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 vintage
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 believe.
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 technicians.
Review of Fundamentals
Before we go on to consider the f-m receiver, a review of the differences
between amplitude and frequency modulation deserves consideration.
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 kc.
General Electric frequency modulation receiver undergoing comparative
listening tests while subjected to million volt lightning discharge.
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
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 for rarefaction.
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 sound.
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 the signal.
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 sets.
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
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 a.v.c. source.
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
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
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
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 by loading.
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 in modulation.
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
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 output voltage.
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