SSM Series High Efficiency
Solid State Modulators
for 100 kW, 300 kW, 500 kW, 600 kW, 1000 kW Broadcast Transmitters
Continental Electronics high
efficiency solid state modulators provide outstanding efficiency
and reliability. In addition, their audio quality sets new
standards of performance for high power broadcasting. Other
features such as installation flexibility, size, ease of
maintenance, cooling system simplicity, etc., make these
and their associated transmitters the clear choice of the
broadcasting community. Along with the conventional AM (A3E)
mode of operation, the modulators also provide operation
in the Controlled Carrier-Level Modulation (CCM) mode, and
the h3E and R3E Single Sideband (SSB) modes.
The modulator provides high level anode modulation of the
associated RF amplifier, and as such, supplies the DC plate
voltage to develop the RF carrier power and the audio voltage
to develop the modulation sidebands. Functionally the high
efficiency solid state modulator serves as both the RF amplifier
anode power supply, and the high level audio power source.
SOLID STATE
The modulator consists of 48
series connected modules which are switched on or off to
provide the high voltage DC and the superimposed high level
audio voltage. The switching is accomplished with Insulated
Gate Bipolar Transistors (IGBT). A low pass filter follows
the series connected modules which removes the switching
signals and allows the DC and audio signals to pass to the
RF amplifier. Because each of the modules is either in full
conduction with very low loss, or turned off, again with
very low loss, the overall modulator efficiency is in excess
of 97%.
Control of each module is accomplished from an assembly
of printed circuit cards that are contained in a control
module located within the RF amplifier cabinet. Interconnection
between the control module and each of the 48 series connected
modules is through fiber optic cables that insure positive
control while totally eliminating the possibility of conducted
RF interference. Patented* circuits, using simple discrete
logic, are employed to insure that each module contributes
equally to the overall output, and in the unlikely event
of a module failure, to effectively remove the module from
compromising the overall output voltage.
*U.S. PATENT NOS. 4,896,372; 5,099,203; 5,200,707
Each of the modulators are capable of producing a maximum
of 16,500 VDC average for the carrier condition, and 33,000
V peak at the positive crest of the modulating signal. Each
IGBT has a collector-emitter rating of 1,200 VDC which give
an ample safety factor for the 700 VDC that each module
can produce. The actual voltage for each module is somewhat
less than 700 VDC and is dependant on the desired anode
voltage desired for the particular RF amplifier tube. Normally
this unmodulated voltage is in range of 14 kV to 15 kV.
The current rating of the IGBT switch elements are considerably
higher than necessary for the peak current demands. 50 ampere
devices are used for the 100 kW modulator, 150 ampere device
for the 500/600 kW modulator, and 300 ampere devices for
the 1000 kW modulator.
The simplified schematic of each switch module is shown
in Figure 1, 48 modules are arranged in series as shown
in Figure 2. The pictures shown above show the 48 modules
arranged in 4 columns of 12 modules each, and a close up
view of 6 of the modules. The small inductor which is part
of the series connection between each module is used to
reduce the transient charging currents when an individual
module is switched on, and these inductors form part of
the input inductance of the low pass filter. The audio response
and total harmonic distortion (THD) of the recovered audio
from a modulated RF amplifier is depicted in Figure 3. There
is essentially no difference in this performance at the
equivalents of 25% to 95% modulation.
CONTROLLED CARRIER-LEVEL MODULATION
Controlled carrier-level modulation
(CCM), a standard feature of the solid state modulator maintains
the carrier at a sufficient level to be 100% modulated by
the incoming audio. In operation, the input audio level is
set so that 100% modulation is achieved on program peaks with
the transmitter carrier output at the full level. With this
audio level setting and CCM systems enabled, the carrier level
will fall to a preset level in the absence of modulation,
and rise to a level compatible with the instantaneous peak
level of the program audio.
The level to which the carrier will be reduced is adjustable
at the front panel of the transmitter in 1 dB steps from 0
to 6 dB.
Activation or deactivation of CCM is also selectable at the
transmitter front panel by a single switch, and may be accomplished
at any time without interruption of programming. The primary
reason to employ CCM is to effect an input power saving, and
this should be accomplished without reducing listener satisfaction.
Our tests and reports indicate that the use of the CCM is
virtually undetectable to the audience. In practice, actual
power saving over non CCM usage is highly affected by program
content, with talk programs giving more power saving than
most music programming. Power consumption tests using processed
music programming and a transmitter with 70% efficiency consistently
produced a power saving of 18%, when CCM with a 6 dB carrier
level drop was used; talk programs produced power savings
in excess of 22%.
SINGLE SIDEBAND
The SSB modes of h3E and R3E
are available as an optional feature when the solid state
modulator is used with the CEC supplied frequency synthesizer.
The SSB system is the Envelope Elimination and Restoration
technique which employs phase modulation of the frequency
synthesizer in conjunction with amplitude modulation of the
RF amplifier with an analog of the SSB signal. With this system
the high efficiency of the Class C RF amplifier is retained
and tuning of the RF amplifier is the same for both AM and
SSB. SSB performance with the Continental Electronics Corporation
solid state modulator is shown in Figure 4. The two audio
tone spectrum was produced with audio tones of 2,000 Hz and
2,165 Hz at a carrier suppression level corresponding to -19
dB relative to the Peak Envelope Power (PEP) of transmitter.
The very low evidence of intermodulation distortion and unwanted
sideband indicates the exemplary performance of the system.
In the SSB modes all of CCIR recommendations are met or exceeded.
POWER SUPPLY
Each of the modulator power
transformers have primary taps for extended delta connections
so that the phase relationship of each transformer winding
is shifted +/-15 degrees from the power line phase. With this
arrangement one transformer is connected for +15 degrees shift,
and the other transformer shifted -15 degrees. (In the 1000
kW modulator two transformers are at +15 degrees and two are
at -15 degrees.) This phase shifting produces the effect of
12 pulse rectification and greatly improves power line utilization.
An additional benefit is that rectification harmonics conducted
back to the power source are attenuated significantly below
that which would be produced by the conventional 6 pulse rectification.
Power transformers can be provided to operate from any of
the normal 3-phase line voltages, for either 50 Hz or 60 Hz
operation, and are sized for the required transmitter power
output.
INSTALLATION
Typical installation layouts
for the various modulator power levels are shown as a guide.
It is important that the power transformer and the columns
of solid state switches be in the same relative position as
shown; otherwise the modulator assembly can be placed almost
anywhere with respect to the associated RF amplifiers.
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