June 1972 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
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. 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
"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
"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
" '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
"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 August 22, 2017
Mac's Radio Service Shop Episodes on RF Cafe
This series of instructive stories was the brainchild of no other than John T. Frye,
creator of the Carl and Jerry series that ran in
Electronics for many years. Mac's Radio Service Shop began life in
Radio & Television News magazine (which
itself started as simply Radio News), and then changed its name to Mac's Service
Shop after the magazine became
World. 'Mac' is electronics repair shop owner Mac McGregor, and Barney is his eager,
if not somewhat naive, technician assistant. 'Lessons' are taught in story format with dialogs
between Mac and Barney.