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June 1972 Popular Electronics
 Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Popular Electronics,
published October 1954 - April 1985. All copyrights are hereby acknowledged.
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If you need a little brushing
up on your basic single sideband (SSB) operational theory versus straight amplitude
modulation (AM), then let this dissertation by Mac to Barney be it. The piece was
the topic of "Mac's Service Shop" feature in the June 1972 issue of Popular
Electronics magazine. There are no circuit details, just talk about how power
from the carrier and dual sidebands is reallocated to a single sideband, thereby
improving efficiency. I like the 'dried milk' analogy Mac uses in reference to SSB
being transmitted sans carrier (i.e., water), with the receiver being responsible
for reintroducing the carrier in order to demodulate the signal. Although I cannot
personally comment as to its validity, many people familiar with comparing DSB AM
to SSB AM say there is a certain je ne sais quoi that is missing in the
tonal quality of SSB.
Mac's Service Shop: Single Sideband for the CB'er
 By John T. Frye, W9EGV, KHD4167
"Mac," Barney said to his employer working at the bench beside him, "I want to
ask you about the olden days."
"Do tell, Sonny," Mac answered, laying aside the diddle stick he was using to
adjust a sound trap on the color chassis in front of him and affecting the cracked
falsetto voice of old age; "how come and how 'olden'?"
"While I was eating at Burger Chef this noon, a young guy with a whip on his
car noticed my ham call license plates and pulled up beside me and began peppering
me with questions about the relative merits of single sideband as compared to amplitude
modulation. He was an avid CB'er and was thinking of going SSB."
"Well, you should have been a gusher of information. After all, you're yakking
it up on SSB all the time."
"That's just the point. Hams had already switched to SSB when I got my ticket.
I've never operated anything but SSB and a little FM on two meters. On the bands
I work, you hear very few AM stations. I felt like the man who, when asked how his
wife was, answered 'Compared to what?'"
"Did you confess you didn't know?"
"Are you kidding? A ham never admits ignorance to a CB'er. I told him I had to
get back to work but that I'd see him at the same place tomorrow and give him the
scoop. I knew you have been an avid shortwave listener ever since you heard Marconi
send his first message, and I figured you could fill me in."
"Thanks a bunch! But aren't you taking a chance in asking a senile old man for
information? It just so happens, though, that I was listening during the time the
big changeover from AM to SSB occurred on the ham bands, and I heard the pro and
con of both systems debated heatedly over and over again. I will not be surprised
if CB follows along much the same pattern in the next few years."
"I gather AM didn't give up easily."
"You gather correctly. There was a great deal of bitterness and name calling
when the first few SSB stations came on the air. AM operators sneeringly referred
to the new signals as 'Donald Duck' and 'slop bucket' modulation. The SSB boys retaliated
with scornful references to 'Ancient Modulation.' Each group accused the other of
putting out broad, interfering signals, and 'each group" deliberately interfered
with the other.
"Then the two types of modulation aren't very compatible."
Advantages of SSB. "Not very - at least not on the ham bands when you're receiving
unwanted SSB signals on an AM receiver, and vice versa. But before we go into the
"why" of that, let's talk about the claimed advantages of single sideband. The first
is talk power, and this adds up to a whopping 9 dB. Here's how.
"A very efficient 5-watt AM-CB transceiver might produce a 4-watt carrier without
modulation. On a panoramic receiver which displays r-f voltage on the vertical axis
of a scope tube and frequency on the horizontal axis, this carrier would be a single
vertical line rising from the base line at the carrier frequency. We can adjust
this line to a convenient one-unit length with the receiver controls. Now if we
modulate this carrier 100% with a 1000-Hz sine wave, we see two other vertical lines,
each 1/2 unit in length, spring up on either side of the carrier signal at a distance
of 1 kHz from it. The carrier line remains unchanged. However, if we simultaneously
examine the modulated envelope of our signal, we find modulating the carrier 100%
caused the peak envelope voltage to double on positive peaks and to fall to zero
on negative peaks.
"Our peak voltage is now 2 units, and since the power across the fixed antenna
resistance is equal to the square of the voltage, the peak modulated power is 2 x 2
or four times the unmodulated carrier power: 4 x 4 or 16 watts. The final
stage and power supply must be able to deliver this amount of power.
"The question is: how much of this 16 watts is actually talk power. Since the
only change we saw under 100% modulation was the appearance of the sidebands-the
carrier amplitude did not change at all - the talk power must be in these. Each
sideband was 1/2 unit high. That means each contained (1/2)2 or 1/4 the
carrier power or 1 watt. Their total was 2 watts.
 "Inasmuch as the carrier contributes nothing
to the intelligence of the signal and the sidebands do not actually need anything
to 'carry' them through the air, suppose we eliminate the carrier and divide the
power it wasted between the two sidebands. When half the cannibalized carrier voltage
is added to each sideband, each becomes one unit in length and their total power
becomes twice the original carrier power, or 8 watts. We now have a double-sideband-suppressed-carrier
signal with four times the talk power of the AM signal.
"But hold on! The two sidebands are as identical as the two faces of the god
Janus. They repeat, in unison, the same message. So why not eliminate one and use
the power saved to amplify the other? When we pick up one sideband and stack it
on top of the other, this lone voltage grows to two units in length. That means
the power of this single sideband is (2)2 x 4 or 16 watts, and it is
all talk power. The two watts of talk power of the AM signal has increased eight
times, or 9 dB, with the same power consumption."
"Hey, how about that!" Barney exclaimed.
"That's not all. Remember each sideband is separated from the carrier by a distance
equal to its frequency. The width of an AM signal, therefore, is twice the highest
modulating frequency. Voice modulation with frequencies up to 3,000 Hz means a 6-kHz
wide signal. But SSB, transmitting only one sideband, occupies only half this bandwidth.
Two SSB signals fit neatly into the 6 kHz taken up by a single AM signal."
What About the Carrier? "If we don't need the carrier, why did we get tangled
up with it in the first place?" Barney asked.
"I never said we didn't need it. I said we didn't have to transmit it. We need
the carrier for a reference at the receiver to recover the modulating frequencies.
You recall a 3000-Hz modulating frequency was converted into a radiated r-f frequency
removed from the carrier by ±3 kHz. Similarly all transmitted sideband
components are keyed to the original carrier frequency; that is, the frequency difference
between anyone of them and the carrier indicates the audio frequency producing that
particular component. Therefore we must have either the original carrier at the
receiver or another carrier of exactly the same frequency. An AM transmitter simply
sends along the original carrier. SSB utilizes the 'dried milk' technique: all the
water is taken out for shipping and then is replaced by the consumer to reconstitute
the original. In the same way the product detector of the SSB receiver produces
a carrier that can be inserted into the incoming sideband signal precisely where
the original carrier was. Difference beats between the components and this carrier
reproduce the original modulating frequencies."
" 'Precisely' is the right word," Barney offered. "If that inserted carrier is
off more than 50 Hz from the point the original carrier occupied in the signal,
voices do not sound right-and that means maintaining a frequency error of less than
two-parts-per-million at 27 MHz. Tuning errors you'd never notice on AM render SSB
unintelligible. However, since both transmitter and receiver on CB are crystal controlled,
I assume this presents no problem."
"Don't be too sure," Mac warned. "From the heterodynes I hear on a CB channel
when skip is coming in, it's evident that not all transmitters on the channel are
on the same frequency. CB crystals must be within 0.005% of the specified channel
frequency, but that means a permissible error of 1350 Hz at 27 MHz. A transmitter
operating at one end of this error limit and a receiver at the other would be 2.7
kHz apart. Some form of fine tuning is obviously a must for the receiver portion
of a SSB CB transceiver."
"Since a SSB signal is only half as wide as an AM signal, why does it seem wider
to AM operators?"
"Because of the AM receiver's slower attack type of avc and the wider passband.
The powerful pulsing type of signal from a SSB station overloads the front end of
these receivers that run at full r-f gain on weak signals. Reducing r-f gain and
using a beat frequency oscillator to insert a carrier makes it possible to receive
SSB on an AM receiver and gives a much more realistic idea of signal bandwidth."
"Don't forget an AM carrier puts a nasty heterodyne into a SSB receiver unless
that carrier is exactly zero beat with the SSB station," Barney pointed out. "No
wonder the two get along like cats and dogs. But SSB, with eight times the talk
power and taking up only half as much room, has a lot going for it."
"There's more. With stages in the SSB transmitter operating in a linear fashion,
there is less distortion to produce TVI causing harmonics. That does not mean, I
hasten to add, that SSB can't cause TVI by front-end overload of the TV receiver.
Also, the only time a SSB transmitter consumes appreciable power is when it is actually
being modulated. There is no carrier to waste a high percentage of full-modulation
power while you're thinking.
These make possible the use of smaller output tubes and lighter power supplies.
And there are no high-power audio amplifiers. You never saw any table-top kilowatts
until SSB came along. Finally, push-to-talk with full break-in is a natural with
the SSB mode of operation."
Any Drawbacks? "Doesn't SSB have any drawbacks? Why hasn't it caught on faster
on CB?"
"It has drawbacks. I've already mentioned the much more stringent frequency stability
requirements. Circuitry in both transmitter and receiver is more involved. These
things add to the expense. But the hobby type of operation of most CB operators
is the greatest deterrent. They don't want to talk just to their own units; they
want to talk to other CB stations, most of whom can't receive SSB. A SSB station
on CB at present is like a frog sending out a mating call in the desert: he doesn't
get many answers! Other stations can't understand his Donald Duck squawking. That's
the way it was on ham radio at first, but once hams grasped the advantages of single
sideband, that mode of operation snowballed."
Posted March 13, 2024
(updated from original post
on 8/22/2017)
Mac's Radio Service Shop Episodes on RF Cafe
This series of instructive
technodrama™
stories was the brainchild of none other than John T. Frye, creator of the
Carl and Jerry series that ran in
Popular Electronics for many years. "Mac's Radio Service Shop" began life
in April 1948 in Radio News
magazine (which later became Radio & Television News, then
Electronics
World), and changed its name to simply "Mac's Service Shop" until the final
episode was published in a 1977
Popular Electronics magazine. "Mac" is electronics repair shop owner Mac
McGregor, and Barney Jameson his his eager, if not somewhat naive, technician assistant.
"Lessons" are taught in story format with dialogs between Mac and Barney.
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