User's Manual

300-Watt Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT830A, Rev. 1 4-29
4.3.11.1 Phase Control
The phase control pot on the front of the
tray connects to J8-1 (+26 VDC), J8-3
(phase I/P), and J8-4 (RTN) on the
board. The phase-control output connects
from J7 on the board to the input of the
phase-control circuit on the variable
gain/phase board (1265-1425).
4.3.11.2 Automatic and Manual Gain
Control Circuits
The amplifier control board contains the
AGC function for the UHF amplifier tray in
which it is mounted. An AGC reference-
level input from the UHF exciter tray is
applied to J3, pin 1, and is amplified by
U3B. The output of U3B is connected to
J5, which is wired to the front panel gain
pot that sets the output power level of
the tray when the AGC is in the Auto
position. The voltage at the arm of the
front panel gain pot is amplified by U2D
and is compared to a sample of the
output power of the tray in U2A. The
error voltage from U2A is sent through
Auto/Manual switch S1 to J10, which
connects to the pin-diode attenuator
circuit on the variable gain/phase board.
A sample of the AGC voltage level is
connected to position 1 on the front
panel meter switch. The tray can also be
operated in manual gain by switching S1
to the Manual position and adjusting R16
for the desired output power level.
FETs Q1 and Q3 delay and slowly reapply
the AGC voltage to the variable
gain/phase board when the system is
switched on or when the board is
switched from Auto to Manual (or back)
to prevent the overdriving of the tray.
4.3.11.3 Overdrive Circuit
A sample of the output of the single
stage amplifier assembly, class A, from
the coupler board assembly connects to
J11 on the amplifier control board. The
sample is peak detected by CR7 and U3A
and the output is connected to U3D. If
the input drive level increases above the
overdrive threshold reference set by R71,
which is the drive level needed to
produce 110% output power, the output
of U3D goes high and is split three ways.
One of the highs is connected through
R38 and CR3 to U3C, causing its output
to go high and lighting the red Overdrive
LED DS2 on the front panel. Another of
the highs is connected through R74 and
R75 to Q6, which is biased on and causes
its output to go low. The low is connected
through J10 to the variable gain/phase
board and cuts back its output power.
The final high from U3D is connected
through CR2 and R37 to U1D, which is
biased on, and causes its output to go
high. The high is connected to U2A,
whose output decreases and cuts back
the output power. If this path is not
present, the AGC, because the forward
power decreases, will try to drive the
variable gain/phase board harder,
creating a positive feedback loop that
could damage the amplifier tray.
4.3.11.4 Metering Circuits
The +26 VDC that is connected to the
board from the switching power supply is
applied to jack J6, pin 1, of the board
and connected through R63, R96, R66,
R65, and R76 to the front panel meter for
monitoring. R65 can be adjusted to
calibrate the voltage reading to +26 VDC
on the front panel meter. This calibration
was completed at the factory and should
not need to be adjusted at this time.
A forward power sample of the output of
the tray is applied to jack J1-1 and J1-2
of the board from the dual peak detector
board, single supply. The forward power
sample is connected through R1 and R2
to U1A, a buffer amplifier. The output of
U1A is split, with one part going to the
AGC circuits, another sample connected
to J1-5 for remote metering, and the final
sample applied to the meter at position 3
on S2, the front panel meter switch. R2
can be adjusted to calibrate the %
Forward Power indication on the front
panel meter.