Circuit Description; Combo Board - BW Broadcast TX5 FM Broadcast Transmitter Technical Manual

3.2

CIRCUIT DESCRIPTION

The transmitter consists of three main circuit boards, two power supply modules and the associated wiring and
connectors. The power supplies are fully approved modules and won't be discussed electrically except for the
connections to the other boards where appropriate.
The three PCB's previously mentioned are the combo board, the power amplifier board and the control / LCD
board. The wiring and connection between the boards can be seen in the internal case diagram.
The combo board contains
1. DSP Audio limiter with pre-emphasis capability
2. High spec over-sampled digital stereo encoder
3. High spec low distortion PLL exciter
4. Power supply circuitry to supply various voltages to the different sections
5. A logic / control section to interface to external equipment
6. Control connections to the LCD control board
The Power amplifier board contains
1. A 2 stage 150/300 Watt FM amplifier
2. Low pass harmonic filter
3. VSWR bridge / coupler and power sniff circuitry
4. Temperature sensing circuitry
The Control / LCD board contains
1. A 122x32 LCD graphics display
2. Three front panel buttons for LCD control
3. Power control adjustment and transmitter fault / protection circuitry
4. An 8 bit microcontroller to control all the LCD functions, metering and alarm monitoring
We will describe the electrical workings of the transmitter from the audio input through to the RF output and will
attempt to explain how the three boards tie together and form the complete unit.
We will start at the audio input sockets on the back panel which are part of the combo board.

3.21 COMBO BOARD

Audio limiter description
Left and right audio signals are applied to balanced input XLR sockets on the back panel. The balanced audio
signals are fed to IC14 and IC13 which are configured as differential amplifiers to convert the balanced inputs to
unbalanced for the rest of the limiters circuitry. The output from the differential op-amps feed an oversampled ADC
(IC15). The digital input feeds into an AES/EBU decoder chip (IC12) with built-in SRC via an isolating transform-
er (TR1). The backpanel input source switch selects which input is fed into the four DSPs (IC8-IC11). Following
the digital signal processing the DAC (IC15) returns the signal to the analog domain in preparation for the stereo
encoding.
Stereo encoder description
The stereo encoder section is based around a high speed switch which is used to generate the multiplex at 38
kHz. The switch is controlled by microcontroller IC5 which supplies the timing signals to the switch at 304 kHz.
The high speed enables an over-sampled 38kHz subcarrier to be generated that is rock steady and spectrally
clean. As the signal is generated digitally no adjustments or setups are required for the encoder. The only adjust-
ments on the encoder section are the stereo/mono control and the level of the stereo subcarrier. The 19kHz pilot
tone is also generated by the microcontroller and because of this the stereo separation is excellent due to the per-
fect timing between the pilot and the subcarrier. The 19 kHz pilot is also generated by over-sampling techniques
to produce a very low distortion pilot tone.
The audio from the DAC is buffered through IC2 and IC7. These buffer op-amps drive into the analogue switches
(IC6). The switches are controlled by the microcontroller IC5 and the output from the switches feed into three
BW Broadcast technical manual
Technical data
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