August 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.
This is the second and final installment
of an article on the topic of frequency modulation (FM) that began with
in the July 1941 edition of Radio-Craft magazine. Author Raymond Guy, a
radio facilities engineer at the National Broadcasting Company (NBC), covers all
the fundamentals of FM (a relatively new concept at the time, invented by
Armstrong) not just from a functional circuits perspective but also pointing
out a broadcaster's concern for channel spacing and broadcasting ranges. Transmitter
pre-emphasis, receiver de-emphasis, noise thresholds, and adjacent channel and co-channel
broadcasting strategies are discussed here.
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, last month,
generalized on the topic and discussed the results of measurements made on the transmissions
of N.B.C. Station W2XWG; Part II (conclusion), presented here, describes transmitter
pre-emphasis and receiver de-emphasis, noise threshold phenomena, and simultaneous
Raymond F. Guy
Radio Facilities Engineer, N.B.C.
Fig. 4 - High-frequency pre-emphasis is used in the transmitter
and de-emphasis is used in the receiver to improve the signal/noise ratio. The importance
of this characteristic in ultra - H.F. broadcasting and its greater effectiveness
in F.M. are illustrated at left. Comparison is on the same scale of relative amplitude.
Fig. 5 - dB RMS signal-to-noise ratio at receiver output.
Fig. 6 - Average FM service broadcast ranges.
Transmission and reception on ultra-short wavelengths is not a new idea - not
even sound programs utilizing the technique of Frequency Modulation. The truth of
this statement was discussed in Part I of this article (Radio-Craft, last month)
in which it was shown that the basic method of F.M. for voice transmission and reception
was the subject of patents issued in 1905. From this general introduction, the writer
proceeded to discuss subsequent technical developments culminating in the wide-band
system of F.M. espoused by Major Armstrong. The pros and cons of such characteristics
as fidelity, noise threshold, triangular noise spectrum, deviation ratio, field
intensity, etc., upon which tests were made at a cost of $30,000 by NBC, were described.
We now continue with the further details of these tests, including discussion
of pre-emphasis, de-emphasis, F.M. noise threshold (the effects of ignition interference,
etc.), and the simultaneous operation of 2 F.M. stations on the same channel.
When the high frequencies are attenuated in a receiver, the high-frequency noise
is, of course, attenuated by the same amount. This may make a noisy signal more
pleasant to the ear, but it degrades the fidelity. However, if the high frequencies
are increased in amplitude in the transmitter, the overall fidelity will be restored.
Nevertheless the noise which comes in at the receiver remains attenuated and therefore
a reduction of noise results from this practice.
The use of a 100-microsecond filter to accomplish this purpose has been adopted
as standard practice in Television and ultra-H.F. sound broadcasting by the Radio
Manufacturers Association and recently by the F.C.C. It has actually been in use
for several years. A 100-microsecond filter is a combination of resistance and capacity
which will charge to 63% of maximum, or discharge to 37% of maximum in 100 microseconds.
It was shown that in F.M., the noise amplitude decreases as its frequency decreases
whereas in A.M. it doesn't. Therefore, de-emphasis is more effective in F.M.
Consider Fig. 4. The full rectangle at the left is the A.M. noise spectrum.
The full triangle at the right is the F.M. spectrum. The application of de-emphasis
reduces these areas to those combining the hatched and black sections. Squaring
those ordinates gives the black areas, corresponding to power, or energy. Extracting
the square root of the ratios of these black areas gives the r.m.s. voltage advantage
of F.M. over A.M. It is 4, corresponding to 12 db. Bear in mind that this 12 db.
includes the gains contributed by both the triangular noise spectrum and de-emphasis.
The spectrum advantage was 4.75 db. Hence the de-emphasis advantage is 12 db. minus
4.75 db. or 7.25 db.
All commercial F.M. receivers include de-emphasis and all F.M. transmitters include
pre-emphasis, It's an F.C.C. requirement.
Now let's sum up. We saw (Part I) that the F.M. noise spectrum advantage was
4.75 db., the de-emphasis advantage was 7.25 db. and the deviation ratio of "F.M.
75" was 14 db. Combining these gives us 26 db.
Let's now see what advantage we actually measured as part of the field test project.
Your attention is directed to Fig. 5 which has on it a great deal of information.
It actually condenses to one illustration much of the data we sought and obtained.
Many pages could be devoted to it. The curves may be extended to the upper-left
in parallel lines as far as desired. The actual field intensity of the noise can
be determined from the A.M. curve. For instance, for 10 microvolts at the receiver
terminals the A.M. signal-to-noise ratio is about 25 db. or 18 to 1. Hence the noise
is 1/18 of 10 microvolts, or 0.6-microvolt r.m.s.
The ordinates are identified in receiver input microvolts, microvolts-per-meter
and miles distance. Use the one you are most interested in. If you want condensed
distance tables refer to the bar chart, Fig. 6.
Compare the measured gains with the calculations we went through. They look to
be the same. They are. That means we found that the theoretical gain of F.M. can
be and was obtained in practice.
Note the dotted sections of the F.M. curves. They are dotted to indicate that
operation is not only below the "noise threshold" but is far enough below it that
a noticeable increase of noise results as soon as modulation occurs. The dotted
sections represent noise in the unmodulated condition. During modulation they break
even sharper than indicated. Since there is no such thing as a noise threshold in
A.M. there is no such break. Wherever usable A.M. entertainment service is provided
"F.M. 15" is 12 db. quieter and "F.M. 75" is 26 db. quieter.
Fig. 7 - dB RMS signal-to-noise ratio.
Fig. 8 - dB peak noise below 100% - 100 cycle modulation.
F.M. Noise Threshold
An interesting series of events takes place in a Frequency-Modulated system when
the noise peaks equal or exceed the peaks of the carrier. The result is a rapid
increase of the noise level or decrease of the signal-to-noise ratio with modulation.
In Frequency Modulation wherein the maximum swing is 150 kc. the point where
this begins to occur is reached when the unmodulated signal/noise ratio is about
60 db. When the unmodulated signal/noise ratio is less than about 60 db., or 1,000
to 1, the noise level rises with modulation, and as the noise peaks exceed the carrier
peaks by a considerable amount, this noise level may go up 20 db., or 10 times.
When operating above the threshold limit the noise changes very little as the station
is modulated. Below the threshold limit the effect is not unlike harmonic distortion
in an overloaded amplitude transmitter.
In Frequency Modulation of a lesser swing, such as 30 kc., a similar effect occurs.
In this case, however, the threshold limit occurs at about 35 db. signal/noise ratio.
Figure 7 shows the results of some of the measurements we made. In order that the
noise would not be confused with the small amount of inherent distortion in a practical
F.M. system, the measurements were made in such a manner that the effects of distortion
were eliminated. This was done by modulating the transmitter with a 17,000-cycle
tone and eliminating at the output of the receiver with a 14,000-cycle low-pass
filter, not only the fundamental modulating tone but all distortion products, leaving
only the noise.
This effect has no doubt been observed by many without being understood. It is
inherent in a frequency modulation system.
The noise threshold in the case of an "F.M. 40" system having a total band width
of 100 kc. occurs at about 43 db. Since this provides a very good signal/noise ratio
and the required band width is only 100 kc., F.M. 40 is believed by many to have
more overall merit than F.M. 75 when the comparative gains and limited space in
the allocation spectrum are considered.
So far as is known, the data on the F.M. threshold effect presented here, and
data published by Murray Crosby of R.C.A.C. constitute the only measured data ever
Figure 8 shows ignition noise measurements with peak noise input microvolts plotted
against peak signal to noise ratio, based upon the signal resulting from maximum
400-cycle modulation. The "F.M. 15" threshold is shown. The F .M. 75 threshold is
not shown because at the time the measurements were made A.C. hum within the system
made the accuracy of S./N. measurements in the 60-db. region uncertain.
It should not be assumed that peak S/N ratios of 20 or 30 db. are unusable when
the noise arises from ignition systems because it isn't true. The relative infrequency
of ignition peaks produces an audible result which is very deceiving. Ratios as
low as 10 db., while distracting, do not entirely ruin service as is the case with
It will be noted that the curves of ignition noise threshold flatten off at the
bottom. This is to be expected from the character of ignition noise. The impulses
are very short in duration, very high in amplitude and (relatively) widely separated.
They literally blank-out any small portions of the signal waves, without impairing
the remainder. The short, blanked-out intervals of the signal change little over
a wide range in noise peak amplitude. Once an ignition peak has risen to the value
required to control the receiver and blank-out the signal a further rise in the
noise level will not occur until the peak increases in breadth, or duration, or
until there is a sufficient rise in certain low-amplitude components of ignition
noise having fluctuation noise characteristics.
The peculiar shapes of such curves below the threshold values are due to the
wave shapes and crest factors of ignition noise, but they are also influenced by
the method of measurements.
Fig. 9 - Amount desired carrier exceeded undesired in dB.
Fig. 10 - Adjacent-channel operation FM band.
Operation of Two F.M. Stations on the Same Channel
By referring to the section covering noise interference it can be seen that the
worst condition of shared-channel operation occurs when both stations are un modulated
and a fixed beat-note, therefore, results. It will also be seen that the higher
this beat note the greater will be its amplitude. Figure 9 was made on the basis
of the worst conditions, which occur when the difference in carrier frequency reaches
approximately 5,000 cycles. Were it not for the effect of de-emphasis in the receiver
the beat-note amplitude would rise with frequency. However, de-emphasis of the high
frequencies prevents that from happening and the effect may be further understood
by referring to the section on pre-emphasis and deemphasis. It will be noted that
the noise on the desired station caused by the undesired station varies inversely
with the deviation ratio; F.M. 75 has a deviation ratio of 5 compared with 1 for
When either of the stations producing the beat-note becomes modulated, the beat-note
disappears because one carrier sweeps across the other one. When the desired station
is approximately 20 db. stronger than the un-desired station, interference and cross-talk
effects become unnoticeable. At 12 db. difference they are noticeable but it is
the opinion of some engineers that the 12-db. ratio would be tolerable. Frequency
Modulation offers a great advantage over Amplitude Modulation in the allocation
of stations on the same frequency. In A.M. the carrier amplitude of the desired
station must be 100 times, or 40 db. greater than the undesired carrier amplitude
for a 40-db. signal to beat-note ratio. For F.M. 75 it need be only 10 db., or 3
times greater. For F.M. 30 it need be only 17.5 db., or 8 times greater. For F.M.
15, it need be only 24 db., or 10.5 times greater.
The result is that F.M. stations can be located much closer geographically, and
therefore many more station assignments can be made per channel. All interference
due to sky-wave transmission from distant stations is automatically rejected in
F.M. because the interfering signals never reach the high amplitude required. This
is not so in A.M. transmission.
Figure 10 shows the results of adjacent-channel measurements using one of the
RCA Field Test receivers and 2 commercial models of other manufacture. It should
be noted that the undesired station was modulated with fixed tone of uniformly high
modulating level. As a result the interference was probably somewhat more severe
than would be the case for program trans-mission in which the average modulating
level is rather low.
Posted March 16, 2023
(updated from original post