Broadcast Warehouse TX 150/300, Technical Manual, Page: 26 / 41

TX FM Transmitter technical manual
page 26
Technical data
3.2 CIRCUIT DESCRIPTION
The transmitter consists of three circuit boards, two power supply modules and the associated wiring and connec-
tors. Both of the power supplies are fully approved bought in 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. 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.
COMBO BOARD
The description of the circuitry describes the right channel path where stereo sections are mentioned. The compo-
nent for the left channel is bracketed next to the right channel component.
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 IC1(IC4) which are configured as differential amplifiers to convert the balanced inputs to unbal-
anced for the rest of the limiters circuitry. The output from the differential op-amps feed a pre-emphasis filter
which can be switched in or out of circuit via the on board jumpers J2(J3). The audio is then fed from the pre-
emphasis filters to one half of dual op-amps IC2(IC3) where input gain can be adjusted through the rear panel
input gain control VR1(VR2) which forms the feedback path for the op-amp. The audio is then fed into the other
half of IC2(IC3). These dual op-amps are also the limiter gain control and output for the limiter.
The output of the input gain op-amps also feed the full wave rectifier circuitry which provides a DC representation
of the audio signal. This DC voltage is applied to a time constant circuit which provides the attack and release
parameters for the limiter. The time constant capacitors C14 and C15 together with R16 and R17 provide a pro-
gramme dependent interactive time constant for clear punchy sound at all levels of limiting. After passing through
the time constant filter the DC signal is buffered by IC7 and then fed to IC8 which drives PNP transistors T1(T2)
which controls the current passing through the gain control element, transconductance amplifier IC5.
Transconductance amplifier IC5 is configured as a variable resistor which is placed in the feedback path of the
gain control op-amps. IC5 varys its resistance in accordance with the level of audio drive so that the output of the
gain control op-amps is fixed at 0dB. LED1(LED2) provide clipping of any overshoots that get through the
limiter.This clipper can be switched in and out of circuit by jumper J6(J7). This provides significantly more loud-
ness than can be obtained by making the attack time quicker to catch the overshoots. The amount of clipping can
be controlled by J2( J3). These provide more drive to the gain control op-amps by putting resistor R8(R29) in par-