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SSM Series, High Efficiency Solid-State Modulators  
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Solid-State Modulator

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

Solid-State Modulator

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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Features

High Efficiency Solid-State Modulators