User's Manual

Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT835A, Rev. 1 4-39
The collector voltage is applied at E101.
The collector voltage is connected
through R108 to the collectors on the
two devices that make up Q101. R106
can be adjusted to set up the operating
currents. The collector circuit is RF
bypassed by C112 to C115, C117,
C118, C121, C124, C130, and C131.
Q201 and Associated Circuitry
The other output of the splitter is
applied through AC coupling and DC
blocking capacitor C201 to L201 and
associated circuitry. This forms a balun
that converts the input signal from a
50-Ω unbalanced impedance to a 12.5-
Ω balanced impedance configuration.
C206 and C205, which are adjusted for
peak output, are for impedance
matching to the input of the parallel
transistors (Q201).
The bias voltage to the bases of the
paralleled transistors, Q201, is applied
at E201. The transistors are protected
from overvoltage by Q202, Q203,
R204, R205, and R206, which can be
adjusted to set 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, C223, and
C219, which are adjusted for peak
output with the 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 static
currents, with no RF applied. The
collector circuit is RF bypassed by C212
to C215, C217, C218, C221, C224,
C230, and C231.
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, which is
connected to J2, the SMA output jack
on the dual stage amplifier assembly.
4.8.11 (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 sent to
pins 1 to 4 and the minus (-)
connections sent to pins 5 and 6. The
+26.5 VDC is connected across the .01-
W/3-Ω 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.