July 1941 Radio-Craft
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
Here is the first of a two-part article on frequency modulation
(FM). FM was a very welcome option
for entertainment radio listeners who had grown weary of static
mixed in with their music and syndicated adventure, drama, and comic
The Green Hornet,
Lights Out, and
The Life of Riley, respectively. Amplitude modulation
(AM) is susceptible to all sorts of
interference from car ignition systems, arcing in electric motors,
light switches being turned on and off, lightning, and a host of
other sources. A commercial radio with good noise and adjacent channel
rejection was relatively expensive. Permanent magnet speakers did
not become a standard feature for first few decades of radio
1941 vintage Crosley radio speaker for an example), so
the speaker coils themselves ended up carrying a lot of the same
static biases that the sound signals contained. Combine far-away
transmitters because of wide spacing between broadcasting facilities
with poor receiver sensitivity and the opportunities for interference
was large. FM solved most of the problem both because it inherently
was immune to amplitude modulation from noise and because by the
time it entered the commercial market, component and circuit design
had advanced considerably from the early AM days. As author
Raymond Guy puts it, "Frequency Modulation is a weapon against noise,
a sword if you please, with advantages which can be calculated accurately
An Engineer Analyzes the How and Why of Frequency Modulation
By special permission of the Association of Technicians,
Radio-Craft here presents an article on F.M., from the A.T.E. Journal,
which covers the engineering aspects of Frequency Modulation more
completely than any previously published in Radio-Craft, and does
it in a thoroughly understandable manner. Part I, presented here,
generalizes on the topic and discusses the results of measurements
made on the transmissions of N.B.C. Station W2XWG.
Raymond F. Guy
Radio Facilities Engineer, N.B.C.
Fig. 1 - "W2XWG's field intensity pattern in micro-volts/meter
(irrespective of the type of modulation). These measurements
represent a power of 1,000 W., a transmitting antenna height
of 1,300 ft., a receiving antenna height of 30 ft. and an
operating frequency of 42.6 megacycles."
There is widespread belief that many present-day developments
are fundamentally new within the last few years.
On the contrary, the bases of many so-called new developments
date back a great number of years. It is true, however, that only
recently has it been possible to utilize to fuller advantage the
possibilities of many of the ideas passed down to us by early investigators
of the Radio Art. New instrumentalities have made it possible to
explore these fundamental ideas to much greater extent. Transmission
and reception at ultra-short wavelengths, or ultra-high frequencies,
is one such outstanding example.
The use of the ultra-high frequencies for sound broadcasting
offers technical advantages, not only to the broadcaster but to
the public, which is much more important. The technical advantages
consist of (a) escaping the 10-kc. channel limitation, (b) getting
away from static. and (c) eliminating all except spasmodic long-distance
We've known this for years, have experimentally operated low-power
U.H.F. stations since Way Back, and have enjoyed the experience
of receiving Clean Stuff from our little ultra-high frequency transmitters
when QRN, with devastating wallops washed out our temporarily muscle-bound
60 kw. steamrollers. Five years ago the F.C.C. had applications
for, or had licensed over 100 ultra-high frequency transmitting
plants and it seemed that a trend was developing toward ultra-high
frequency broadcasting, but this trend was not sustained. Interest
has been revived in recent months through the promotion of F.M.
on the ultra-high frequencies.
What Does F.M. Offer?
Frequency Modulation is a weapon against noise, a sword if you
please, with advantages which can be calculated accurately and simply,
as we shall see. But unreasonable powers should not be attributed
to it. The pen should not be mightier than the sword.
Your scribe bows low and humbly attempts, with these hesitant
strokes, to bring to you gentlemen of the A.T.E. Journal what the
Lower Classes vulgarly call the Lowdown. A snack of ins ide dope.
Let's get to the point. What advantages does F.M. really give
over A.M.? Using the frequency deviation approved for the industry
by the F.C.C., F.M. Under the Optimum Conditions gives (a) an advantage
of 20 to 1 in background noise suppression, (b) an advantage of
at least 30 to 1 in rejection of shared-channel interference, depending
on the beat frequency, and (c) some advantage to the broadcaster
in capital expenditures and operating costs. There you have it.
F.M. In 1902!
One frequently meets laymen who have the mistaken idea that F
.M. is a revolutionary new invention. The justly proud father of
your profoundly humble scribe bought him his first lace velvet pants
in 1902. Most of you were still unborn during that antediluvian
It was in that year that a gentleman named Ehret applied for
a patent which was issued in 1905 covering the basic method of F.M.
for voice and code transmission and reception!
Mr. Ehret proposed to shift the carrier frequency by means of
a voice-actuated condenser. He proposed an off-tuned circuit in
the receiver for converting the frequency-modulated waves into waves
of varying amplitude.
With certain improvements these are the methods now used. For
code signaling he proposed to key the transmitter inductance or
capacity to change the carrier frequency. Before the No.1 war this
method was very widely used for many years on longwave transmitters.
Remember how .discombobulated one could become by trying to read
the back wave when fatigued?
"Wide Swing" F.M.
Frequency Modulation research. has been carried on for over 30 years
and, except for 1918, 1920 and 1924, patents have been issued on
F.M. methods and devices each year for the last 25 years. They were
granted mostly to a number of inventors in the employ of organizations
which spend large sums on research, such as G.E., Westinghouse,
A.T.&T. and RCA, and to a few individuals, particularly Major
Edwin H. Armstrong who has promoted use of the feature of "wide
swing" in F.M.
Other features are important in F.M. such as limiting. Gentlemen
named Wright and Smith filed a patent application covering, it 15
years ago. Fourteen years ago, and subsequently, patent applications
were filed and granted to Westinghouse. A.T.&T. and RCA on balanced,
or "buck-to-buck" F.M. demodulators. The most commonly used discriminator
today was patented by S. Seeley of RCA. Frequency multiplication
of an F .M. wave to increase the frequency shift is covered in patents
issued to Westinghouse, and G.E., for which applications were filed
in 1926 and subsequent years, High frequency pre-emphasis and de-emphasis
circuits were patented by S. Seeley and others of RCA. Its introduction
to the industry was due in considerable part to the efforts of N.B.C.
At the close of 1939 more than 250 patents had been granted on
either Frequency or Phase Modulation, of which more than 160 covered
F.M. About 10 years ago R.C.A.C. was trying F.M. on channels between
our East and West Coasts. About 12 years ago your scribe cooperated
with Westinghouse in F.M. tests between New York and Pittsburgh.
So you can see F.M. isn't new.
There is a popular impression that by use of F.M. and "wide swing"
the public may only now enjoy high fidelity. The facts are that
with ultra-high frequencies the fidelity can be made as good as
anyone wants it to be with either frequency or amplitude modulation.
Any improved fidelity is made possible by getting a way from the
10-kc. channel allocations of the Standard Broadcasting Band and
not by using F.M.
Furthermore, to get "high fidelity" in A.M. or F.M. receivers
the listener must pay exactly the same high price for high-power,
low-distortion audio amplifiers, loudspeakers and acoustical systems.
However, the time may come when High Fidelity will receive the widespread
recognition it merits. There is much more interest now in low receiver
prices which preclude high fidelity. This is unfortunate but incontestably
true regardless of any wishful or idealistic thinking to the contrary.
There is no lack of satisfactory fidelity in present-day transmitters
because, if for no other reason, the F.C.C. requires it. The loss
of fidelity rests in the home receivers. Medium-priced receivers
satisfy the public demand and high fidelity cannot be obtained in
those models. The price paid for so-called high-fidelity amplifiers
and loudspeakers is in itself more than the cost of most receivers.
Possibly 1 person in 6 has a receiver of good fidelity. Many of
these listeners normally operate with the tone control adjusted
for the lowest degree of fidelity possible with such receivers.
It appears that the public is not suffering any lack of fidelity
because of the present broadcasting system.
We in N.B.C., and others, have been providing transmission of
excellent fidelity for at least 15 years (network lines excepted)
and will continue to do so. We believe in it and endorse it. But
we have no illusions about the public reaction toward it.*
Frequency Modulation would under favorable conditions, but not
all conditions, reduce static about 20 to 1. But what static are
we talking about? Static practically doesn't exist on ultra-high
frequency. Therefore. isn't its absence mainly due to the shift
to the ultra-high frequency band? It is.
Don't think that your humble servant is bearish on F.M. because
that would be incorrect. It is cold professional realism, not bearishness.
An F.M. station will provide noise-free service to a much greater
distance than an A.M. station of equal power because F.M. can suppress
receiver hiss noise, auto ignition noise and other ultra-high frequency
disturbances about 20 to 1 if the carrier is stronger than the noise
and if the receivers have enough gain to make the limiters limit
at low field intensities. Some F .M. receivers begin to slack off
at about 100 microvolts. To obtain the full benefit of F.M. out
to the "noise threshold" limit they should hold up down to 10 micro-volts.
This noise threshold is strictly an F.M. phenomena; more on this
We are all confident that Television has a most brilliant future.
We are not entirely clear on the position that Ultra-High Frequency
Sound Broadcasting will have with respect to it. Those of us who
have lived with television for many years feel that sound is supplemental
to sight but definitely second in importance. When television hits
its stride, sound broadcasting may assume the status of silent pictures.
Who knows? Nobody does. In any event, sound broadcasting will be
with us for many more years and we should give full opportunity
to improved methods and devices. F.M. is one of them. N.B.C. has
one F.M. station and will build more. F.M. is being given its chance
to prove itself.
The N.B.C. has for many years viewed realistically the advantages
of the ultra-high frequencies and has been confident that the industry
would, in time, do likewise. Five years ago Mr. Hanson and your
profoundly humble scribe wrote a long report on the subject forecasting
the growth of ultra-high frequency Sound Broadcasting by 6-month
intervals and hitting very close. Frequency Modulation had such
promising theoretical advantages that we undertook a full-scale
field test to determine the extent to which they could he realized
$30,000 Worth of Tests
As a result we completed, last year, at a cost of over $30,000,
the most thorough field test of F.M. ever undertaken and we have
the information we sought.
It was obtained, not by laboratory work, which had been done
before by others, including R.C.A.C., nor merely by operating an
F.M. station, but by building special transmitters, receivers, measuring
instruments, etc., and then painstakingly making thousands of measurements
at distant points over many months and under a variety of conditions.
A special 1,000-watt transmitter was ordered from the R.C.A.M.
Company. It had facilities for both A.M. and any degree of F.M.
deviation or "swing" desired, with remote control facilities for
instantaneously switching to either system. Since the F.M. deviation
varies directly with the audio input level, remote controlled pads
could be and were used to select the deviation desired.
W2XWG was installed in the Empire State Building. Special authority
was obtained from the F.C.C. to use amplitude modulation as well
as F.M. on 42.6 mc. for the term of the project. The television
video antenna, having a pass band extending from 30 to 60 megacycles,
was used for most of the W2XWG transmissions although a special
folded dipole was used when the video antenna was transmitting "pictures."
W2XWG was equipped with means for continuous variation of power
between 1/10 watt and 1,000 watts, and a vacuum-tube voltmeter for
accurately measuring the power.
The modulation conditions selected were A.M./F.M. 15 (deviation
of 15 kc. or total swing of 30 kc.), and F.M. 75 (deviation of 75
kc. or total swing of 150 kc.). Tone modulation was used for most
measurements. For measuring distortion, or noise levels with modulation
present, the tone output of the receivers was cleaned up by passing
it through filters and then impressed upon RCA noise and distortion
Four special receivers were built by the R.C.A.M. Company for
this project. Each was equipped for instantaneous selection or A.M./F.M.
15 or F.M. 75. Two complete I.F. systems were built-in, one 150
kc. wide and one 30 kc. wide, each having 6 stages, with both A.M.
and F.M. detectors. All receivers contained meters, controls, de-emphasis
circuits with keys, 8-kc. cutoff filters with keys, separate high-quality
amplifiers and speakers, cathode-ray oscillographs, etc. Each receiver
had sufficient R.F. gain to give full output with limiting at input
levels much lower than required, theoretically doing so with only
1/10·microvolt input. These receivers were made as good as receivers
can be built in order that our conclusions on F.M, would not be
clouded by apparatus shortcomings. Sacrificing good receiver design
to price will not permit the full gain of F.M., as reported herein,
to be realized.
As a part of the project, a field intensity survey was made of the
W2XWG transmissions. The map is included herein for 1,000 watts,
1,300 feet antenna height, and 0.7 antenna gain. It is Fig.1.
Measurements and electrical transcriptions were made
under a variety of conditions at the following locations:
Collingswood, N.J .............................. 85 Miles
Hollis, L.I. ........................................
Floral Park, L.I. ................................. 15
Port Jefferson, L.I. ............................ 50
Commack, L.I. ................................. 36 Miles
Riverhead, L.I. ................................. 70
Hampton Bays, L.I. ........................... 78 Miles
Bridgehampton, L.I. ........................... 89 Miles
Eastport, L.I. .....................................
N.B.C. Laboratory ................................ 1
Bellmore, L.I. .................................... 23
All above stations are temporary, with the exception
of the last two, which are permanent.
Most of the measurements were made at the Bellmore station. For
the temporary stations, 2 automobiles were equipped and used, one
a Radio Facilities Group measuring car, the other a borrowed R.C.A.C.
truck full of recording gear. The receiving stations represented
a cross-section of rural and suburban Americana.
Let's next see what theoretical advantage F.M. has in noise suppression
and how it is obtained. Later we will see what we measured.
In. F.M. the deviation of the carrier frequency can be made as
great as desired. If it is 15 kc. and the audio bandwidth is 15
kc. the deviation ratio is 1, corresponding to the deviation divided
by the audio bandwidth. If the deviation is 30 kc. the deviation
ratio is 2, etc.
The advantages of F.M. over A.M. in noise suppression are contributed
by 3 factors:
(1) The triangular noise spectrum of F.M.
(2) Wide swings, or large deviation ratios.
(3) The greater effect of de-emphasis in F.M. compared to A.M.
Let Us consider them In order.
F.M. system with a deviation ratio of 1 has an advantage in signal-to-noise
ratio of 1.73 or 4.75 db. for hiss or other types of fluctuating
Since the figure 1.73 applies to such noises as tube hiss, which
is comparatively steady in amplitude, we will consider this type
of noise. It differs from impulse noise such as is produced by automobile
Tube hiss consists of a great many closely overlapping impulses
or peaks, There are so many of them at all audio frequencies, we
are concerned with, that the noise has a steady characteristic.
When combined with a steady carrier of fixed frequency, the noise
peaks beat with the carrier. The noise peaks also beat with each
other. When the carrier is considerably stronger than the noise
peaks, beats between the noise peaks become negligible in amplitude
and the predominating noise is due to the combination of carrier
and noise peaks.
Since a combination of 2 carriers differing in frequency produces
a similar phenomenon, we will treat both cases at the same time.
The effect is most easily shown and understood by means of a simple
The strongest carrier vector continuously rotates through 360°
and is indicated on Fig. 2. The weaker carrier, or the "noise voltage,"
rotates around the carrier vector at a frequency which is equal
to the difference between the desired carrier and undesired frequency.
It will be seen that amplitude modulation is produced. If the
undesired frequency is 50% as strong as the desired frequency, 50%
amplitude modulation results. As the undesired vector rotates around
the desired vector, phase modulation also is produced between the
limits A and B. The faster the undesired vector rotates, or the
faster the rate of phase change becomes, the greater becomes the
momentary change in frequency and therefore, the greater the frequency
modulation becomes, because Frequency Modulation is a function of
the first differential of phase modulation. Therefore, the amplitude
of the frequency modulation noise or beat note varies directly with
beat frequency. With both frequencies exactly the same there is
no amplitude modulation nor is there any frequency modulation.
Such being the case, the noise frequencies close to the carrier
produce little frequency modulation noise but as the noise components
further from the carrier combine with it they produce more frequency
modulation. Therefore, the higher the noise beat frequency the higher
its amplitude. This results in a frequency modulation noise spectrum
in which the noise amplitude rises directly with its frequency.
In other words, it is a triangular spectrum.
In amplitude modulation there is no such effect as this. All
noise components combine with the carrier equally. Therefore in
amplitude modulation there is a rectangular noise spectrum. The
ratio of noise voltages in F.M. and A.M. is therefore the ratio
between the square root of the squared ordinates of a triangle and
a rectangle. This ratio is 1.73 or 4.75 db.
an F.M. System the suppression of fluctuation noise is directly
proportional to the deviation ratio.
On Fig. 3 the A.M. noise spectrum corresponds to the total hatched
area below 15 kc. because the I.F. system would cut off there. The
F.M. 75 receiver I.F. system actually accepts noise out to 75 kc.
and it has the usual F.M. triangular characteristic. However, the
receiver output and the ear responds only to noise frequencies within
the range of audibility, around 15 kc., and rejects everything else.
Therefore, the F.M. 75 noise we actually hear corresponds only to
the small cross-hatched triangle and and the rest is rejected.
The maximum height of this F.M. triangle, corresponding to voltage,
is only 1/5th of the height of the A.M. rectangle. Such being the
case the F.M. 75 advantage is 5 to 1, or 14 db. Simple?
*See the first item under "Sound" in the June, '41, issue of
Radio-Craft, pg. 715. - Editor
Posted January 19, 2015