User Manual

UHF Analog Driver/ Transm it ter Chapter 4, Circuit Descript ions
LX Series, Rev. 0 4- 28
regulated voltage that is about 5 volts
less than the + 32 VDC supply,
approxim ately + 26.25 VDC. When the
+ 32 VDC supply is enabled, the circuitry
around U13B is used 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
am plifier # 1 should be greater than 5
Am ps. U11B is used to turn off the Q10
FET, if high current is detected in
am plifier # 2. U11A is used to turn off
the Q10 FET, if high current is detected
in am plifier # 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 Am ps.
U14B is used to turn Off the gate of Q10
in the event of high power supply
voltage, approxim ately + 35.4 VDC.
U14A is used to keep the FET disabled in
the event of low power supply voltage,
approxim ately + 25.4 VDC.
Current m onit oring sections of the board.
The ICs U16, U17 and U18 along with
associated com ponents set up the
current m onitoring sections of the board.
R67, R68 and R69 are 0.01Ù/ 5W 1%
through hole resistor is used for
monitoring the current through several
sections of the am plifier. The voltage
developed across these resistors are
am plified for current m onitoring by U16,
U17 or U18. The LT1787HVCS8 precision
high side current sense I C am plifier
accepts a m axim um voltage of 60 VDC.
The 43.2 kÙ 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 m atches the resistors of this
part but their actual resistance value is
not closely defined. A trim m ing resistor
is suggested to give a tem perature
stability of –200 ppm/ C, but instead the
microcontroller will determ ine the exact
gain of the circuit and use a correction
factor for m easurements. Circuit loading
com ponents are located in the lower
portion of each current m onitoring circuit.
These com ponents 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 Ù 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 am ps + 1%
through the load resistors.
A6 is a tem perature sensor thermistor
that is used to monitor the tem perature of
the module's heat sink. It connects to J6
pins 1 & 2 on the board wand is wired to
the com parator IC U10B. If the
tem perature increases above 75° C the
output will go Low that is used as a
tem perature fault output, which generates
a Fault alert at U15A and disables
Am plifier # 1.
Aural, Visual/ Average and Reflected power
det ect or sect ions of the board.
Page 3
A Forward Power Sam ple 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 Sam ple
Jack, located on the front panel of the
assem bly. The other part of the split
forward power sam ple 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 this 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 elim inate the 4.5
MHz aural intercarrier, if present. The
Average power sample is am plified by
U26D and connected through the average
calibration pot R166 to U26C. The output