User's Manual

300-Watt Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT830A, Rev. 1 4-27
the bias, operating currents of the
transistors. The base voltage is RF
bypassed by C229, C202, C207, C208,
C209, and C210 and applied to the bases
through R202 and R203.
The collectors are impedance matched to
12.5Ω by C216, C220, C222, C223, and
C219, which can be adjusted for peak
output with best linearity and lowest
current. C225 provides AC coupling and
DC blocking for the output signal to the
combiner. L202 and associated circuitry
form a balun that transforms the signal
back to an unbalanced 50-Ω impedance
signal.
The collector voltage is applied at E201.
The collector voltage is connected
through R208 to the collectors on the two
devices that make up Q201. R206 can be
adjusted to set up the operating
currents. The collector circuit is RF
bypassed by C212 to C215, C217, C218,
C221, C224, C230, and C231.
4.3.8.4 Output Description
The outputs of the two sides are
combined by a 2-way wilkinson
combiner, using R2, and applied to the
RF output of the board at E2. The output
is connected to J2, the SMA output jack
on the dual stage amplifier assembly.
4.3.9 Generic Dual Stage Amplifier
Board, Class AB (1265-1404;
Appendix D)
The generic, dual stage UHF amplifier
board, class AB, is used as the basic
board in building the dual stage amplifier
assembly, class AB.
Mounted on the generic board are the
components that have no frequency-
determining factors. The low-band, mid-
band, and high-band assemblies are
produced by adding the specific
frequency-related components to the
generic board to make the specific-
frequency assembly.
4.3.10 (A7) Amplifier Protection
Board (1265-1412; Appendix D)
The amplifier protection board distributes
the biasing voltages to the transistor
amplifier devices that are mounted on
the amplifier boards in the UHF amplifier
tray. It also protects the transistor
devices from overcurrent conditions using
the board-mounted 7-amp fuses F1 to
F12. F13 is a board-mounted 3-amp fuse
that protects the +26.5 VDC that is
applied to the amplifier control board and
is needed for the operation of the board.
F14 and F15 are 7-amp spare fuses.
The +26.5 VDC from the switching power
supply enters the board at TB1, with the
plus (+) connections to pins 1 to 4 and
the minus (-) connections to pins 5 and
6. The +26.5 VDC is connected across
the .01W/3Ω voltage-dropping resistors
R14 to R26 that are used to set up the
idling currents for the transistor devices;
the fuses F1 to F13 that protect the
transistor devices during an overcurrent
condition through the outputs of the
board at TB2, TB3; and also to output
jack J1. Table 4-1 indicates the fuse, the
amplifier device it protects, and the idling
current settings for the class AB amplifier
devices.