December 1957 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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"Whenever some science fiction movie or TV show is intended
to appear 'ultra-scientific,' or when a sponsor wants to clinch
his sales pitch with some phony-technical display, chances are
that an oscilloscope will be shown with a modulated wave pattern
jumping around on the screen." Funny that this was going on
back in 1957 when this article appeared in Popular Electronics,
and that it still occurs today. How many modern sci-fi movies
or techie action movies have you seen that still show Lissajous
patterns squirming around on oscilloscope screens? The only
difference is today's hoaxes have rectangular displays whereas
the old ones used round displays.
Even though this article is over 50 years old, it still has
some good pointers for newbie o-scope users. I'm guessing there
are even a few veterans out there who have never actually driven
the x- or z-axes of a scope themselves and seen what it does
to the display. Maybe now's a good time to give it a whirl.
Last month's article featured
square waves.
Oscilloscope Traces: RF Measurements
Wherever signals go "on the air" these tests will
tell you their nature
By Howard Burgess
Whenever some science fiction movie or TV show is intended
to appear "ultra-scientific," or when a sponsor wants to clinch
his sales pitch with some phony-technical display, chances are
that an oscilloscope will be shown with a modulated wave pattern
jumping around on the screen. Such 'scope traces are, as it
were, the popular image of scientific work.

Checking the carrier frequency of a portable transmitter
by the techniques described here will assure its proper functioning
when it is taken into the field. Perhaps this particular trace
has gained its popularity because it is so useful. It can be
immensely valuable to anyone working with a modulated carrier.
The carrier need not be one of a high power transmitter. It
can originate from a wireless record player, a serviceman's
signal traces can be obtained to well over one hundred megacycles.
One use of the oscilloscope which generally requires this
type of connection is the checking of frequency multipliers
in transmitters. Many of the modern transmitters, both amateur
and commercial, have one or more frequency multiplier stages
following the crystal oscillator. If the output of the transmitter
is to be on the correct frequency, each multiplier stage must
increase the frequency by the desired number of times.
Counting Loops. The output frequency of each stage can be
checked with a good calibrated wavemeter if one is available.
A double-check, and in many cases a better test, can be made
with an oscilloscope. This method makes use of the Lissajous
generator, a transmitter for opening garage doors or controlling
models, or a ham transmitter.
Direct Connection. Very few amateurs own
an oscilloscope with an internal amplifier able to pass the
higher radio frequencies. However, in most cases this problem
can be solved by feeding the r.f. signals directly to the deflection
plates of the oscilloscope cathode-ray tube. Most oscilloscopes
have these connections brought out to a small terminal board
in the rear of the 'scope's cabinet. With the signal fed directly
to the cathode-ray tube, pattern which is formed when two signals
of different frequencies are applied to the vertical and horizontal
plates of the tube. (See POPULAR ELECTRONICS, March, 1957, page
63.) If one set of deflection plates is coupled to the input
of the multiplier stage under test and the other set of deflection
plates is coupled to the output of the same stage (Fig. 1),
the pattern formed will indicate the number of frequency multiplications
taking place in the stage.
The pattern most likely to be formed will be found among
those shown in Fig. 2. They range from a multiplication factor
of 1 (shown at A) to factor of 4 (at D). For those who are not
familiar with this type of trace, examination of the figure
will show that the multiplication factor is found by dividing
the number of loops in the width of a pattern into the number
of loops in its height. All of these patterns are one loop wide.
In many cases the loops will not be as rounded and clear-cut
as shown here; however, it is the number of loops and not the
shape that matters in this case.
Modulation Check. Anyone that works with
electronic equipment for any length of time sooner or later
finds himself confronted with modulation problems. Even the
experimenter who is not a licensed amateur would like to be
able to check the modulation of stations that he hears. If precise
checking isn't necessary, the simple test to be described will
be adequate. The setup in Fig. 5 will usually do the job.
The horizontal input to the 'scope is coupled to the plate
of the last i.f. amplifier of the receiver through a small blocking
capacitor. This added load will probably require a slight amount
of retuning of the last i.f. transformer. The sweep generator
in the 'scope must be turned off. Now connect the vertical input
to the horizontal input through a 100,000-ohm potentiometer.
Then, with a good strong signal tuned in on the receiver, adjust
the vertical and horizontal gain controls of the 'scope and
the variable resistor until a circle is produced on the screen.

Fig. 1. The oscilloscope is coupled to the
input and output of a frequency multiplier stage in order to
check its operation.
Circle Check Pattern. If the signal is not
being modulated, the circle will be a sharp, well-defined pattern.
As soon as modulation is applied to the carrier, the circle
will alternately expand and contract to form a pattern similar
to a doughnut. As the percentage of modulation is increased,
the hole in the center will gradually close until it is completely
closed at 100% modulation. If more than 100% modulation is present,
a bright dot will form in the middle. The circle should move
out the same distance that it moves in toward the center.

Fig. 2. Scope patterns created by an r.f. stage in the hookup
described in Fig. 1: (a) without frequency multiplication; (b)
with frequency doubling; (c) with frequency tripling; (d) with
frequency quadrupling. These patterns provide a quick check
for frequency multiplier stages.

Fig. 3. The circular patterns produced by r.f. on the 'scope
screen are shown here for three different cases: (a) no modulation;
(b) 50% modulation, and (c) 100% modulation of the carrier frequency.
Caution: Before alienating your transmitting friends by criticizing
the signal they send out, make sure that the receiver you used
for this test is in good condition. A properly modulated wave,
upon passing through a poorly adjusted receiver, can appear
either over-modulated or undermodulated.
This system can also be used to check the output of signal
generators which are operating on frequencies too high to be
fed directly into the oscilloscope. Phono oscillators and similar
gadgets can be checked for percentage of modulation in the same
manner. The oscillator is tuned in on the broadcast receiver
and the 'scope connected to the i.f. system of the receiver
as outlined above.
Trapezoid Check Pattern. Another modulation
pattern which is formed in a manner similar to the circular
trace is that of the trapezoidal figure. This type of measurement
requires a direct power takeoff from the transmitter tank and
a sample of the audio used to modulate the transmitter. Because
of this, it is not suitable for measuring the modulation of
distant stations. The circular pattern serves that purpose.

Fig. 4. The trapezoidal pattern of r.f. as
it appears with: (a) no modulation, (b) 50% modulation, (c)
100% modulation, and (d) overmodulation of the carrier frequency.
However, for continuous monitoring of a transmitter, the
trapezoidal pattern is the simplest to obtain since it requires
no amplifier or sweep circuits. Connections are made directly
to the deflection plates of the cathode-ray tube of the oscilloscope
as shown in Fig. 6.
When the transmitter is not modulated, the .f. carrier will
produce only a vertical line. When modulation is applied, it
furnishes the sweep to form a triangle as in A of Fig. 4. If
transmitter is modulated less than 100%, the triangle will not
be filled out, as at B. Too much modulation will form a pinched-off
pattern, as at C, with a bright trailing line. If the sloping
sides of the triangle are curved rather than straight, the transmitter
under test is distorting due to nonlinear operation.
General Hints. There are many other r.f.
measurements that can be made with the oscilloscope which may
be discussed in future articles. Whenever the 'scope is used
for r.f., it is a good idea to keep leads short, with as little
shielding as possible. And don't jump at conclusions. You will
see a good many "queer" traces during your first investigations
but, with growing experience in interpretation, important clues
about the condition of your radio-frequency equipment will be
clearly revealed to you.
YYou will find that familiarity with radiofrequency traces
will come in handy on many occasions. Wherever signals are actually
sent out "on the air," it is necessary to check r.f. stages
both at the transmitter and at the receiver, as well as to
monitor the degree of modulation wherever the signal itself
is more complex than simple c.w.

Fig. 5. This simple circuit will usually
produce a circular pattern at an r.f. frequency.

Fig. 6. Coupling the oscilloscope to produce
a trapezoidal pattern on the screen.
Posted July 22, 2011