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Dec. 31, 1946.
2,413,543
W. L. CARLSON '
REDUCTION OF SELECTIVE FADING D_ISTORTION
Filed Sept. 22, 1943
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. WENDELL L
BY
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,1’ 2,413,543
Patented Dec. 31, .1946
UNITED STATE
2,413,543
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REDUCTION OF sELEcnvE
DISTORTIQN
_
_1 "
.
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Wendell L. Carlson, Princeton, N. J ., assignor_ to 1’
Radio Corporation of America, a corporation
of Delaware
I Application September 22, 1943; Serial No. 503,465
5 Claims. (01. 250-20)
1
'
~
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2
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,
,
diatefrequency networks thereof, and, which
commonv path has a 'pass band which is suffi
My present invention relates generally to sys
tems for reducing selective fading distortion.
In areas fairly distant from standard broadcast
ciently wide to pass each intermediate frequency
stations ' (530—1700 kilocycle broadcast band)
with its modulation side bands.
distortion occurs by virtue of the well-known
phenomenon known as “selective fading.” Dis
tortion of this type is actually a distortion of
audio frequency modulation due to interference
of sky and ground waves at a particular signal
wave collector of a receiver‘. Selective fading 10
- distortion is also encountered on the frequency
‘
Still other ‘objects of my invention are to im
prove generally the efficiency and reliability of‘
systems for reducing distortion accompanying:
selectivefading, and more especially to provide:
methods ,of reducing selective fading distortion
whichv are economical so far as apparatus is concerned.
-
The novel features which I believe to be char»
bands higher than the broadcast frequency range.
acteristic of my invention are set forth with.
One important object of my invention is to
particularityin the appended claims; the invenprovide means for automatically rejecting a dis
torted signal and to select an undistorted signal. 15 tionitself, however, as to both its organization.
and method of operation will best be understood‘,
Another important object of this invention is
by'reference to the following description taken:
to provide a novel method of reducing selective
in connection with the drawings in which I have
fading distortion in receivers, wherein vcommon
indicated diagrammatically circuit organizations.
audio frequency modulation signals are trans
mitted on‘ adjacent channel carriers, the recep 20 whereby my invention may be carried into ef
tion of such adjacent channel carriersbeing car
fect.
ried out in a system having a pass band wide
-
'
~In the drawings:
'
,
Fig. 1 schematically shows'a transmitter sys
enough topass both carriers up to the demod
tem adapted to radiate a pair of adjacent chan
ulator whereby when one carrier fades substan
tially below the strength of the other carrier, 25 nel carriers of common modulation,
‘ Fig.- 2 shows in a pictorial manner the mod
then the efficiency of recti?cation is reduced for
ulated carriers radiated by the system of Fig. 1,
the ‘fading carrier and the audio frequency out
Fig. 3 schematically shows a receiver adapted
put from the fading signal is abnormally reduced.
to receive the'radiated waves from the system of
Still another important object of my invention
'
.
is to provide a method of reducing distortion due 30 Fig. 1,
> Fig. 4 shows the pass band curve‘ of a receiver
to selective fading which is based on the ob
constructed in accordance with my invention,
served phenomenon that at any given reception
Fig. 5 schematically‘ illustrates a modi?ed em
point where skyand ground waves from a given
bodiment of the invention wherein a single chan
transmitter are of the same order of magnitude,
v
'
'
combining these waves in, two different phase 35 nel carrier is received.
Referring now to the accompanying drawings,
relations will» cause one of the resulting waves
wherein like reference‘ characters in the di?er_
to be undistorted and strong while the other is
ent'?gures designate similar circuit elements, I
distorted and weak’. The method employed in
volves conversion of the two waves into two dif
have shown in Fig. 1 in purely schematic man
ferent intermediate frequency waves, and passing 40 ner a~transmission system adapted to be em
ployed in one method of reducing distortion
them through a wide transmission path to a de
caused by selective fading. In this transmitter
tector, or to employ separate detectors for the
two waves with means for increasing the dif
system a pair of master oscillators I and 2 is
ference in amplitudes of the signals impressed
utilized. >These oscillators are constructed and
on the detectors.
'
45 designed to'produce oscillations having frequen-‘
cies on adjacent frequency channels. _Thus, os-.
cillator' l operates at'frequency F1 whereas os-.
cillator 2 operates at a frequency F2. Assuming
that the transmitter is producing modulated car_
their planes at right angles to each other; each
of the loops feeding separate converter networks 50 rier waves in the standard broadcast, band, it
so as to provide intermediate frequencies locat
will be understood that F1 and F2 will be the
A more speci?c object of one‘ form of this in-'
vention is‘ to provide a broadcast receiver with
a pair of loops which are oriented so as to have
ed‘on adjacent frequency channels but having
common modulation signals thereon, the de-'
modulator ofrthe system having a common trans?
misslon‘path from the pair ‘of parallel interme‘i
carrier frequencies of immediately adjacent
channels. These channels are normally spaced
10 kilocyclesikc.) apart.
‘
' -~
The’ source of audio‘frequency modulation 3 ‘is
2,413,543
4
tion side bands. In designing the receiving sys
tem in Fig. 3 it is to be understood that each of
the selector transmissionnetworks from the sig
nal collector 6 to the input terminals of demodu
utilized concurrently or synchronously to modu
late the oscillations produced by each of oscil
lators l and 2. Thus, the modulator 4 is employed
to modulate the oscillations of oscillator l in ac
cordance with the modulation signals of source 3.
The modulator 5 is adapted to modulate the os
lator I!) should have a pass band width which
will be sufficiently wide to pass a radio frequency
cillations of oscillator '2 in accordance with, the
modulation signals of source 3.
band 20 kc. wide.
The separate
.
The demodulator I 0 may be of any well-known
form.
modulated carrier energy of oscillators l and 2
That is to say, it can be a diode detector,
may then be transmittedto one or more separate: 10 a grid leak recti?cation type of detector, or a plate
circuit recti?cation form of detector. It is a well
stages of ampli?cation, and ?nally the separate
known fact that with such detectors the stronger
modulated carrier waves are radiated from sepa
of two signals will dominate, and that the weaker
rate radiators. There is shown in Fig. 2 in purely
- signal will appear in the detector output only as
illustrative manner the relation which exists be
tween the modulated carrier waves which are 15 frequencies resulting from the heterodyne beat
radiated from the separate radiators of the sys
tem of Fig. 1. It will be seen that there is a 10 kc.
from reaction with the stronger carrier. These
beats will in this case all be of frequencies above
spacing between the carrier frequencies F1 and
F2, whereas the modulation side band components
pass band of the audio system employed. As one
5,000 cycles, and, therefore, will be outside the
are exactly the same for both carrier frequencies.
At the receiver, which is schematically repre
sented in Fig. 3, there is shown a signal collector
device 6 which may be the usual grounded an
tenna circuit. The receiving system is repre
sented as being of the superheterodyne type, since 25
signal intensity falls, the detector efficiency falls
at a rapid rate. This has the signi?cance that
when one Of the signals F1 or F2 falls substan
tially below the strength of the other signal,
then the e?iciency of recti?cation is reduced for
the fading signal and the audio frequency output
derived from the fading signal is abnormally re
the latter type of receiver is practically uni
duced. It will now be appreciated that a simple
versally used to receive radio signals in the stand
and effective manner .of reducing distortion
ard broadcast band. However, it is to be clearly
caused by selective fading is readily provided by
understood that any other type of receiver, such
as one of the tuned radio frequency ampli?er type, 30 my invention. Let it be assumed that the re
or the superregenerative type, may be employed.
ceiver of Fig. 3 is located at such a distance from
the transmitter of Fig. 1 that severe selective fad
Assuming that the receiving system is of the su
ing occurs in that receiving locality.
perheterodyne type, there is employed the usual
?rst detector or converter 1 which has a tunable
If the ground wave and sky wave of carrier F1
input circuit so that the receiver may be tuned to
frequencies F1 and F2. The usual local oscillator
8 is also tunable, and the locally produced oscil
cancel at the position where collector 6 is located,
it is most unlikely that the same cancellation will
lations of a predetermined frequency are applied
to the ?rst detector 1. In the output of the latter
occur for carrier F2. Indeed, actual observation
demonstrates that simultaneous selective fading
of adjacent carriers is most improbable. Hence,
there is produced the intermediate frequency (IF)
if, for example, carrier F1 should fade sufficiently
energy, which may be ampli?ed by one or more
so that detection thereof at the demodulator ID
would cause audio distortion, with my invention
such audio distortion will be greatly reduced and
even substantially eliminated. This follows from
the fact that when the carrier F1 fades severely
relative to its modulation side bands, the recti?ca
stages of IF ampli?cation 9. The ampli?ed IF
energy may then be applied to a second detector
or demodulator ID whose input is tuned to the
operating IF value. The modulation output of
the second detector It) may then be transmitted
through one or more stages of audio frequency
ampli?cation, and the ampli?ed audio energy is
then reproduced in any desiredlform of repro
ducer such as a loudspeaker.
>
_
It will be recognized that the aforedescribed
networks of the receiver of Fig. 3 are purely con
tion ef?ciency of demodulator It! will drop very
rapidly and the audio output due to the carrier
F1 will substantially disappear. However, since
the network which feeds the demodulator is su?i
cientlybroad to pass the carrier F2, it follows
that the audio output reaching the loudspeaker
ventional. Indeed, my invention is readily ap
plied‘ to any conventional receiver of the standard
broadcast type. The only change'that need be
will be sufficient because the audio output due to
F2 is the same audio output which would have
been derived from F1. It is emphasized that since
made in the conventional superheterodyne re
F2 is not fading relative to its modulation side
ceiver is that the pass band characteristic of the
bands, the detector efiiciencylwith respect to, the
receiver up to the input terminals of the second
detector 10 be suf?ciently wide so as to pass the
modulated carrier wave F2 will be at a high value
and una?ected by the reduction of the detector
energy of carriers F1 and F2 and their associated 60 efficiency with respect to’the wave F1.
In the system of Fig. 5 there is employed a
modulation side bands. In Fig. 4 I have shown
~ anillustrative representation of the type of trans
mission characteristic which the receiving system
of Fig. 3 should have. Thefullline curve, H__de.-.
singlev carrier transmission channel in contrast
to the dual channel transmission of ‘Figs. 1 and 3.
There‘ isradiated from the broadcast transmitter
notesgthe ideal transmission, characteristic of the
65 the usual and standard modulated carrier wave
receiver up to the input terminals’ of the second
having a channel width of ‘about 10 kc. 1 At the
receiverv there are located a pair of loops which
detector. It will be noted that the pass ‘band
width is 20 kc. This means that the radiated care
riers F1 and F2 and their modulation sidebands
may readily be passed through the various net
works from the signal collector. $.to the input
terminals of the demodulator ill. ‘The dotted
lines l2 in Fig. 4 show the positions of the car
riers F1 and F2 and the 10. kc. spacing between
each. ‘ii the terriers. F1 emits‘ and their module
are oriented 90.". in respect to each other for re
ception of the broadcast band, It will be un
derstood that the'loops i3» and IA can here
179'te§1'a<1‘~Q‘lnd 9' Vgrtical axis, shown 913 9e, ‘wilted
line, and at any angle ‘in respect to each other.
One ,of these, loops will collect the “modulated
carrier wave energywvhich appears as a ground
wave and. also, as a sky.‘ wave, while the other
2,413,543
5
at least two directional signal collectors arranged
loop will collect solely the sky wave energy. Let
it be assumed that there is being received the
signals from a transmitter operating at 1000 kc.
One of the loops can be pointed toward the
transmitting station, while the other loop is
in predetermined relation, means coupled to one
collector to reduce modulated carrier Waves to a
?rst lower carrier frequency, means coupled to
the second collector to reduce modulated waves
of the same carrier frequency to a second lower
turned broadside to the station. With this‘condi
tion the receiving channel connected to the loop
carrier frequency different from the first lower
pointing towards the station would have the
frequency, means amplifying said waves of both
highest average signal and less average distor
lower carrier frequencies in a common amplij
tion. Observation has shown that during periods 10 ?er stage, and a common demodulator coupled
when the transmitted signal faded and distor
to the latter stage.
I
tion occurred on this channel, the other channel
2. In a receiver of modulated carrier waves, at
with the loop broadside usually had strong non
least two directional signal collectors arranged
distorted signals. It has been observed that the
signal received by one of the loops which has its 15 in predetermined angular relation, converter
means coupled to one collector to reduce modu
lated carrier waves to a ?rst lower carrier fre
quency, a second converter means coupled to the
second collector to» reduce modulated waves of
the same carrier frequency to a second lower
modulated carrier wave energy subject to se
lective fading is most probably subject to can
cellation effects due to the combined action of
the ground and sky waves. In that case the
other loop most likely is collecting sky Wave en 20
carrier frequency different from the ?rst lower
ergy, andis providing a substantially uniform
frequency, means amplifying said waves of both
intensity of desired signals.
lower carrier frequences, and a common demodu
Each of the loops feeds its collected signal
lator for the ampli?ed waves.
energy to a respective converter stage. Thus,
3. In a receiving system for modulated car
loop I3 is connected to the converter IS. The 25
rier waves subject to selective fading of the car
local oscillator I5’ feeds its locally produced
oscillations to converter l5.
rier relative to the modulation side bands, means
The local oscilla—
separately effecting at least two collections of
said modulated carrier waves in space quadrature
of signals of 1000 kc., the local oscillator I5’ is 30 relation, means translating the separate waves to
different frequencies, means separately amplify
adjusted to frequencies of 1450 kc. while the
ing the waves of different frequencies, means
oscillator I6’ is adjusted to a frequency of 1470
transmitting the ampli?ed Waves through a
kc. The output energy of converter 15 will have
common path adapted to pass the separate Waves,
an IF‘ value of 450 kc., while the IF Value of
and
means rectifying the latter waves in a com
converter I6 will have a value of 470 kc. Sepa
mon detector.
rate IF ampli?ers l1 and I8 are utilized to am
4. In a receiver of modulated carrier Waves, at
plify the separate modulated IF carrier waves.
least two directional signal collectors arranged in
Above the ampli?er I‘! there is shown graphical
90 degree relation, means coupled to one col
ly the appearance of the pass band of the am
pli?er l1, ‘and below the ampli?er box I8 there 40 lector to reduce modulated carrier waves to a
?rst lower carrier frequency, means coupled to
is shown the pass band of that ampli?er.
the second collector to reduce modulated waves
After separate ampli?cation in networks I‘!
of the same carrier frequency to a second lower
and I8, the combined energies of these two-net
carrier frequency different from the ?rst lower
works are fed to a common IF ampli?er and
frequency, means amplifying said waves of both
transmission network H] which has a pass band
lower carrier frequencies in a common ampli
sufficiently wide to pass each of the IF car
?er stage, said ampli?er stage having a pass band
riers of 450 kc. and 470' kc. and the associated
width chosen to include both lower frequencies,
modulation side bands of each carrier. Above
and a common demodulator coupled to the lat
the network I9 there is shown in idealized form
ter stage.
the appearance of the pass band characteristic
5. In a system for receiving a high frequency
of the network l9. It will be seen that it is suffi
carrier modulated by signals of a predetermined
ciently wide so as to pass the output energy of
frequency range, a pair of loop collectors ar
each of ampli?ers I‘! and I8. Subsequent to
ranged on a common axis in quadrature rela
the network [9 the demodulator will be similar
to the demodulator l0 of Fig. 3. The action is 55 tion whereby one loop collects the sky wave and
the other combined ground and sky waves, a
precisely the same. If there is selective fading of
?rst converter connected to one loop and adapted
the energy collected by loop I4, then the de
to convert received carrier frequency to a lower
modulator e?iciency for the modulated IF en
value, a second converter connected to the sec
ergy of 470 kc. will decrease sharply. Hence,
ond loop and adapted to convert the received
the audio output will be that due to the 450 kc.
carrier energy to a lower value differing from
energy, which is representative of the sky ‘wave
the ?rst lower value by the ‘said modulation fre~
energy which was collected by loop l3.
quency range, separate ampli?ers for respec
While I have indicated and described several
tor I6’ feeds its locally produced oscillations to
converter 16.
In the assumed case of reception
systems for carrying my invention into effect, it
will be apparent to one skilled in the art that
my invention is by no means limited to the par
65 ampli?ed energies in an ampli?er whose pass band
ticular organization shown and described, but
that many modi?cations may be made without
departing from the scope of my invention.
What I claim is:
tively and separately amplifying the energies of
lower frequency, means combining the separately
1. In a receiver of modulated carrier waves, 70
is at least double said modu1ation frequency range
and whose center frequency is a value between
said two lower frequencies, and means for demod
ulating the combined energies.
WENDELL L. CARLSON.
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