August 1941 Radio-Craft
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This is the second and final installment of an article on the topic
frequency modulation (FM) that
began in the July 1941 edition of Radio-Craft. Author Raymond
Guy, a radio facilities engineer at the National Broadcasting Company
(NBC), covers all the fundamentals
of FM 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
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 same-channel operation.
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.
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.
Fig. 5 - dB RMS signal-to-noise ratio at receiver output.
Fig. 6 - Average FM service broadcast ranges.
Fig. 7 - dB RMS signal-to-noise ratio.
Fig. 8 - dB peak noise below 100% - 100 cycle modulation.
Fig. 9 - Amount desired carrier exceeded undesired in
Fig. 10 - Adjacent-channel operation FM band.
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.
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
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 ke., 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 published.
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.
Operation of 2 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 de-emphasis. 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 F.M. 15.
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 undesired 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.
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 transmission in which the average
modulating level is rather low.
Posted January 19, 2015