Aug/Sep 1940 National Radio News
of Contents] These articles are scanned and OCRed from old editions of the
National Radio News magazine. Here is a list of the
National Radio News articles I have already posted. All copyrights are hereby acknowledged.
1940 was a big year in the commercial broadcast industry because
it was when the FCC began licensing stations for FM operation. Amazingly,
that was only four years after Edwin Armstrong first came up with
his frequency modulation scheme - fast moving for the government.
Simultaneously, equipment manufacturers were cranking out transmitters,
receivers, antennas (new frequencies),
writing installation and operation guidelines, training servicemen,
and doing scores of other vitally important tasks. The advent of
FM was considered a very significant technical improvement because
of immunity to electrical noise interference. If for no other reason,
you should look at this article to see the photo of the megavolt
artificial lightning discharge created to test and demonstrate FM's
tolerance of such phenomena.
See all available
Radio News articles.
F-M Receivers and Their Alignment
By Joseph Kaufman
N. R. I. Director of Education
With about thirty frequency-modulated (f-m) broadcasters in operation
and one hundred station permits granted, and with the F.C.C. formally
acknowledging that f-m signals will serve public interests, localized
radio broadcasting enters a new spectrum, namely 42 to 50 megacycles.
Even the most enthusiastic admit that with forty-five million
amplitude-modulated (a-m) receivers in use today in these United
States, wide public acceptance of f-m will take from five to ten
years. At this early date, well-informed receiver merchandisers
say that the evils which f-m is said to overcome are not as "wicked"
as most people would like to have us believe; hence they argue that
f-m may not be the "whirlwind" that initial publicity leads us to
However, it cannot be denied that f-m provides an almost noise-free
signal, to a degree not attainable with the a-m system. To be sure,
local man-made interference has been greatly reduced in our present
system, but atmospheric and lightning disturbances still affect
a-m receivers. With f-m transmission, however, reception can be
noise-free even during local electric storms. Its victory over noise
is the greatest appeal, but f-m can provide high fidelity with depths
of volume never before considered feasible for a-m.
Whether the public as a whole wants true fidelity and natural
reproduction is still a highly debatable subject; a great deal of
evidence indicates that after years of ear subjugation to false
reproduction, broadcasting has developed an ear for reproduction
peculiar to radio.
How f-m will be accepted by the public, only time will reveal;
it is here, however, and offers a real opportunity to trained radio
Review of Fundamentals
Before we go on to consider the f-m receiver, a review of the
differences between amplitude and frequency modulation deserves
With amplitude modulation, a basic r.f. signal called the carrier
is increased and decreased in amplitude, in accordance with the
sound intelligence that is to be conveyed. The basic r.f. signal
amplitude is never increased more than twice the carrier level,
and never reduced to such an extent that an r.f. signal does not
exist for an instant. Amplitude modulation produces side frequencies
with the highest audio frequency determining the band width. For
example, if a 10-kc. audio signal is the limit, modulated on a 1,000-kc.
carrier, the side frequencies will extend from 990 kc. to 1,010
General Electric frequency modulation receiver undergoing
comparative listening tests while subjected to million volt
With frequency modulation, the amplitude of the r.f. carrier
remains fixed at all points in a given communication system. When
no sound is being transmitted, the frequency of the signal is a
definite value which is often referred to as the "resting" frequency.
This frequency is increased and decreased in accordance with the
level (or volume) of the sound being transmitted.
Let us look at it this way; sound is the result of condensation
and rarefaction of air particles. Condensation results in a dense
group of air particles, and rarefaction results in a below-normal
amount of air particles. We could arrange to increase the radio
frequency for conditions of condensation, and decrease the frequency
There must be a limit to the frequency swing from the resting
value, depending on the maximum sound level intended, and this range
is referred to as the frequency deviation. Thus, for the loudest
sound to be transmitted, the swing could be limited to 75 kc. Since
the frequency is varied above and below the resting frequency by
this value, the total deviation will then be 150 kc. For example,
if the resting frequency is 43,000 kc., for the loudest sound the
frequency will swing from 42,925 kc. to 43,075 kc. Should this loudest
sound have a 1,000-cycle pitch, the r.f. signal will vary from 43,000
to 42,925 to 43,075 and back to 43,000 kc., one thousand times in
a second. If this 1,000-cycle sound has a lower level, the swing
could be from 42,995 to 43,005, one thousand times a second.
In frequency modulation, the instantaneous frequency corresponds
to the sound level at that instant, and the rate at which the frequency
is varying above and below the resting value is the pitch of the
As far as fidelity of transmission is involved, the deviation
can be any value; in fact, equally as good fidelity can be obtained
with an overall deviation of 20 kc. as with 150 kc.
For maximum elimination of noise. however, a large frequency
deviation is desirable. A noise pulse received along with the f-m
signal affects the instantaneous amplitude by creating peaks on
the r.f. signal, and also affects the instantaneous frequency of
As we will see later. the amplitude peaks of noise are removed
by the "limiter" in the f-m radio receiver, but any instantaneous
change in the signal frequency will introduce volume pulses after
the f-m signal is converted to amplitude changes. If a large frequency
change is employed to produce an appreciable change in sound level,
the frequency change due to a noise will normally have little effect
in producing noise interference. On the other hand, if full range
in volume is produced with a small frequency deviation, the frequency
change produced by noise pulses will be quite apparent.
Essential Stages in an F-M Receiver
Once you understand the basic principles involving f-m receivers,
you will find these new sets are no more difficult than ordinary
As you will shortly see, no radically new circuits are used in
a f-m receiver. Conventional vacuum tube circuits, designed to meet
special requirements are predominant. The superheterodyne circuit
is employed, usually with a stage of r.f. ahead of the frequency
converter, and with one or more i.f. stages following the converter.
After sufficient amplification has been obtained, a stage which
will convert f-m to a-m is required. This modulation converter must
be followed by a normal amplitude type of detector.
There is, however, a modulation converter which also detects
at the same time. The discriminator circuit used in automatic frequency-controlled
receivers will produce positive and negative voltages, the instantaneous
voltage depending on the deviation in frequency from the reference
frequency. Thus, f-m can be converted directly to audio signals
by an a.f.c. discriminator circuit.
Between the frequency discriminator and the last i.f. stage,
a special tube circuit (called the limiter) is introduced. Although
its elimination would not prevent f-m reception, its use definitely
results in the unique features which make f-m transmission superior
to a-m. A limiter removes all amplitude noise pulses, so the discriminator
output contains negligible noise signal. The limiter keeps all signal
amplitude levels below the permissible swing of the discriminator,
thus preventing amplitude distortion. The discriminator also operates
so as to favor the desired (stronger) signal and suppress the weaker
undesired signal. Finally, the limiter supplies a negative d.c.
voltage proportional to carrier intensity, hence it is used as an
Following the discriminator is a potentiometer which serves as
a volume control; its output feeds into a conventional a.f, amplifier
and loudspeaker. For high fidelity, both the a.f. amplifier and
loudspeaker must be designed to have essentially uniform response
over a wide range of audio frequencies. The loudspeaker system usually
consists of a low-frequency unit and a high-frequency reproducer,
acoustically compensated for high-fidelity reproduction.
For some time, f-m receivers will also include circuits for a-m
reception. The preselector and oscillator coils for any a-m band
will be switched into the circuit; the same switch will also switch
in the 42-50 megacycle coils, and in all probability will also introduce
the low-capacity variable condensers required for band-spread tuning.
Such a combination f-m and a-m circuit is shown in Fig. 1.
In each i.f. section of the circuit in Fig. 1, the secondary
of the i.f. transformer for a-m (usually about 456 kc.) will be
connected in series with the secondary of the f-m i.f. transformer
(usually about 1 to 7 megacycles). By connecting the secondaries
in series and by preventing any mutual coupling, one i.f. transformer
will have negligible impedance at the other frequency, hence its
presence will not interfere with amplification of the desired frequency.
The primaries of the a-m and f-m i.f. transformers will be switched
into the plate circuit of the tube to which they are connected,
as shown in Fig. 2. If this is not done, transfer through both transformers
would exist, and the selectivity of the circuits would be lost.
The i.f. channel for f-m will employ a number of stages, only
one of which is shown in Fig. 2. This i.f. channel will feed into
a limiter-discriminator type of detector for f-m. The input of the
a.f. amplifier will be switched from the f-m detector to the a-m
detector according to the type of reception desired.
F-M Antenna System. The all-wave antenna used for a-m signals
is not designed for high-frequency radio waves. An additional antenna
will be desirable. Half-wave antennas are being widely used; these
are connected directly to the input terminals of the receiver, and
these in turn connect to the primary of the 42-50 mc. antenna coil.
Two antennas will thus be normally used for f-m and a-m reception,
as shown in Fig. 1.
Fig. 1. Preselector
and frequency converter stages of an a-m and f-m combination receiver.
Any of the antennas found acceptable for television reception
can be used. Reflection will not be a problem. The horizontal antenna
should preferably be faced broadside to the f-m transmitter and
adjusted for the least phase cancellation, so the strongest signal
possible is accepted. A horizontal di-pole about 11 1/2 feet long
will be required.
Preselector-Frequency Converter. A stage of amplification preceding
the mixer-first detector is to be expected in an f-m receiver in
order to over-ride converter noise. The tuning condenser will have
a low capacity so the L-C ratio of the tuning circuit will provide
high gain. A remote cut-off pentode tube will be used and will be
a.v.c.-controlled. Sufficient selectivity must be embodied in the
preselector to eliminate image interference and i.f. signal interference.
Fig. 2. Type 6AC7
tube serving as i.f. amplifier tube for both a-m and f-m reception;
primary switching is employed.
Since the output is through independent
a-m and f-m channels, switching is required only at the input
of the combined
i.f. amplifier and at the input of the a.f. amplifier.
A conventional pentagrid converter may be employed in the converter
stage. although a triode-pentode tube combination will serve equally
well. The local ocillator should be stable, so that the i.f. amplifier
and the discriminator characteristics need not be made too broad.
F-M Intermediate Amplifiers. These will be of the conventional
double-tuned type, carefully designed for optimum coupling so that
a resonance curve with a flat top and steep sides will result. As
we will see later, perfectly flat tops are not required for strong
signals, as the limiter will give the entire r.f. system a flat-top
resonance characteristic. However, sufficient broad response will
be required so that amplitude distortion does not arise for weak
signals. To get sufficient broadness in tuning, either the primary
or the secondary of the i.f. transformer will be loaded with a resistance,
about 10,000 ohms. Because of loading and the use of a high i.f.
value, about 1 to 7 mc., low gain will result and at least two i.f,
tubes will normally be required.
Limiter. This stage, important as its functions are, is a simple
tube circuit. An ordinary pentode tube, operating at low plate and
screen grid voltages (about 60 volts) and with no initial grid bias,
is used. Grid current flows upon application of an r.f. signal,
and this rectified current is made to flow through grid return resistors
R1 and R2 as shown in Fig. 3.
Using low plate voltage causes the plate current to cut off at
low negative grid voltage values. The upper limit of plate current
is also kept low by the low plate current, for the space charge
readily prevents the flow of electrons to the plate when low accelerating
potentials exist; excessive. grid current contributes to low plate
current. As a rule, this circuit is designed so that the limiter
output does not vary more than 1 volt for all input r.f. voltages
above the saturation value.
Fig. 3. Typical
limiter and frequency discriminator circuit.
While the limiter circuit prevents excessive rises in the amplitude
of the current output, the grid current in the limiter stage causes
the operating point to move more negative from the no-signal point
shown as a in Fig. 4. The operating point may assume a position
such as b, c or d as the input level of the signal increases. The
normal operating point will be somewhere between c and d, a condition
for high signal input. In such a case, the r.f. signal will undergo
half-wave rectification, and plate current will flow for half a
cycle or less. Since the plate load of the limiter (Fig. 3) will
be a resonant circuit, the voltage developed across the tank circuit
will have both alternations of the cycle with the tuning circuit
possessing the ability to sustain oscillation at its resonant frequency
by virtue of the energy stored in this circuit.
If noise pulses make the input r.f. signal swing positive beyond
point x in Fig. 4, the pulses will be removed by the saturation
effect of this limiter; negative noise peaks will be removed by
the cut-off characteristic of the limiter. Of course, this only
occurs when a strong signal is received, and the a.f. system is
hence designed to load the limiter fully for the weakest signal
to be received.
If the desired signal fully loads the limiter, causing the operating
point to be more negative than point c (see Fig. 4), a weak signal
entering the limiter will either cause no plate current variation
or will reduce the plate current variation to such a low amplitude
that the limiter output resonant circuit will not receive enough
energy to sustain this oscillation. It is possible to design the
limiter so that a desired signal which has twice the amplitude of
an undesired signal, both operating at the same resting frequency,
will so completely over-ride the undesired signal that the latter
will not be heard.
The flow of grid current through grid resistors R1
and R2 in Fig. 3 produces a voltage across the resistors
which self-biases the limiter to cut-off or beyond for normal and
above-normal signal levels. This negative voltage may, in some receivers,
be used to operate an electronic tuning eye,* or to feed an a.v.c.
voltage to any of the i.f. or r.f. stages which it may be desirable
to control. The resulting a.v.c. action will prevent the limiter
from being overloaded too much on very strong signals.
In passing through the r.f. amplifier of the f-m receiver, the
signal is varying in frequency above and below the resting frequency.
If the r.f. system is sharp, as shown by curve 1 in Fig. 5, the
signal amplitude will vary from a to b to c to d to e and back to
a for one audio cycle. To prevent such extreme variation in amplitude,
the frequency deviation would have to be limited so the swing would
be from y to z, or the r.f. system would have to be made much broader
Fig. 4. Limiter characteristic curve.
Fig. 5. How the limiter flattens the r.f. response.
Fig. 6. The S curve of a frequency converter.
As was previously, pointed out, the limiter in itself causes
the over-all response of the r.f. system to act broad. If the level
of the signal is so proportioned that all signal amplitudes above
b (amplitude greater than x in Fig. 4) are in the saturation region
of the limiter, then all such peaks will be removed. The r.f. amplifier
and the limiter together will then have the resonant response portrayed
by curve 2 in Fig. 5. The f-m signal may thus embody wide deviation
The r.f. system is never made too sharp for signals which do
not drive the limiter to saturation. Weak signals do not benefit
by this action; to receive them with good response, the r.f. and
i.f. amplifiers must be reasonably broad.
Frequency Discriminator. This circuit does not differ from the
circuit used in automatic frequency control. except that it is designed
for the i.f. value used in f-m receivers. Such a circuit is shown
at the right in Fig. 3.
Note that two diode rectifiers are used, each diode being fed
with one-half the voltage of the final resonant circuit. Being a
split secondary connection, one diode input r.f. voltage is 180°
out of phase with the other diode input voltage. At the same time,
both diodes get the full r.f. voltage which is present at the plate
of the limiter. When the frequency is off the resting value, as
it is during transmission of intelligence, the phase relationship
between the r.f. voltages acting on each diode varies. As a result
one diode gets more r.f. voltage than the other, and the rectified
d.c. voltages differ. The difference in d.c. voltage is the f-m
demodulated signal; its amplitude is proportional to the amount
the signal frequency differs from the resting value and its polarity
depends on whether the signal is above or below the resting value.
Thus, while the f-m signal is varying in frequency due to modulation,
the net d.c, voltage across R3 and R4 in Fig.
3 is changing in amplitude, with point m becoming alternately positive
and negative with respect to ground. Condenser across R3
and R4 remove all r.f. components.
The discriminator must be designed so that increases and decreases
in frequency from the resting value produce proportional changes
in d.c. output voltage, as shown in Fig. 6. This linearity must
extend for the full deviation in frequency. It is important to standardize
on the maximum deviation that will be used, and design the discriminator
accordingly. In fact, the discriminator should be able to handle
even a greater deviation, to take care of the normal drift in the
frequency of the receiver oscillator.
By designing the discriminator for wide frequency deviation,
this stage will function for f-m signals with low frequency deviation.
A discriminator designed for a narrow frequency deviation would
distort when an f-m signal with a wide deviation was received.
Alignment of F-M Receivers
Alignment of an f-m receiver will differ somewhat from the procedures
used for a-m receivers. It may surprise you to learn, however, that
this alignment can be done with standard servicing equipment having
First, the discriminator will be lined up, A high-resistance
d.c. voltmeter, preferably a vacuum tube voltmeter, is connected
across one diode load resistor. To introduce a signal, connect the
service signal generator to the grid-chassis of the limiter tube.
The signal generator should be set exactly to the i.f. value for
f-m, and its output should be as high as possible, about 1 volt.
Adjust the primary of the discriminator transformer for maximum
output. Now connect the d.c, voltmeter across both diode loads,
and adjust the secondary of the discriminator transformer for zero
To align the resonant circuit ahead of the limiter stage, connect
the signal generator (still set at the i.f. value for f-m ) to the
grid-chassis of the stage ahead of the limiter. A 0 to 100 micro-ampere
meter can be connected in the grid return of the limiter, or a high-resistance
voltmeter (or v.t.v.m.) can be connected across the grid return
resistor which produces the a.v.c. voltage. Adjust the resonant
circuit ahead of the limiter for maximum deflection.
When a peak reading is obtained, the output reading should be
high enough to indicate that the limiter is being saturated. To
do this, the signal generator output should be set up to a high
output value. This can be checked by noting the output across one
diode load in the discriminator; increased input to the limiter
should show little rise in output voltage. This condition is essential,
for it is necessary to have the same loading of the limiter on the
resonant circuit as would exist in normal operation. This loading
affects the response of the resonant circuit. If you align this
circuit with little load, a different peak setting will result.
Advancing the signal generator one stage at a time aligns each
resonant circuit for maximum limiter grid current or self-rectified
d.c. voltage. The i.f. channel for f-m will be aligned when the
signal generator is connected to the input of the mixer-first detector,
Next is the alignment of the preselector and oscillator. For
this adjustment, the signal generator is connected to the two antenna
posts. The oscillator is always aligned first, and the preselector
is adjusted for maximum grid current or voltage in the limiter.
Alignment will, of course, depend upon the type of tracking employed.
One method worth mentioning involves the iron-core coil in the oscillator.
The signal generator and the receiver dial are set at a low frequency
(about 42 to 43 mc.), and the oscillator core aligner is adjusted
for maximum output. Then the signal generator and receiver are set
to a high frequency (about 49 to 50 mc.) and the trimmer shunting
the oscillator variable condenser is adjusted for maximum output.
*An f-m receiver should be tuned for least
noise, not for maximum sound level. An electric eye working on peak
limiter grid current offers an excellent tuning indicator.
Posted March 21, 2014