User's Manual

Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT835A, Rev. 1 4-21
sample that is split is connected to
U2B, pin 5.
If the reflected sample level increases
above the level set by R22, the VSWR
cutback pot, the output of U2B at pin 7
goes high. The high is connected to
Q11 through CR11, which is biased on,
making U2C, pin 10, low and causing
U2C, pin 8, to go low. This low is split
and fed out of the tray at J1-6, J1-7,
J1-8, and J1-9; these are ALC outputs
to the amplifier trays that cut back the
output power of the amplifier trays. The
low from U2C, pin 8, is also fed through
coaxial jumper W2 on J13 and J14 to
R73. R73 is a bias adjust pot that sets
the level of the pin-attenuator bias that
is available as an output at J16.
The high at U2B, pin 7, is fed to the
base of Q14 and Q13, which are
forward biased, and produces a low at
the drains. The low connects to the
front panel amber VSWR Cutback LED
DS7 and causes it to light, indicating
that the tray is in cutback, and to
output jack J8-37 for a connection to a
remote VSWR cutback indicator.
(Optional) Receiver ALC Fault
If a receiver tray is part of the system,
a sample of the ALC voltage from the
receiver tray is connected to J8-11 on
the board. If the receiver is operating
normally, the ALC level applied to U3C,
pin 9, remains below the trip level set
by R35; the output at pin 13 stays
high. The high is applied to the red ALC
Fault LED DS8, which is off. The high
also connects to U3A, pin 2, and to
Q15. Q15 is biased on and the drain
goes low. The low connects to U5B, pin
6. U5B normally has a low that is
connected to U5B, pin 5, and produces
a high at the output pin 4. The high is
wired to Q18, which is biased on, and
makes its drain low. The low connects
to U3D, pin 12, which, because the
level is below the preset, its output at
U3D, pin 14, goes low. A low at this
point indicates a no-fault condition. The
high connected to U3A, pin 2, causing
its output to go low. The low is
connected to Q25, which is biased off.
The low is removed from J8-12, which
will not light any remote receiver fault
indicator that is connected to it.
If the receiver should malfunction, the
ALC level applied to U3C, pin 9, goes
high; because this is above the level
set by R35, the output at pin 13 will go
low. The low is applied to the red ALC
Fault LED DS8, which lights. The low
also connects to U3A, pin 2, and to
Q15. Q15 is biased off and the drain
goes high. The high connects to U5B,
pin 6, which produces a low at its
output at pin 4. The low is wired to
Q18, which is biased off, and makes its
drain high. The high connects to U3D,
pin 12; because the level is above the
preset, the output at U3D, pin 14, goes
high. A high at this point indicates a
fault condition that switches the
transmitter to Standby. The low
connected to U3A, pin 2, causes its
output to go high. The high is
connected to Q25, which is biased on,
causing its drain to go low. The low is
connected to J8-12, which can light any
remote receiver fault indicator that is
connected to it.
4.3.10.5 Metering
The front panel meter connects to J3-
1(-) and J3-2(+) on the board; this is
the output of switch S3. The front panel
meter has four metering positions that
are controlled by switch S3: % Forward
Power, % Reflected Power, % Exciter,
and ALC.
The reflected sample connects to the
board at J2-9 and is connected through
buffer amplifier U1B and 100-Ω resistor
R84 to position 3 of switch S3. The
forward sample connects to the board
at J2-5 and is connected through buffer
amplifier U1D and 100-Ω resistor R86
to position 4 of switch S3. The exciter
sample connects to the board at J2-3
and is connected through buffer