 Meters
in this chapter.
While systems used for telephone relays
used pilot carriers so that the signal could
be reproduced without distortion, hams
chose to eliminate the carrier entirely. This
required generating a reference frequency
at the receiver which, if accurate to within
20 Hz, allowed intelligible speech to be
recovered. Since amateur regulations have
long prohibited transmission of music, the
distortion produced by loss of the exact
phase and frequency reference was not
serious. The loss of the amplitude refer-
ence was overcome with the development
of the “hang” AGC, which works on the
average value of the received sideband
which is constantly changing. While not as
fast or accurate as the carrier-based AGC
available in AM, this has proven satis-
factory, if proper attention is given to its
design (See the
Receivers and Transmit-
ters
chapter of this
Handbook
.)
Thus, SSB, while giving up some fidel-
ity and increasing complexity, has proven
superior to full-carrier AM for speech
communication because of its power and
bandwidth efficiency. And under certain
circumstances, such as selective fading, it
can actually have less distortion than AM.
On HF, it is possible for the carrier to fade
in an AM signal leaving less than is needed
for envelope detection. AM broadcasts
often have this problem at night. It can be
overcome with “exalted carrier detec-
tion.” Synchronous detection is a refine-
ment of this method. (See the
Mixers,
Modulators and Demodulators
chapter
of this
Handbook
.) SSB, in effect, uses
exalted carrier detection all the time.
An SSB signal is best visualized as an
audio or
baseband
signal that has simply
been shifted upwards into the radio fre-
quency spectrum as shown in
Fig 9.2
. The
relative frequencies, phases and amplitudes
of all the components will be the same as
the original frequency components except
for having had a fixed reference frequency
added to them. Surprisingly, this process,
called heterodyning, is not done by directly
adding the signals together, but by multi-
plying them and subsequently filtering or
phasing out the carrier and one of the side-
bands. The
Mixers, Modulators and
Demodulators
chapter of this
Handbook
explains this interesting process in detail.
The relative frequencies within the band
of information being transmitted may
appear inverted; that is, lower frequencies
in the original audio signal are higher in the
RF signal. When this happens, we call the
signal lower sideband or LSB. LSB is pro-
duced when the final frequency is the result
of subtraction rather than addition. If a tone
of 1 kHz is heterodyned to 14201 kHz by
mixing with a 14200 kHz carrier, the result
will be upper sideband, since 14200 + 1
gives us that result. When the same tone
appears at 3979 kHz by mixing it with a
3980 kHz carrier, we know that an LSB
signal was produced since 3980 – 1 gives
us the 3979 result. Whenever the audio tone
needs a minus sign to find the result, we are
on LSB. In most mixing schemes there will
be three frequencies involved (carrier,
VFO, and band select crystal) but the prin-
ciple still holds.
The frequency of an SSB transmission
is designated as that of the carrier, which
is the frequency (or the sum of several fre-
quencies) used to shift the baseband infor-
mation into the RF spectrum. In a good
SSB signal, little or no energy actually
appears on the frequency we say we are
using. It is strictly a reference. For this
reason, some radio services have chosen
to designate SSB channels by the center of
the occupied bandwidth rather than the
carrier frequency. Ham practice is to des-
ignate the carrier frequency and whether
the upper or lower sideband is in use. An
interesting exception is the new five-chan-
nel, 60-m amateur band (a secondary allo-
cation) where the FCC specified a 2.8-kHz
maximum BW centered on five frequency
segments: 5332, 5348, 5368, 5373 and
5405 kHz. Only USB is permitted. Most
Fig 9.2 — How an occupied radio
frequency spectrum shifts with application
of an audio (baseband) signal. The dotted
line represents the RF carrier point, or in
the case of the 3-kHz audio signal, the
reference frequency, 0 kHz.
Fig 9.3 — A method of changing
sidebands with virtually no change in
the frequency spectrum occupied. Note
that the carrier point position has
changed concurrent with the change
from LSB to USB.
Modes and Modulation Sources
9.3
hams will find it more natural to remem-
ber USB at corresponding carrier frequen-
cies of 5330.5, 5346.5, 5366.5, 5371.5 and
5403.5 kHz. Since the USB or the LSB is
considered “normal” for each of our
bands, it is assumed that the sideband in
use is understood. We need to remember
when switching sidebands that we will be
occupying a different portion of the spec-
trum than before the switch, and we may
inadvertently cause QRM, unless we
check for a clear frequency. If one wishes
to change from LSB to USB without
changing the spectrum occupied, we must
retune our dial down about 3 kHz as a care-
ful study of
Fig 9.3
should make clear.
This principle applies to digital as well as
voice modes, but usually not to CW, where
modern rigs make the above adjustment
for us. This means that the frequency read-
out with a CW signal will be the actual
frequency occupied, but with analog voice
and digital modes this will probably not be
the case.
Another need for understanding where
sideband signals actually fall is in operat-
ing close to the edge of a band or subband.
For example, on 20 meters where USB is
used, one must not operate above approxi-
mately 14.347 MHz, since the transmis-
sion will be outside the band if one
operates much higher. Operation on ex-
actly 4.0 MHz could be legal on LSB if the
signal is very clean, but is not recom-
mended. Most modern rigs which prevent
out of band transmissions
do not
preclude
the above cases of illegal operation.
Today there are many new modes for
text, speech and image transmission, and
more will be developed. Often these are
transmitted using SSB. Knowing exactly
where the signal will appear on the band
depends on understanding how LSB and
USB signals are produced. These modes
use either a separate circuit or more re-
cently a computer sound card to produce
audio frequency tones which represent the
information in coded form. This is then
fed into the audio input of an SSB trans-
mitter. They are then heterodyned to the
desired amateur band for transmission. In
a transceiver, the incoming signals are
similarly heterodyned back to the audio
range for processing in the computer
sound card or other circuitry. Some com-
puter–based digital modes allow reading
the actual signal frequency off the screen,
provided the transceiver dial is properly
set.
Voice signals and some text and image
modes require linear amplification. This
means that the amplifiers in the transmit-
ter must faithfully represent the amplitude
as well as the frequency of the baseband
signal. If they fail to do so, intermodu-
lation distortion (IMD) products appear
and the signal becomes much wider than it
should be, producing interference (QRM)
on nearby frequencies. CW and FM do not
require a linear amplifier, but one can be
used for them also, at a small price in effi-
ciency. Some VHF “brick” amplifiers
have a choice of either the more efficient
class C amplification or the more linear
class B amplification. The linear or SSB
mode must be chosen if SSB voice and
some digital modes are being used. When-
ever linear amplification is needed, flat-
topping must be prevented. This results
from overdriving the amplifier so that it
goes above the design power limit and
becomes non-linear at that point.
SSB transmitters and most linear ampli-
fiers use automatic level control (ALC) to
prevent overdrive and flattopping. How-
ever, there are limits to ALC and flattop-
ping can still occur if the amplifier is
grossly over driven. The surest way to cre-
ate ill will on any band is to cause spatter by
over driving your amplifier, regardless of
the mode. Amplifiers suitable for both
linear and non-linear signals are discussed
in the
RF Power Amplifiers
chapter of this
Handbook
. The effects of non-linear am-
plification are also further treated in the
Mixers, Modulators and Demodulators
chapter herein.
have infinitely steep skirts, the response
of the filter must begin to roll off within
about 300 Hz of the phantom carrier to
obtain adequate suppression of the un-
wanted sideband. This effect limits the
ability to transmit bass frequencies, but
those frequencies have little value in voice
communications. The filter rolloff can be
used to obtain an additional 20 dB of car-
rier supression. The bandwidth of an SSB
filter is selected for the specific applica-
tion. For voice communications, typical
values are 1.8 to 3.0 kHz.
Fig 9.4
illustrates two variations of the
filter method of SSB generation. In A, the
heterodyne oscillator is represented as a
simple VFO, but may be a premixing sys-
tem or synthesizer. The scheme at B is
perhaps less expensive than that of A, but
the heterodyne oscillator frequency must
be shifted when changing sidebands in
order to maintain dial calibration.
SSB Generation: The Phasing
Method
Fig 9.5
shows another method to obtain
an SSB signal. The audio and carrier sig-
nals are each split into equal components
with a 90 degree phase difference (called
quadrature
) and applied to balanced
modulators. When the DSB outputs of the
modulators are combined, one sideband is
HOW AN SSB SIGNAL IS
PRODUCED
When the proper receiver bandwidth is
used, an SSB signal will show an effective
gain of up to 9 dB over an AM signal of the
same peak power. Because the redundant
information is eliminated, the required
bandwidth is half that of a comparable AM
(DSB) emission. Unlike DSB, the phase
of the local carrier generated in the re-
ceiver is unimportant.
Table 9.1
shows the
qualities of a good SSB signal.
Table 9.2
Unwanted Sideband Suppression
as a Function of Phase Error
Phase Error
Suppression
(deg.)
(dB)
0.125
59.25
0.25
53.24
0.5
47.16
1.0
41.11
2.0
35.01
3.0
31.42
4.0
28.85
5.0
26.85
SSB Generation: The Filter Method
If the DSB signal from the balanced
modulator is applied to a narrow band-
pass filter, one of the sidebands can be
greatly attenuated. Because a filter cannot
10.0
20.50
15.0
16.69
20.0
13.93
30.0
9.98
45.0
6.0
Table 9.1
Guidelines for Amateur SSB Signal Quality
Parameter
Suggested Standard
Carrier suppression
At least 40-dB below PEP
Opposite-sideband suppression
At least 40-dB below PEP
Hum and noise
At least 40-dB below PEP
Third-order intermodulation distortion
At least 30-dB below PEP
Higher-order intermodulation distortion
At least 35-dB below PEP
Long-term frequency stability
At most 100-Hz drift per hour
Short-term frequency stability
At most 10-Hz P-P deviation
in a 2-kHz bandwidth
9.4
Chapter 9
SSB on 20 and
75 meters —
the 9 to 5 connection
SSB experiments began on
75 meters because it was the
lowest frequency phone band in
widespread use. Due to perpetual
crowding and its DX potential,
20 meters also seemed to call for
use of SSB. Some early rigs
included only these two bands.
The popular homebrew W2EWL rig
was built on the chassis of a war
surplus ARC-5 transmitter using its
5 MHz VFO, and generated the
sideband signal on 9 MHz using
the phasing method. Nine plus five
is 14 MHz, and nine minus five is
4 MHz, yielding 75 or 20 meter
coverage by choosing which of the
two mix products we would filter
out and amplify. Thus, two bands
were covered with the same VFO/
IF combination. Other rigs used a
tunable IF from 5.0 to 5.5 MHz.
This was subtracted from a 9 MHz
crystal to obtain 4.0 to 3.5 MHz,
and added to 9 MHz to cover 14.0
to 14.5 MHz. This process re-
versed the sidebands, and eventu-
ally led to the convention of using
LSB on the lower bands and USB
on the higher bands. This also
explains why on some vintage rigs
the 75 meter band dial reads
backwards!—K4ERO
Fig 9.4 — Block diagrams of filter-method SSB generators. They differ in the
manner that the upper and lower sideband are selected.
reinforced and the other is canceled. The
figure shows sideband selection by means
of transposing the audio leads, but the
same result can be achieved by switching
the carrier leads. The phase shift and am-
plitude balance of the two channels must
be very accurate if the unwanted sideband
is to be adequately attenuated.
Table 9.2
shows the required phase accuracy of one
channel (AF or RF) for various levels of
opposite sideband suppression. The num-
bers given assume perfect amplitude bal-
ance and phase accuracy in the other
channel. The table shows that a phase
accuracy of 1° is required to achieve un-
wanted sideband suppression of greater
than 40 dB . It is difficult to achieve this
level of accuracy over the entire speech
band. Note, however, that speech has a
complex spectrum with a large gap in the
octave from 700 to 1400 Hz. The phase-
accuracy tolerance can be loosened to 2°
if the peak deviations can be made to occur
within that spectral gap. The major advan-
tage of the phasing system is that the SSB
signal can be generated at the operating
frequency without the need of heterodyn-
ing. Phasing can be used to good advan-
tage even in fixed-frequency systems. A
loose-tolerance (4°) phasing exciter fol-
lowed by a simple two-pole crystal filter
can generate a high-quality signal at very
low cost.
Fig 9.5 — Block diagram of a phasing SSB generator.
Modes and Modulation Sources
9.5
Plik z chomika:
gaszek.karol
Inne pliki z tego folderu:
05.pdf
(7477 KB)
00.pdf
(2136 KB)
01.pdf
(2045 KB)
04.pdf
(5689 KB)
02.pdf
(1183 KB)
Inne foldery tego chomika:
2006
2007
2009
2010
2012
Zgłoś jeśli
naruszono regulamin