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Патент USA US2093780

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Sept. 21, 1937.
w. A. E. QUILTER
2,093,780
MODULAT ION SYS TEM
Filed Aug. 25, 1933
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INVENTOR
MAL/AM A. . 0 7474-19
BY
,
-
ATT
RNEY
Patented Sept. 21, 1937
ATEN "
' UNITED sr
FEE
2,093,780
MODULATION SYSTEM
William Alfred Edward Quilter, Bushey, England,
assignor to Radio Corporation of America, a
corporation of Delaware
'
Application August 23, 1933, Serial No. 686,365
In Great Britain September 2, 1932
5 Claims.
(or. 179-171)
‘This invention relates to modulated carrier
wave transmitters and has for its object to pro
vide an improved system capable of dealing with
high degrees of modulation while maintaining a
5 relatively high efficiency and substantial recti
a condenser, the center point of the inductance
being earthed.
The load circuit is represented in the accom
panying drawing at F as consisting of an in
ductance shunted by a condenser and a resistance
in series. This load circuit may, of course, in
linearity of response.
practice
be any suitable load circuit, such, for
With modulated carrier wave transmitters of
the so-called power ampli?er type, i. e., of the example, as an aerial.
Coupling between the load circuit F and the
type wherein the modulated carrier wave is am
tank circuit A is accomplished by means of a
10 pli?ed in a so-called power ampli?er prior to
coupling valve arrangement whose effective cou
radiation, it is very difficult to obtain more
pling co-e?icient is varied in accordance with
than a comparatively low power e?iciency in the
modulated potentials applied at J. This cou
power ampli?er stages, the reasons for this be
pling system consists of two thermionic tubes
ing that the efficiency of conversion in those V3, V4 arranged back to back and providing a 5
15 stages is a substantially rectilinear function of substantially purely resistive coupling which is
the grid excitation .and it is necessary that the effective both in the direction A to F and vice
grid excitation in the said stages be kept low
in the carrier condition so as to allow for the versa.
Low frequency modulated potentials, for ex
increase in excitation which will occur due to ample, voice frequency potentials, are applied at
2 0 modulation.
J to the primary of a transformer K having two
The present invention provides a system where
secondaries. These secondaries apply modulat
in these difficulties and disadvantages are avoid
ing potentials as shown to the grids of the tubes
ed and wherein the grid excitation is maintained V3 and V4 and, in order to exclude radio fre
high under all conditions and consequently the quency potentials from the modulating trans 25
2 5 overall power efficiency can be maintained high.
former K, choke coils L are provided in series
According to this invention a modulated car
with the secondaries of the said transformer K.
rier wave transmitter includes a power ampli
Of course, the source of modulated potentials
?er to whose input terminals the carrier wave connected at J may be and in practice generally
only is applied and the output of this power will be constituted by the output of a chain of 30
3 O ‘ampli?er is coupled to the load, e. g., to an
low frequency ampli?ers, not shown.
aerial circuit, by means of one or more thermi
The tubes V3 and V4 have their ?laments
onic tubes whose internal resistance is varied in heated by high frequency currents by means of
dependence upon the signals, 1. e., the modulat
coils G and G’, which are coupled respectively
ing potentials.
The invention is illustrated in the accompany
ing drawing and shows schematically one ar
rangement in accordance therewith.
Referring to the drawing unmodulated radio
frequency carrier voltages are applied in pushpull
O to the grids of two high frequency tubes V1, V2
vhaving a common split input circuit C. The
grids of the valves V1, V2 are biased negatively
from a source of potential D, the said grids be
ing biased to the point of cut-off of anode cur
4 01 rent so that the ampli?er constituted by the
tubes V1, V2 operates as a so-called “class B”
ampli?er. With this arrangement and assuming
an unmodulated carrier wave input at C, as re
quired by this invention, the el?ciency of conver—
sion from direct current power to high frequency
power accomplished by the tubes V1, V2 may be
as high as ‘70%.
The anodes of the tubes V1, V2 are coupled
through coupling condensers B to a tuned tank
circuit A consisting of an inductance shunted by
to the inductance in the circuit A and the in
ductance in the circuit F. Alternatively the ?la
ments may be directly connected across a few
turns of the inductances in the circuit A and F
respectively, or the ?laments may be heated
from "any other suitable source of heating cur
rent provided that the said source is suitably in
sulated from ground or effectively insulated as
regards radio frequency potentials by means of
choke coils or the like.
When the modulating potentials from J on the
grid of V3 swing the same in the positive direc
tion V3 becomes more conductive and a path for
radio frequency energy is provided. This path
includes G, the anode of V3, the point on H’
ground and back to 1-1.
When the modulating "
potentials on the grid of V4 cause V4 to be more
conductive, a radio frequency path is also pro
vided between A and F. This path is from'H to
ground to H’ to G’ and the cathode of V4 to the
anode of V4 and to the point on H.
2
2,093,780
In order that there may be no stray undesired
reactive coupling between the circuits A and F
the circuit A with its associated components G,
V3 and the appropriate inductances L are en
closed in a screening box E and the circuit F
with its associated components G’, V4 and the
remaining inductances L are similarly enclosed in
a second screening box E’.
Since there is no coupling between the circuits
A and F other than via the substantially purely
resistive coupling provided by the tubes V3 and V4,
the amount of power transferred to F from A
is dependent upon the e?ective resistance of these
coupling tubes. The effect of the application of
signal potentials to the grids of the tubes V3 and
V4 is, of course, to vary their effective resistances
in accordance with signal potential so that modu
lated high frequency currents occur in the circuit
F. Thus the high frequency unit comprising the
tubes V1 and V2 works into a circuit, A, which
has a resistance which is, in effect, varied by
means of the variable load transferred through
the signal varied coupling.
It will be seen that with an arrangement as
above described the effective coupling to the
aerial or load and consequently the effective load
on the ampli?er, will vary in accordance with the
signal. Moreover if the point of maximum cou
pling as determined by the tubes V3 and V4, and
consequently the point of peak modulation, is
maintained always below a critical coupling point
at which over-modulation may occur, the input
and convenient arrangements for neutralizing
the self-capacities of the coupling tubes and/or
the whole coupling circuit arrangement may be in
eifect connected between points which are not
much above earth potential as regards high fre
quency. In the speci?c arrangement above de
scribed it has been assumed that the coupling
tubes in effect connect a point near the high
potential end of the inductance in the output
circuit A of the power ampli?er with a point near
the high potential end of the inductance in the
aerial or load circuit F, but it is not necessary
that the eifective connection be made between
these high potential ends. The effect of making
the eifective connection, as regards coupling, be
tween points of lower potential is to lower the
actual resistance required for coupling and to
render the self-capacities of the coupling tubes
less important relative to the useful and desired
coupling impedances provided thereby.
Thus,
frequency capacity of the coupling tubes to
ground, the coupling tubes may be connected to
points such as the points H shown in the ?gure
instead of as actually illustrated, these points, H, 30
being so selected that the eifective resistances of
the coupling tubes desired to be used is compar
to the aerial or load circuit, should vary in a linear
able or equal to the effective resistance of the
manner in accordance with the signal input and
coupling theoretically required.
thus provide good modulation.
Furthermore, the grid of the power ampli?er
tubes may be arranged to be excited up to the
limits set in practice by considerations of har
monic distortion, i. e., the power ampli?er may
ordinarily be set for an e?iciency as high as
about 70%.
It will be noted that the load on the recti?ers
or similar devices supplying anode potential to
the power ampli?er tubes will at all times be pro
portional to the instantaneous value of the signal
and this involves that the source of anode poten
tial supply must be of good regulation both as re
gards audio frequency and direct current loads, if
dif?culties due to anode voltage variations are to
be avoided.
These requirements can be met by employing
as the source of anode potential apparatus hav
ing an inherently low impedance characteristic,
e. g., the mercury vapor recti?ers now available
having low voltage drop and good direct current
regulation, the requirements as to audio fre
quency being met by providing the customary
very large condenser ordinarily provided at the
present time in power ampli?ers. Alternatively
any suitable source of anode potential may be
utilized if there be associated therewith voltage
stabilizing means for maintaining the applied
anode voltage substantially constant.
A further; di?iculty which may arise is that,
even at the point where the coupling tubes are
operating on that part of the modulation cycle
at which they are biased to “cut-01f” so that no
current flows between the anode and cathode of
said tubes, the inherent coupling between the
anode and cathode of the tubes may be such as to
leave a residual reactive coupling between the
circuits A and F thus tending to prevent very
“deep” or complete modulation. This di?iculty
can be avoided either by providing any known
20
where it is required to proportion the eifective
resistance of the coupling end to use tubes readily
avail ble commercially as coupling tubes, or again
where the coupling tubes have cathodes heated
other than by radio frequency currents and it is 25
accordingly desired to reduce the effective radio
Any alternative method of applying the modu 35
lating signal to the coupling tubes may be used,
for instance, a resistance-capacity coupling cir
cuit may be substituted for the low frequency
transformers shown in the drawing.
Having thus described my invention and the
operation thereof, what Iclaim is:
1. Means for applying linear modulation at
signal frequency to a carrier wave comprising, a
pair of thermionic tubes each having an anode,
‘a cathode and a control grid, a tuned circuit con
nected between the control grids of said tubes,
said circuit being tuned to the carrier wave and
energized thereby, a source of potential connect
ing a point on said tuned circuit to the cathodes
of said tubes, said source of potential being of 50
such a value as to render said tubes conductive
only for a portion of the carrier wave cycle,
a circuit coupled between the anodes of said
tube, said circuit being tuned to the frequency
of the carrier wave, a load circuit, an additional '
pair of thermionic tubes, a connection between
the cathode of one of said additional tubes and
a point on said circuit connected between the
anodes of said ?rst named pair of tubes, a con
nection between the cathode of the other of said
additional pair of tubes and said load circuit,
a circuit connecting the anode of said last named
tube of said additional pair of tubes to the
cathode of said ?rst named tube of said addi-,
tional pair of tubes, a circuit connecting the 65
anode of said ?rst named tube of said additional
pair of tubes to the cathode of said last named
tube of said additional pair of tubes, and a
source of modulating potentials connected with
the control grids of said additional pair of tubes. 70
2. A device as recited in claim 1 in which the
cathodes of said additional pair of tubes are
each connected with a heating circuit and in
which one of said heating circuits is coupled to
said circuit coupled between the anodes of said 75
3
2,093,780
?rst named pair of tubes, while the other of said
heating circuits is coupled to said load circuit.
3. A device as recited in claim 1 in which the
circuit connected between the anodes of said
?rst named pair of tubes and one of said addi
tional tubes is enclosed in a conductive screen,
while the other of said additional pair of tubes
and said load circuit are enclosed in another
conductive screen.
10
4. A device as recited in claim 1 in which radio
frequency choking inductances are inserted in
the connections between the source of modulating
potentials and the control grids of the additional
pair of tubes.
5. Means for applying linear modulation at
signal frequency to carrier waves comprising, a
pair of thermionic tubes each having an anode,
a cathode and a control grid, a tuned circuit
connected between the control grids of said tubes,
20 said circuit being tuned to the frequency of the
carrier wave to be modulated and energized
thereby, a source of negative potential connected
between the control grids and cathodes of said
tubes, said source of negative potential being of
such a value as to bias said tubes to be con
ductive for a half of the cycle of the carrier
wave only, a tuned circuit connected between
the anodes of said tubes, said tuned circuit in
cluding an inductance, a load circuit including
an inductance, a pair of modulator tubes, a circuit
connecting the cathode of one of said modulator
tubes to the ?rst inductance and the cathode 10
of the other of said modulator tubes to the sec
ond inductance, circuits connecting the anode of
said last named modulator to the cathode of said
?rst named modulator tube and. the anode of said
?rst named modulator tube to the cathode of 15
said last named modulator tube, a source of
modulating potential, circuits connecting said
source of modulating potentials between the con
trol grid and cathode of each of said modulator
tubes, and radio frequency choking inductances 20
in said last named circuits.
WILLIAM ALFRED EDWARD QUILTER.
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