4.7 Driver Switch Logic Circuit Board
The Driver Switch Logic board is mounted on the bottom side of the transmitter chassis with
the two power supplies. The Driver Switch Logic board performs the following five different
functions, program audio detection, audio fail timer, PA current monitor, PSU2 voltage monitor,
and driver switch.
The program audio detection circuit and the audio fail timer is made up of U3,U4, and U5. U3A
and U3B and associated circuitry discriminate between normal program material and white
noise ( such as might be present from a studio transmitter link during program failure) or si-
lence. U3A and surrounding components form a band-pass filter with a Q of 3 tuned to about
5kHz. U3B is a first-order low pass filter. Red (DS3) and green (DS4) LEDs on the board indi-
cate the presence or absence of program determined by the balance of the detected signals
from the two filters. U5 and U4B form a count-down timer. The time between a program fault
and shutdown is selected by jumpering pins on header JP1. For times, see section 5.7. The
times are proportional to the value of R28 ( that is, times can be doubled by doubling the value
of R28). When the timer circuit times out, a red LED (DS2) will illuminate giving a visual indica-
tion of the audio fail timers status.
The PA current monitor circuit consists of R45(R51) U6(U7), U8 and associated circuitry.
Note that components listed in parentheses ( ) are used in the PA current monitor for PA2.
Current passing through R45(R51) creates a voltage drop that is measured by the current
monitor U6(U7). U6(U7) has a gain of 5. The output voltage from U6(U7) is delivered to op-
amp U8A (U8B) which has a gain of 4. The outputs of U8A and U8B are summed together
through R47 and R53. The summed output equals the total currents drawn by both RF power
amplifiers PA1 and PA2. This total current draw is displayed on the front panel as PA DC
Amps.
The PSU2 voltage monitor is used to monitor the status of the high voltage power supply and
determines if it is stable and operational. The PSU2 voltage monitor circuit is made up of R29,
R30, U9A ,U11 and associated circuitry. R29 and R30 create a voltage divider circuit that
scales down the voltage from the high voltage power supply. The scaled down power supply
voltage is fed to comparator U9A. When the high voltage power supply has an output voltage
above 21 volts, the output of comparator U9A goes high. The output of U9A is fed to inverter
U11B which inverters the signal creating a logic low output. This output is fed to NOR gate
U2C in the driver switch circuit. The logic high signal from U9A is also fed to U11A which en-
ables the green LED (DS8) indicating that the voltage from the high voltage power supply is
OK.
The driver switch circuit is the last circuit on the driver switch logic board that determines if the
driver amplifier should be enabled based on the status of the above mentioned circuits. The
driver switch circuit consists of U2C, Q1, Q2 and associated circuitry. Logic gate U2C is a
NOR gate. The inputs to U2C are the outputs of the PSU2 voltage monitor and the audio fail
timer circuits. If the outputs from the PSU2 voltage monitor circuit and the audio fail timer cir-
cuit are at a logic low, the resulting output of U2C will be a logic high, enabling transistor Q1
and turning on green LED (DS1) indicating that the driver amplifier should be enabled. When
Q1 is turned on, this will place about –8.8 volts (w/respect to the source of Q2) on the gate of
the P-channel MOSFET Q2. This will cause MOSFET Q2 to conduct, delivering +24 volts to
the driver amplifier, thus enabling the RF output.
4-11 Principles of Operation
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