Note: Descriptions are shown in the official language in which they were submitted.
9~376
1 -1- RCA 71,646
~ADIO TRANSITTER ENERGY RECOVERY SYSTEM
This invention relates to an arrangement for
improving the efficiency of converting DC illpUt power to
radio-frequency output signal of a radio transmitter.
The importance of reducing the waste of energy
takes on continually increasing importance. To this end,
highly efficient solid-state components have been used
in radio transmitters, although in some cases the output
stages continue to be the somewhat less efficient thermionic
vacuum tubes. In amplitude-modulation transmitters, it is
known to use a radio-frequency power amplifier to amplify
the signal from a radio-frequency source by an a~ount
dependent upon the energizing voltage applied across the
radio-frequency (RF) power amplifier. The magnitude of the
energizing voltage is varied at an audio rate in order to
achieve audio modulation of the RF signal. The energizing
20 voltage is varied at an audio rate by an audio-frequency
power amplifier, and large amounts of audio-frequency
modulating power are required in order to modulate a
high-power radio-frequency amplifier.
Class A audio amplifiers have a theoretical
25 maximum conversion efficiency of DC power to AC signal of
50~, and class B amplifiers have a theoretical maximum
efficiency of 78. 5~. Where large amounts of audio power
are involved, significant amounts of energy may be wasted
as heat dissipation in the audio amplifier. This has been
30 corrected in the past by the use of so-called class D
amplifiers, in which the audio signal to be amplified pulse-
width or pulse-duration modulates a subcarrier. The
width-modulated pulses are used to drive a high-power
switch-mode amplifier to produce high-power pulses at the
35 subcarrier rate, the duty cycle of which varies at the
audio information rate. ~ low-pass filter coupled to the
power switching amplifier integxates the duty-cycle
modulated signal to remove the carrier and to reproduce the
audio signal. Such amplifiers have a theoretical
40 conversion efficiency approaching lO0~.
'` ~ !
,
., , , ' ' .
.
' " ' ,', ' ~ ', ' . :, :
'' ' ' ' ' " ' -' ' ' ` -
87~
1 -2- RCA 71,646
As described in U.S. Patent 4,122,415 issued
~October 24, 1978 to A.C. Luther, et al., such switching
audio amplifiers may introduce a distortion of the
modulating signal during those intervals in which the
modulation process produces short-duration pulses having a
low duty cycle. This distortion results from the finite
switching time of the audio power switch transistors. The
aforementioned Luther, et al. patent describes an arrangement
for correcting the distortion. The correction arrangement
includes a threshold device in the form of a diode coupled
to the filter and to an offset voltage source. In one
of the embodiments shown in the Luther et al. patent, the
offset voltage device is a zener diode. During those
intervals in which the power switch is open, the reaction
voltage resulting from current flow through the inductances
of the low-pass filter of the audio amplifier cause a
current flow through the threshola diode and through the
zener diode which raises the average voltage applied to the
filter in such a manner as to compensate for the
modulation distortion caused by the switching time of the
switched transistors of the switch-mode amplifier. Another
embodiment of the offset voltage source includes a capacitor
for storing energy in the form of charge resulting from
current flow through the threshold diode, together with a
resistor for allowing the voltage across the capacitor to
be reduced during the intervals between current pulses.
In still another embodiment, the offset voltage source
includes a battery or an additional power source. The
zener diode and capacitor-resistor embodiments of the
offset voltage source dissipate power during operation.
The power dissipated may be substantial. The battery
embodiment of the offset voltage source is undesirable for
reliability reasons, but more -importantly it also
dissipates power, because a resistor in parallel with the
battery is necessary to prevent destruction of the battery
by increasing without limit the energy stored therein
during operation. The power supply embodiment mentioned
.
: '
76
1 ~3~ RCA 71,646
in the specification of the Luther et al. patent as
being equivalent to the battery embodiment is subject to
the same disadvanta~es. A more efficient and reliable
radio transmitter is desirable.
In accordance with the principles of the present
invention,an improved AM generator for transmitting a
carrier signal modulated with intelligence from an
intelligence signal source includes a pulse-wid-th
modulator coupled to the source of intelligence signals for
producing recurrent bilevel pulses having a first state and
a second state the duration of which depends upon the
intelligence. A controllable switch is coupled to the
pulse-width modulator for controlling the impedance of a
controlled main current path of the switch in first and
second states in response to the bilevel pulses. The first
state is a high impedance state and the second state is low
impedance. The generator includes a radio-frequency
modulator for producing radio-frequency signals at an
amplitude which depends upon the energizing level applied
across first and second terminals of the radio-frequency
modulator. The generator also includes an energizing
source, and at least an inductance for coupling the first
and second terminals of the radio-frequency modulator,
the energizing source and the controlled current path in
a series path for the flow of current. The inductor
develops a voltage pulse during the first state of the
switch. The generator includes an offset voltage
generator and threshold switch coupling the offset
voltage generator to the inductor for allowing current flow
in the offset voltage generator for controlling the voltage
at the inductor in such a manner as to reduce distortion
of the modulation of the carrier signal by the intelligence
35 when the duration of the second state of the controllable
switch is low. The imp,rovement lies in that the offset
voltage generator includes an energy storage arrangement
coupled to a point of reference potential for storing
energy as a result of the current flow in the offset
' .... . ' '~.' : ' '
:. :
1376
1 -4- RCA 71,646
voltage generator, and in that the offset voltage generator
further includes an energy coupling arrangement for coupling
energy from the storage arrangemen-t to the source of
energizing potential.
In the Drawings:
YIGURE 1 is a diagram, partially in block and
partially in schematic form, of an AM generator or
transmitter according to the prior art; and
FIGURE 2 illustrates a generator embodying the
invention.
FIGURE 1 illustrates a prior art amplitude-
modulation (~) generator or transmitter 10 for producing
a carrier signal modulated with intelligence from an
audio signal source (not shown). Transmitter 10 includes
a radio-frequency (RF) signal generator or drive source 13
which provides a carrier signal at a predetermined radio-
frequency carrier frequency. Radio-frequency drive
source 13 may include one or more driver amplifiers for
increasing the carrier signal power output to the radio-
frequency power amplifier, and may also contain phase and
amplitude predistortion circuits for compensating for the
distortion in later stages. The output of drive source 13
is applied to an RF power amplifier 15, the output of
which is fed to a radiating antenna, not shown. The RF
carrier being amplified by power amplifier 15 is modulated
by the magnitude of the applied energizing potential
between energizing terminals 15a and 15b of the amplifier.
Terminal 15b is coupled to a terminal 30 to which is
applied B~ from a power supply or source of energizing
potential (not shown). Terminal 15a is coupled to ground
by way of a switching audio power amplifier including a
35 pulse duration modulator (PDM) 17, a power audio switch
or switch-mode amplifier 25 and a low-pass filter 27. The
RF signal passing through power amplifier 15 is modulated
by reconstructed audio applied to terminal 15a.
The audio signal to be modulated onto an RF
~' ' , .
- - . ,
B76
--5-- RCA 71,646
carrier is applied to an input terminal 22 of PDM 17 from
an intelligence or audio signal source, not shown.
subcarrier signal which rnay have a triangular or sawtooth
voltage time waveform is generated by a subcarrier signal
source 19 and is coupled to terminal 20 of PDM 17. The
triangular subcarrier signal may be of the type illustrated
in U.S. Patent 3,943,446 of Quidort. Pulse duration
modulator 17 may comprise a comparator such as that
described in connection with U.S. Patent 3,943,446 which
produces a pulse at the subcarrier frequency the duration
or pulse width of which is established by the instantaneous
amplitude of the audio signal. Pulse duration modulator 17
may include amplifiers for increasing the power of the
pulses to a level suitable for driving the bases of
paralleled transistors Ql-Qn of modulation switch 25.
Since the pulse rate is constant at the subcarrier rate and
the pulse duration varies at the audio rate, the duty cycle
is modulated. The emitters of each of parallel-connected
transistors Ql-Qn are coupled to ground or to reference
potential. The bases of transistors Ql-Qn are coupled to
each other and to the output of PDM 17, and the collectors
are connected by way of a common point 26 to the input of
low-pass filter 27. Filter 27 comprises as is known
cascaded sections of series inductance and shunt
capacitance, and the end remote from circuit point 26 is
the outp`ut terminal of the audio amplifier. The output of
filter 27 is coupled to terminal 15a. During the positive-
~` 30 going portion of the two-level or bilevel pulses produced
by PDM 17, the base-emitter junctions of Ql-Qn are forward-
~; biased and the transistors are turned ON, which represents
a low impedance of their main current (collector-emitter)
paths.
The circuit as described, without more, will
efficiently modulate the RF carrier. The method of
::
modulation may be understood by noting that energizing
current flow through power amplifier 15 must flow through
filter 27 and through the main current path of switch 25. `
~- ~ 40
......... ~ . . . ::
.: ' :
--: ~ . .
', : .: -' :
.
. : .
~9t3~6
1 -6- RCA 71,646
Switch 25 opens and closes at the subcarrier rate, which
may be at a frequency such as 70 kilohertz, with a varying
duty cycle depending upon the magnitude of the audio. The
low-pass filter averages the duty cycle variations to cause
a variable voltage across amplifier 15 at the audlo rate
to create the desired modulation of the RF carrier.
As described in the aformentioned Luther, et al.
patent, at low duty cycles corresponding with low depth of
modulation, the modulation linearity may be affected by
the switching time of the transistors of switch 25. With
short duration pulses applied from PDM 17 to the common
bases of the transistors of switch 25 the desired duty cycle
is low, but the finite turn-on and turn-off time of
switch 25 causes a spreading or stretching of the effective
pulse width and an effective increase of the duty cycle
from the desired duty cycle. This will result in a
compression of the modulated carrier signal compared with
the audio modulating signal This is overcome in the
aforementioned patent by the use of an offset voltage
generator 35 together with a threshold device in the form
of a diode 34. During those intervals in which switch 25
is conductive, eurrent flows through inductors 28 and
energy is stored in the magnetic fields associated with the
inductor. When switch 25 becomes open or goes to a high-
impedanee eondition, the stored energy ereates a voltage
which is of sueh polarity as to cause the eurrent to
continue to flow. The voltage at eireuit point 26 begins
to rise as a result of the induetive energy, and rises
until it beeomes equal to the sum of the B+ supply voltage,
the offset voltage of zener diode 38 and the forward
junction voltage of diode 34. Current is then able to
circulate through a series path ineluding B+, offset
generator 35, diode 34, induetor 28 and filter eapaeitors 29
until the energy stored in induetor 28 deereases to zero
or until switeh 25 eloses, allowing the current to again
begin to inerease. During long duty cyele portions
of the operation, the average voltage at eireuit point 26
''
:-
.
~9876
1 -7- RC~ 71,646
is well below B+ and may in fact approach zero volts. This
allows maximum current flow through amplifier 15 and
corresponds with a maximum carrier level. As the duty
cycle becomes shorter, the average voltage at circuit 26
rises until near the shortest duty cycles, the voltage
rises above B+ as a result of the mechanism described.
The tendency of the average voltage at circuit point 26
to rise above B+ at very short duty cycles tends to cause
the voltage at terminal 15a to rise to B+ notwithstanding
the finite switching times of transistors Ql-Qn. With
terminal 15a at B+, the difference voltage applied across
the energizing terminals of power amplifier 15 is zero,
and the amplifier is completely deenergized and incapable
- of producing any carrier. Thus, threshold device 34 in
conjunction with offset voltage generator 35 compensates
for the residual carrier which results at low duty cycles
from the finite switching characteristics of switch 25.
The current flow through diode 34 and through
zener diode 38 represents, as mentioned, a power which
is converted to heat. Similarly, the alternative
embodiments of the offset voltage generator of FIGURE 1
dissipate as heat the power resulting from the flow of
current therethrough. The amount of power dissipated as
heat may be considerable.
FIGURE 2 illustrates a transmitter 210 embodying
the inve`ntion. In FIGURE 2, elements corresponding to
those of FIGURE 1 have the same reference number. In
FIGURE 2, the RF drive source, RF power amplifier, audio
amplifier and threshold diode 34 are identical with those
of FIGURE 1. The offset voltage generator is illustrated
as a block 243 and includes a capacitor 212 which responds
to the flow of current through diode 34 to produce a
voltage of the polarity shown. Comparison of the polarity
with the polarity illustrated in FIGURE 1 shows that the
offset voltage is in the same direction as that produced
by zener diode 38. Capacitor 212 stores an increasing
amount of enexgy with time, as the periodic opening and
376
1 -8- RCA 71,646
closing of switch 25 causes periodic pulses of unidirectional
current through threshold diode 34 as a resul-t of the
energy periodically stored in inductor 28. As mentioned, if
allowed to continue, the voltage across capacitor 212
would rise continually. This is undesirable because lt
might damage the capacitor or other components, and it
continually changes the operating state of the modulator.
Consequently, a DC-to-AC inverter illustrated as a
block 214 is coupled across the terminals of capacitor 212.
Inverter 214 may be of any known type suitable for
accepting direct current from capacitor 212 and for
producing an alternating voltage across output terminals
216 and 218. The primary winding 220a of a voltage step-up
transformer 220 is coupled across terminals 216 and 218
for being energized by the alternating voltage produced by
inverter 214. The stepped-up voltage appearing across a
secondary winding 220b of transformer 220 is applied across
a pair of terminals of a bridge recifier designated
generally as 230. Another terminal of bridge rectifier 230
is grounded, and the fourth terminal is connected to
terminal 30, the source of B+ energizing potential.
In operation, the voltage across capacitor 212is
controlled by drawing charge from the capacitor in the form
of current flow through inverter 214 The energy
represented by this flow of direct current through inverter
214 is converted to the form of an alternating current
applied to winding 220a, and is further transformed by
transformer 220 to high-voltage secondary 220b. On the
secondary side, a pulsating direct current is formed by
rectifier 230 and the pulsating direct current is applied
by way of terminal 30 to the source of B+. The flow of
current through the source of B+ as a result of the
operation of inverter 214, transformer 220 and
rectifier 230 is in a direction to restore energy to the
source. Thus, the energy which in the prior art was
wasted as heat in the offset voltage generator is in the
arrangement of FIGURE 2 returned to B+, the energizing
source.
.
: ~,
.
- . : :
76
1 -9- RCA 71,646
Various taps are provided on secondary winding
220b to which rectifier 230 may be connected to better
match the transformed voltage produced by inverter 214 to
the value of the B+ voltage.
The operation of inve:rter 214 to produce a
pulsatlng direct current through terminal 30 to source B+
of energizing voltage causes a fluctuation of the B+ at
twice the alternating-current frequency of inverter 214.
This voltage fluctuation is caused by the flow of the
pulsating direct current through the internal resistance
of the B+ supply. This unavoidable pulsating voltage
appears at terminal 15b and will result in a residual
modulation of the carrier by power amplifier 15. Similarly,
low-pass filter 27 cannot remove every vestige of the
subcarrier signal from the audio signal appearing at
terminal 15a, and the RF carrier has residual modulation
at the subcarrier rate. If the frequency of the inverter
is not controlled, the frequency may vary as the voltage
across capacitor 212 varies. The interaction of the
residual modulations due to the inverter-induced B+
variations and to the unfiltered subcarrier signals may
interact to cause audible beat notes of low amplitude. In
order to avoid such beats, inverter 214 may be synchronized
with the subcarrier by means of a conductor 240. In this
manner, inverter 216 may be locked to a multiple or
submultiple of subcarrier frequency for preventing changes
in the difference frequency which may result in audible
whistles. Such synchronization is well-known in the art
and requires no further explanation.
.
