The new RCA BTF-1D FM transmitter was designed for both conventional and multiplex operation. It uses the standard BTF-10B Exciter, (see note 1), as its modulating and frequency control element. For simplicity of operation, it was desired to reduce the additional RF circuitry following the exciter to a minimum. The ideal solution was to use only one tube. By choosing the 4CX1000A, a modern high-gain air cooled tetrode, a power gain of 200 including all circuit losses is achieved. This made it possible to eliminate intermediate stages and to resistance load the 4CX1000A input circuit for added stability. Further simplification is achieved in the control circuits and power supplies by using silicon diodes.
Controls and meters are mounted on the vertical panel on the right side of the cabinet. Amplifier tuning controls, a multimeter, and the exciter controls are accessible through a non-interlocked front door (see Fig. 1). The exciter is mounted so it may be tilted out giving complete accessibility to all components (see Fig. 2). All other transmitter components are accessible from the back through an interlocked door.
The output of the exciter is approximately 10 watts at the carrier frequency and is fed by 125-ohm coaxial cable to the input of the 4CX1000A amplifier tube (see Fig. 3). The amplifier input circuit is a simple parallel resonant circuit, tuned by a variable inductance with resistance swamping for stability of operation (see Fig. 4). This stage is neutralized by varying inductance in series with the screen. The output circuit is a modified pi network, having a variable inductance across the tube capacity-which is used to adjust the loading. All capacitors in the final stage are of the fixed ceramic type. A small blower on the back of the RF compartment provides sufficient filtered air for cooling the 4CX1000A for operation up to 7500 feet. The filament transformer is of the regulator type and keeps filament voltage regulation within one percent.
The high-voltage power supply uses a bridge circuit of silicon diode rectifiers (see Fig. 7). Each leg consists of 18 silicon diodes in series, each diode shunted by 27,000 ohms to insure equal voltage distribution. Printed circuit construction is used for each leg, making a unit with a peak inverse voltage rating of 7,200 volts. This supplies more than ample safety factor when used in a 2,700 volt supply. The screen silicon supply uses four diodes to supply the screen voltage for the 4CX1000A stage. A variable transformer in the primary of this supply provides control of the screen voltage, which is used to control the power output of the transmitter.
Overload protection for the transmitter is supplied by a line circuit breaker and two overload relays in the ground leads of the two power supplies. These overload relays also protect the 4CX1000A tube, since it constitutes the total load for both supplies. The blower is protected by a thermal overload, and the tube by an air interlock. Only two fuses are used, both are in the crystal heater circuit.
Remote Control Provisions
The BTF-1D FM transmitter incorporates connections for remote control and remote meter reading, when combined with a remote control system such as the BTR-11B. To control transmitter power level remotely, an accessory motor drive may be connected to the screen supply control. Furnished with the RCA harmonic filter, the BTF-1D meets current FCC and industry requirements with regard to spurious emission, cabinet, and harmonic radiation. Hence, this new unit is an excellent low-power, low-cost transmitter for both conventional and multiplex FM operation.
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