UHF Analog Driver/Transmitter/
Chapter 4, Circuit Descriptions
Translator
LX Series, Rev. 3
4-23
to provide gate voltage to Q10 that is 5
volts greater than the source pin of this
FET. The gate of Q10 can be turned Off
by any one of a few different circuits.
U10A is used to turn Off the gate of Q10
in the event of high current in amplifier
#1. At 0.886 VDC the current to
amplifier #1 should be greater than 5
Amps. U11B is used to turn off the Q10
FET, if high current is detected in
amplifier #2. U11A is used to turn off
the Q10 FET, if high current is detected
in amplifier #3. With 2.257 VDC at Pin 5
of U11B or Pin 3 of U11A, the voltage
output of current sense amplifier U17 or
U18 at high current shut down should be
greater than 15 Amps.
U14B is used to turn Off the gate of Q10
in the event of high power supply
voltage, approxi35.4 VDC.
U14A is used to keep the FET disabled in
the event of low power supply voltage,
approxi25.4 VDC.
4.6.6.3 Current monitoring sections of
the board.
The ICs U16, U17 and U18 along with
associated components set up the
current monitoring sections of the board.
R67, R68 and R69 are 0.01O/5W 1%
through hole resistors used for
monitoring the current through several
sections of the amplifier. The voltage
developed across these resistors are
amplified for current monitoring by U16,
U17 or U18. The LT1787HVCS8 precision
high side current sense IC amplifier
accepts a maximum voltage of 60 VDC.
The 43.2 kO resistor from pin 5 to
ground sets the gain of the amplifier to
about 17.28. This value is not set with
much accuracy since the manufacturer
internally matches the resistors of this
part but their actual resistance value is
not closely defined. A trimming resistor
is suggested to give a temperature
stability of –200 ppm/C, but instead the
microcontroller will determine the exact
gain of the circuit and use a correction
factor for measurements. Circuit loading
components are located in the lower
portion of each current monitoring circuit.
These components allow for short duration
high current loading of the supply. By
measuring the current through the sense
resistor with and without the additional
four 30.1 O 1% resistors. For very short
duration pulses, a 1206 resistor can handle
up to 60 watts. The processor requires
226 uSec per conversion. A supply voltage
of +32 VDC will pass 1.06 amps + 1%
through the load resistors.
A6 is a temperature sensor thermistor
that is used to monitor the temperature of
the module's heat sink. It connects to J6
pins 1 & 2 on the board and is wired to
the comparator IC U10B. If the
temperature increases above 75°C the
output will go Low that is used as a
temperature fault output, which generates
a Fault alert at U15A and disables
Amplifier #1.
4.6.6.4 Schematic Page 3, Aural,
Visual/Average and Reflected power
detector sections of the board.
A Forward Power Sample enters the board
at SMA Jack J3 and is split. One part
connects to J4 on the board that is cabled
to J1, the SMA Forward Power Sample
Jack, located on the front panel of the
assembly. The other part of the split
forward power sample is detected by CR17
and the DC level amplified by U25A. The
output of U25A at pin 1 is split with one
part connected to the Aural Power sample,
which is not used in a digital transmitter.
The other split output connects to U265A
that is part of the Forward Average Power
circuit. The detected level is connected to
L4 that is part of an intercarrier notch filter
circuit that is tuned to eliminate the 4.5
MHz aural intercarrier, if present. The
Average power sample is amplified by
U26D and connected through the average
calibration pot R166 to U26C. The output
of U26C is connected to the comparator IC
U26B that has Aural Null and Offset Null, if
present in the system, connected to the
other input. The output Average Forward
power level connects to J9 pin 2 of the
board.
