Chapter 1: AM Transmitters Chapter 3: FM Transmitters Chapter 4

218 Pages·2014·5.16 MB·English

Chapter 1: AM Transmitters
Chapter 2: AM Stereo Systems
Chapter 3: FM Transmitters
Chapter 4: Subcarrier Transmission and Stereophonic
Broadcasting
Chapter 5: Television Transmitters
Chapter 6: Multichannel Television Sound
Chapter 7: Transmitter Control Systems
Chapter 8: Frequency and Aural Modulation Monitoring

3.1
AM Transmitters
George W. Woodard, P.E.*
Radio Free Europe /Radio Liberty
Washington, DC
Broadcasting to the general public began with Modulation band of 535 to 1605 kHz in North
the process known as Amplitude Modulation near America, and slightly wider in other parts of the
the start of the second decade of this century. world. From almost the very beginning of broad-
Then, as now, the system of modulation chosen cast technology, it was known that AM was not
for transmission was heavily dependent on prac- a very efficient mode of transmission, either in
tical and economic aspects of receiver technology. spectrum usage or in transmission of intelligence.
The evidence is clear that amplitude detection was The pioneers in AM broadcast transmitter and
the only known practical method of signal de- receiver engineering technology had a deep sense
modulation when the ideas of radio communica- of responsibility toward developing this "mar-
tion began to be formulated in the late 1800's velous medium" to enhance the lives of the gen-
and early 1900's. It is apparent from some of the eral public with informative, educational, and en-
earliest technical writings on radio communica- tertaining programs. Some of the early commu-
tion that a general mathematical knowledge of nication technology pioneers in the U.S. and
angular modulation, i.e.; frequency and phase abroad had great vision for both the technical
modulation, did not exist until the mid 1920's.' and programming aspects of public broadcasting.'
These modes of transmission and the necessary As time and technology progressed, engineers
receiver technology were not to be proven prac- could no longer ignore the problems of transmis-
tical until long after amplitude modulation had sion efficiency. When, in the 1930's, the telephone
become the standard of the radio communication industry was planning a major switch from DSB-
and broadcast technological art. Amplitude FC-AM in favor of Single SideBand- Suppressed
modulation in broadcasting is today more ac- Carrier (SSB -SC) for both its long distance wire
curately referred to as DSB- FC -AM, an acronym and wireless services because of its higher trans-
for Double SideBand- Full Carrier- Amplitude mission efficiency and channel capacity', the
Modulation, however, the abbreviation "AM" broadcast industry was necessarily committed to
has been generally accepted to describe the mode the continuance of conventional AM because of
of transmission currently used throughout the
world in the standard broadcast Amplitude
'Inaugural Address of Dr. Lee DeForest upon becoming the
President of the I.R.E. Proc. IRE, VOL. 18, No. 7, Jul. 1930,
*Formerly with Continental Electronics Manufacturing Com- p. 1121.
pany.
"Production of Single Side -Band for Trans -Atlantic Radio
3
' "Notes on Modulation Theory ". John R. Carson, Proc. Telephony ". R. A. Heising, Proc. IRE, Vol. 13, No. 3, June
IRE, Vol. 10, No. 1, June 1925, p. 57. 1925, p. 291.
3.1 -1
3.1 -2 Section 3: Transmitters
the need for economic public receiver compatibili- transmitted. Linear or quasi -linear undulations
ty. Therefore, the broadcast engineers attention in the amplitude of the carrier wave, however,
naturally turned to improvement of transmission are normally used to transmit the analog infor-
standards such as fidelity, efficiency of transmis- mation present in speech and music. The radio
sion, reliability of transmission, and co- channel carrier wave signal onto which the analog ampli-
and adjacent channel interference, etc. Of these, tude variations are to be impressed is expressed as
efficiency and reliability of transmission have seen
and continue to see the greatest improvement in e(t) = A x Ec x Cos (coct) [1]
the views of many transmitter designers and users.
Transmission fidelity, always important, reached Where:
a plateau in the late 1940's which has not been
significantly improved upon even in today's latest e(t) = instantaneous amplitude of carrier
transmitter designs; though pre- transmitter pro-
wave as a function of time (t)
gram processing equipment and philosophy, A = a factor of amplitude modulation of
which influence perceived reception fidelity, con- the carrier wave
tinue to change. ac = angular frequency of carrier wave
In the early 1980's, stereo transmission on the (radians /second)
AM band became a widespread reality on the Ec = peak amplitude of carrier wave
North American continent. This mode of trans-
mission has become known and is referred to as If A is a constant, the peak amplitude of the
AM- Stereo. Some AM broadcasters see AM- carrier wave is therefore constant and no modula-
Stereo as a necessary means to compete with FM- tion exists. Periodic modulation of the carrier
Stereo which continues to gain increased public wave exists if the magnitude of A is caused to
acceptance for all types of programming. AM- vary with respect to time as, for instance, a sinu-
Stereo transmission was approved on a general soidal wave, i.e.;
basis by the FCC in 1981, but unlike FM- Stereo
or Color Television, the FCC did not adopt a A = 1 + (Em /Ec) x Cos (omit) [2]
transmission standard for this potentially revolu-
tionary development in the standard broadcast Where:
band. Instead the FCC allowed any commercial-
ly developed AM- Stereo transmission system, Em /Ec is the ratio of modulation
which will pass FCC type approval requirements amplitude to carrier amplitude
regarding mono compatibility and interference,
to compete for the broadcasters and consumers therefore;
favor in order to establish a de -facto standard
for AM- Stereo transmission. Some receiver e(t) = Ec x (1 + (Em /Ec) x Cos (wmt))
manufacturers have decided to build mass pro- x Cos (coct) [3]
duction receivers for only one of the various pro-
this is the well known basic equation for periodic
posed systems based on their public and private
amplitude modulation and when all multiplica-
study of the systems merits and market poten-
tions and a simple trigonometric identity are per-
tial. Other receiver manufacturers have decided
formed the result is
to build mass production receivers which will
automatically and /or manually decode any of the
e(t) = Ec x Cos (act) +
several proposed systems. Which receiver market-
(M/ 2) x Cos(act + amt) +
ilnogn gp hteilromso ipnhteyr ewsitl l opfr tehvea illi satnedn inwgh aAt Mis ipnu tbhleic baerset (M /2) x Cos (act -amt) [4]
yet unknown. Several AM- Stereo systems are pre- Where:
sented in the chapter following this one.
M = the amplitude modulation factor Em /Ec
AMPLITUDE MODULATION Equation [4] can be represented graphically
THEORY three familiar ways; in the time domain represen-
tation as shown in Fig. 1, in the frequency do-
Amplitude modulation of a radio signal occurs main as shown in Fig. 2, and as relative vectors
in at least two basic forms, coded (digital) and as shown in Fig. 3. The graphical representations
linear (analog). The first amplitude modulation shown are for a single tone modulation index (M)
processes for long distance communication in- of 0.7, i.e.; the peak modulating voltage is 70%
volved on /off keying of a radio carrier wave. The of the peak carrier wave voltage (Em /Ec = 0.7).
pattern or "code" of the on /off keying process Fig. 2 shows the occupied bandwidth of an AM
determined the content of the information being signal with single tone modulation. From this Fig.
Chapter 1: AM Transmitters 3.1 -3
( +1 VOLTAGE IOR CURRENT)
ENVELOPE OF E= Em
Em
En = PEAK CARRIER VOLTAGE
Etim M=O DPUELAAKT IMOOND U= LA(ETmINrGE d*V O10L0T AGE
( -I VOLTAGE MR CURRENT)
Fig. 1. Time domain representation of a carrier wave signal amplitude modulated
by a sinusoidal audio signal to a peak modulation depth of 70%.
- Ec = 1.0
MODULATION =
Em = ELSE + ELISE
Em x 100 = 70%
Ec
f
1 Em = 0.35
2
0.354
ELSB = EusB = 0.35
J (i-)re>que ncy
1L
- J
fCARRIER fAUDIO CARRIER CARRIER + fAUDIO
MINIMUM
OCCUPIED BANDWIDTH
= 2 X fAUDIO
(ASSUMING NO TRANSMISSION
DISTORTION)
Fig. 2. Frequency domain representation of an amplitude modulated signal showing the
carrier wave signal and two resultant modulation sidebands at 70% modulation.
f
fL
UPPER SIDEBAND -LOWER SIDEBAND
EUSB = 0.35 ELSE = 0.35
Ec
= 1.0
% MODULATION =
ELSe + EUSB x 100
Ec
Em = ELSE + EUSB
Fig. 3. Vector representation of an amplitude modulation signal showing the
carrier wave signal and two resultant modulation sidebands at 70% modulation.
3.1 -4 Section 3: Transmitters
and its defining equation [4], it is clear that the and high efficiency is the vacuum tube class `C'
bandwidth of an AM signal is equal to twice the amplifier. Class `B' RF power amplifiers, very
highest modulating frequency if no system distor- popular in early transmitters, are still used occa-
tion is present. High quality music reproductions sionally at lower power levels and for driver stages
include frequency components as high as 15 kHz of final class `C' or `D' stages. Solid state class
or higher and therefore the required theoretical `D' amplifiers up to 5 kilowatts are becoming
bandwidth of a DSB -FC -AM signal capable of more common as driver stages for final vacuum
high quality music reproduction is at least 30 kHz. tube amplifiers and for the final power amplifier
Harmonic and inter -modulation system distortion modules in transmitters of up to 10 kW carrier
have the effect of widening the effective occupied power. As stated in the introduction, the most
bandwidth of the system. However, most modern common major concerns for both manufacturers
transmitters have sufficiently low distortion and users of modern AM broadcast transmitters
characteristics that bandwidth stretching is not is operating reliability and efficiency, hence
normally a significant problem. The occupied operating cost. To achieve high overall operating
bandwidth characteristics of an AM broadcast efficiency, the stages which consume the most
transmitter are discussed in more detail in the sec- power, the modulator and /or the final RF power
tion on "Factory Tests." amplifier stages must be designed for highest
possible operating efficiency.
RADIO FREQUENCY The basic tuned anode vacuum tube amplifier
POWER AMPLIFIERS is described in graphical form in Fig. 4. The
vacuum tube can be either a triode, tetrode, or
A common AM transmitter major system com- pentode. Tetrode final amplifiers are most com-
ponent is the power amplifier circuit. High power mon in modern high power transmitter designs.
amplifiers that produce 0.25 to 50 kilowatts (kW) The RF excitation voltage is supplied to the grid
of carrier power are common for AM broadcast of the power amplifier tube and the ratio of dc
transmitters in North America. Carrier power grid bias voltage -to -peak RF excitation voltage,
levels up to one megawatt and higher are com- shown sinusoidal in Fig. 4, determines the con-
mon in other parts of the world for medium wave duction angle of anode current, given as
broadcasting. Transmitters delivering these high
power levels should be designed for highest -
0, = 2 x Arc Cos(Ecc /(Eg EcJ) [5]
operating efficiency within the circuit constraints
of the particular manufacturer and /or modula-
tion system used. The most common amplifier where the exciting grid signal is sinusoidal as
used to meet the demands of high output power shown in Fig. 4a.
-X-
=
OPERATING POINT IP
FOR CERTAIN TUBE TRANSFER
CLASSES OF CURVHE eg)/ i
AMPLIFIER OPERATION. IP = I !
..A"
Fig. 4b Fig. 4e
OF
"AB" - - ANODE
1AREDIASSIPA TION
"C" OR "D" e P(min)
eg
II - 0 + 60 N (DEG0 REES) 60 60 B(DEG0 REES) +60
+ EBB iP
-- RESONANT
ANODE LOAD
CIRCUIT RESISTANCE
ÿW Ór-
lCY¢7 Ú'i dfi Flg. 4d - (2 EBB - ep(min))
Oi
U< + Eg2
EBB
ay (ark.)
Fig. 4a
BELOW CUTOFF ABOVE CUTOFF
Ecc Eg,\
I
Fig. 4. Classical vacuum tube Class "C" Amplifier with sinusodial grid
drive, 120° anode current conduction, and resonant anode load.
Chapter 1: AM Transmitters 3.1 -5
The shape of the anode current pulse is deter- DISCUSSION OF
mined by the vacuum tube transfer characteristics BASIC AM SYSTEMS
and input waveshape. The pulse of current thus
generated, Fig. 4c, is supplied by the dc power High Level Anode Modulation
supply, EBB, and passed through the resonant The first practical method of generating the
anode tank circuit shown in Fig. 4d. The reso-
amplitude modulation signal was Heising constant
nant anode tank circuit is assumed to have suffi-
current modulation4, 5, a method of applying
cient operating Q to force anode voltage, ep, to
audio modulation to the anode supply voltage of
be essentially sinusoidal and of the same periodic
a class `C' RF amplifier. This general class of
frequency as the RF excitation voltage and resul-
modulation has since been known as high level
tant anode current pulse. The instantaneous
anode (or plate) modulation. The Heising modu-
anode dissipation, shown in Fig 4e, is the prod- lator was used at least as early as 1920 and was
uct of instantaneous tube anode voltage drop and
usually used to modulate a low power RF ampli-
anode current. The tube transfer characteristic is fier or master oscillator stage which was followed
a variable dependent upon many tube factors as
by several linear amplifier stages until the desired
well as maximum drive signal, Eg. The exact
power level was attained. In some cases the Heis-
shape and magnitude of the current waveform
ing modulator was used to modulate the final RF
is normally obtained from a load -line plot on con-
amplifier stage of lower power transmitters. The
stant current characteristic tube curves supplied
Heising shunt modulator operated in the class `A'
by the tube manufacturer. The resonant anode
mode and therefore was low in operating effi-
load impedance is chosen and adjusted to allow
ciency. The early linear amplifiers were tuned
ep(min) to be as low as possible without causing
class `B' amplifiers operating with carrier level
excessive screen grid (in the tetrode case) or con-
anode efficiencies of 30% maximum. The Heis-
trol grid dissipation. Some manufacturers increase
ing and similar systems of audio amplification
anode efficiency beyond the limits for typical
were also used to modulate the grid bias level of
Class `C' amplifiers by using a circuit employing
RF amplifier stages in order to obtain the AM
a third harmonic resonator between the output
signal to be used for further linear amplification.
anode connection and the fundamental resonant
Heising constant current anode modulation was
circuit. This has the effect of squaring up the
very popular in military and aviation radio sets
anode voltage waveform (ep) thus causing the in-
used through the end of World War II.
tegral of the ep x ip product, or anode dissipa-
tion, to be smaller; resulting in lower anode dis-
sipation for a given RF power output. An ampli- Class `B' High Level Anode Modulation
fier employing the third harmonic anode trap is Historically the most popular method of ap-
commonly referred to as class `C -D'; suggesting plying the audio modulating voltage to the anode
an efficiency rating somewhere between conven- circuit of a class `C' RF power amplifier was by
tional class `C' operation (nominal 120 degree a high power push -pull class `B' audio amplifier.
conduction angle) and true class `D' operation This type of modulation was first used to improve
with rectangular anode or collector voltage wave- the operating efficiency and to increase the out-
forms. Anode efficiencies can be increased put power of AM broadcast transmitters. Class
typically to values of 90 percent for transmitters `B' push -pull audio amplification was first used
up to approximately 10 kW carrier power and
approximately 85 percent for transmitters higher
than 10 kW carrier power by using the third har- 4 "The Equivalent Circuit of the Vacuum Tube Modulator ".
monic trap technique. Table 1 shows a compari- John R. Carson, Proc. IRE, Vol. 9, No. 4, Aug. 1921, p. 243.
son of anode efficiency for six classes of high 5"Modulation in Radio Telephony ". R.A. Heising, Proc.
power tuned RF amplifiers. IRE, Vol. 9, No. 3, June 1921, p. 305.
TABLE 1. Comparison of Tuned RF Amplifier Anode Efficiencies
Conduction Anode
Amplifier Angle Efficiency Defined Conditions
Class (degrees) ( %) of Operation
A 360 30 Eb(min) = 0.10 X EBB
A-B 240 60 Eb(min) = 0.10 X EBB
B 200 67 Eb(min) = 0.10 X EBB
C 120 84 Eb(min) = 0.05 X EBB
C-D 120 90 Eb(min) = 0.05 X EBB
D 120 95 Eb(min) = 0.05 x EBB
3.1 -6 Section 3: Transmitters
to improve distortion and output power of tele- the modulator output circuitry, and the dc cur-
phone transmission amplifiers. The invention was rent to the modulated RF amplifier anode flows
soon recognized by broadcast engineers and ap- directly through the secondary of the output
plied to high level anode modulation. With the modulation transformer. Elimination of these C
final RF power amplifier operating at approxi- and L components is necessary for optimum
mately 8007o anode efficiency and the class `B' operation of modern AM stations. With the ex-
audio modulator total static currents approxi- tra C and L components, the modulator output
mately one -tenth that of an equivalent Heising is effectively a three pole high -pass filter which
modulator, total anode efficiencies at carrier level causes low frequency transient distortion to be
rose to approximately 72% compared to 3707o for generated when driven with the complex
the Heising system and 30% for conventional waveforms that are produced by many modern
linear amplification. A simplified drawing of a and popular program audio processors.
typical high level class `B' anode modulation Eliminating the C and second L component,
system is shown in Fig. 5. The vacuum tubes causes the output modulator circuit to become
shown in Fig. 5 may be either triodes, tetrodes, a single pole high -pass filter, greatly reducing low
or pentodes. The output circuit of the class -B frequency transient distortion.
modulator shows the output modulation trans- Another problem with transformer coupled
former (MT), an audio coupling capacitor (C), high level class- B anode modulation is with high
and a dc shunt feed inductor (L). frequency audio transient distortion. Stray inter-
This arrangement was used in all high level nal winding capacitances and leakage inductances
class -B high power broadcast transmitters until form multi -pole low pass filtering at the high fre-
about 1960 because of a transformer design con- quency end of the audio spectrum. This equiva-
straint that would not economically allow un- lent multi -pole low pass filter generates transient
balanced direct currents to magnetize the overshoot distortion when driven by the same
transformer core material without poor low fre- type of processed complex program waveforms
quency distortion. Advanced technology core mentioned above. Transient overshoot up to 1207o
materials and careful magnetic transformer design is typical for squarewave modulator input signals
allowed elimination of the coupling capacitor and and results in a required modulation level reduc-
the dc feed shunt inductor, first in some 100 kW tion of the same 12% in order to maintain peak
European transmitter designs in the early 1960's, modulation levels within FCC allowed limits. This
and in an American shortwave transmitter design high frequency transient overshoot distortion can
in the later 1960's. Many of the more advanced be effectively minimized by filtering the audio in-
modern AM broadcast transmitter designs still put to the transmitter with linear phase filters,
using high level class -B anode modulation have resulting in somewhat lower high frequency audio
eliminated the extra C and L components from response, and/ or by careful control of the
MT= MODULATION TRANSFORMER
C = AUDIO COUPLING CAPACITOR
L = SHUNT AUDIO REACTOR
MODULATOR
FEEDBACK
SAMPLE C I 1.i
DRIVER
AMPLIFIER(S) R.F.
VACUUM TUBE LOAD
OR T RESISTANCE
SOLID STATE
A.F.
INPUT ïi
f
DRIVER
AMPLIFIER(S)
r VSAOCLUIDUO MRS TTAUTBEE VOHLITGAHG E BYPRAFS S CRLFA SDSR IV"CE"
SIGNAL
D.C.
POWER
MODULATOR SUPPLY
FEEDBACK
INPUT SAMPLE PUSH -PULL
AMPLIFIERS CLASS "B"
MODULATORS
Fig. 5. High level anode modulation employing transformer coupled push -pull Class -B modulators .
Chapter 1: AM Transmitters 3.1_7
modulation transformer equivalent circuit yielding Besides causing switching losses, the high stray
more linear audio phase characteristics for the capacitance to ground is also a cause of major
entire modulator circuitry. Balanced modulator modulator distortion at high negative modulation
negative feedback is used to reduce modulator indices.? The circuit shown in Fig. 6b is one in-
non- linear distortion and noise. Negative feed- genious way to overcome the stray modulator
back, however, usually worsens high audio fre- capacitance problem. It is identical to the circuit
quency transient distortion characteristics. in Fig. 6a, in principle, except that the high
Pulse Width High Level Anode Modulation voltage pulse modulated wave is at a point in the
circuit where the shunt capacitances to ground
Pulse width modulation (PWM) of the dc are inherently minimized. The circuit in Fig. 6c
anode voltage of a class -C RF amplifier was first is basically the same as in 6b except the system
used in commercial high power broadcast ground has ingeniously been placed at another
transmitters in Europe in the early 1960's. It was point in the circuit. PW Modulator anode effi-
the first commercially successful attempt to ciencies approach 92% in some higher power
significantly improve upon the efficiency of the transmitter designs yielding a combined modula-
popular high level class -B modulation system by tor and carrier tube anode efficiency of approxi-
applying and improving basic PWM concepts that mately 74% at all levels of modulation. An added
were described decades earlier.6 Since this first efficiency advantage over high level class -B anode
success, pulse width modulation has become a modulation is that a PWM transmitter may have
preferred method of high level anode modulation only two high power vacuum tubes, one modu-
by many broadcast engineers, and is employed lator and one final RF amplifier, thus eliminat-
in several broadcast transmitter designs by several ing the filament heating power of one large
manufacturers world wide. The basic pulse width vacuum tube required in push -pull modulator
modulation system and two ingenious improve- designs. A significant disadvantage of PWM as
ments to the basic system are shown in Fig. 6. described in Fig. 6b and 6c is that the cathode
The circuit in Fig. 6a graphically describes the and grid circuits of the modulated RF amplifier
basic principle of PWM. An inherent practical are operated at high voltage levels off ground,
deficiency in the basic concept, Fig. 6(a), is caused adding complications to the circuitry in these
by the relatively high shunt circuit capacitance areas that are avoided with the classical PWM
of the modulator tube filament transformer plus circuit of Fig. 6a and conventional high level
stray capacitances. Though special low - class -B anode modulation. Another major disad-
capacitance isolation transformers can be used to vantage of PWM in any form is that of transient
supply modulator filament and auxiliary power distortion caused by the phase non- linearity of
to minimize capacitive switching losses, typical the multi -pole PWM filter, similar to that
realizable values of capacitance can cause exces- previously discussed for high level class -B anode
sive switching losses and audio distortion in lower modulation. The switching frequency is typically
power transmitters.
70 kHz for most transmitters manufactured in
For example, the switching losses at carrier level North America. This frequency is chosen to ease
of a typical PW modulator for a 5 kW transmit- compliance with FCC regulations which requires
ter can be higher than the quiescent modulator all spurious radiation more than 75 kHz removed
losses of an equivalent power class -B modulator from the carrier frequency to be 80 dB or more
even when the stray modulator tube capacitances below the carrier level. Present FCC regulations
are as low as 100 pF. The power lost per modu- require transmitter spurious output between 30
lator switching cycle is kHz and 75 kHz removed from the carrier to be
only 35 dB below the carrier level. To meet
Pmodsw = (CV2/2) + Ptd [5] these spurious requirements (most manufactures
of PWM transmitters far exceed these require-
Where: ments), a very steep cutoff lowpass filter is re-
C is the shunt modulator filament quired at the output of the PW modulator. Most
transformer plus stray capacitance manufacturers of PWM transmitters make a
to ground and; valiant attempt to maintain linear phase charac-
V is the pulse switching voltage to teristics in this filter to as high an audio frequen-
ground at the cathode of the tube. cy as possible, but the laws of nature prevent re-
Ptd is the saturated tube and diode quired linear phase characteristics to the highest
losses during the respective on and third audio harmonic to be achieved within the
off conduction states major attenuation constraints noted above. As
6"Transmission System ". R. A. Heising, U.S. Patent No. 7 "Distortion in Pulse Duration Modulation ". Ernest R.
1,655,543, Jan. 1928. Kretzmer, Proc. IRE, Vol. 35, No. 11, Nov. 1947, p. 1230.
3.1 -8 Section 3: Transmitters
LOW CAPACITANCE
CATHODE AND OUTPUT SWITCHING
DRIVER ISOLATION CAPACITANCE =
TRANSFORMER C.FYR + COW. + CSTP
-U -- U-J - Eirowunow
SERIES PULSE
WIDTH MODULATOR V,Eon
///
LOAD
GREFSISTA NCE
CLASS "C.
RF POWER
ISOLATED AMPLIFIER
P.W.M.
SIGNAL
DRIVER
(FIBER OPTICS)
RF DRIVE
SIGNAL
Fig. 6a. Basic classical high level anode pulse width modulation of a vacuum tube Class "C" amplifier.
SYSTEM
GROUND
QRF
OAD
ISOLATED
DRIVE SIGNAL
(FIBER OPTICS) FLOATING
MSOWDIUTCLAHTINOGR RSF IGDNRAIVL E
OUTPUT
CAPMACINITIMANIZCEED (C) CATHODE OFF
0.0 AND AUDIO
GROUND
MODULATOR
CATHODE Mr
TRAIANSNDOS LFDAOTRROIVMRE ERR MOSDTEUURLBIAEETS O R MNOEDGUEALeeTA ITVEED
Fig. 6b. Collins Radio /Continental Electronics patented modification to basic high level PWM
System to minimize modulator losses by minimizing switching modulator output capacitance.
RF
CLASS "C
RF OUTPUT
AMPLIFIER
FLOATING
SWITCHING RE DRIVE
MODULATOR SIGNAL
OUTPUT
CAPACITANCE ICI
MINIMIZED CATHODE OFF
D.C. AND AUDIO
GROUND
SYSTEM
GROUND
SERIES NEGATIVE
MODULATOR MODULATED
TUBE Eae
Fig. 6c. Harris Corp. patented modification to basic high level PWM System to
minimize modulator losses by minimizing switching modulator output capacitance.