User's Manual

300-Watt Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT830A, Rev. 1 4-32
amplifier control board connects to FL4
and FL5 on the assembly and is wired to
J5 on the board. The phase control input
from the amplifier control board connects
to FL2 and E1 on the assembly and is
wired to J4 on the board. The input fault
to the amplifier control board connects to
FL3 on the assembly and is wired to J6-1
on the board. E1 on the assembly
connects to J3-4 on the board.
The +26 VDC needed to operate the
board connects to FL1 on the assembly
that is wired to J3-3 on the board.
4.3.15 (A2-A1) Variable Gain/Phase
Board (1265-1425; Appendix D)
The variable gain/phase board provides
the circuits that adjust the phase and the
gain of the RF signal for the amplifier tray
in which it is mounted.
The RF input signal at J1 is split, with one
output connected to a detector circuit
consisting of C8, CR4, and U3A. This
detected level is then applied to
comparator U3D, which provides a high
output when the input signal level drops
below a threshold set by R16, R17, and
CR5. This high is applied to the red Input
Fault LED DS1, which lights to indicate an
input fault. DS1 can be seen through the
hole in the lid on the variable gain/phase
Assembly. The high is also connected to
the gate of Q1, which biases it on and
causes its drain to go low. The low is
applied to the pin-diode attenuator circuit
consisting of CR1, CR2, and CR3. The low
to CR3 decreases the current through it
and increases its resistance, decreasing
or completely shutting off the RF that
flows through it.
The other output of the RF input signal
from J1 is connected through C1 to a
voltage-controlled pin-diode attenuator
circuit consisting of diodes CR1, CR2, and
CR3. The diodes are pin diodes in a pi-
type configuration whose resistance
varies inversely with the DC current flow
through them.
As the AGC voltage, the attenuator bias
that is applied to J5 increases and CR3 is
forward biased even more. This increases
the current flow through it by decreasing
its resistance; the RF signal that flows
through it increases in level. CR1 and
CR2 have less current through them; this
raises their resistance, causing the RF
signal that is applied to them to decrease
in level.
The three diodes form a pi-type
attenuator whose attenuation decreases
with the increasing AGC voltage. U4
provides amplification, approximately 8
dB, of the RF signal before it is connected
to the phase-shifter circuit. The phase-
shifter circuit consists of L1, C16, C17,
CR7, and CR8. L1 is a 90°, 2-way
splitter. The signal at pin 1 of L1 is split
and applied to pins 2 and 4. The signal
reflects off CR7 and CR8 and is passed to
pin 3. The phase shift between pins 1
and 3 changes with the voltage applied
across CR7 and CR8. This voltage is
controlled by an external phase-adjust
pot that connects to J4. The +26 VDC
from the external switching power supply
is used as the reference that is applied to
the phase-control pot. The IC U2
provides approximately 10 dB of gain at
the output of the phase-shifter circuit
that connects to two class A amplifier
stages, Q2 and Q3, with a total gain of
approximately 20 dB.
The first amplifier stage, Q2, is biased at
a collector current of approximately 100
mA. This current is set by R29, R30, VR1,
and Q2. VR1 forces the voltage at the
collector to stay at 8.9 VDC. This biases
on Q2 and draws enough current through
R29 and R30 to keep the collector
voltage at 8.9 VDC. The amplified output
connects to the second amplifier Q3. The
bias circuit for Q3 works in a manner
similar to the bias circuit for Q2. VR2 and
VR3 maintain a collector voltage of 21
VDC, while R36 and R37 limit the
collector current to 650 mA.