User's Manual

Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT835A, Rev. 1 4-30
as the heatsink, for the 1-watt UHF
amplifier board (1227-1303) that is
mounted inside the assembly.
Depending on the frequency of the
channel of operation, the assembly has
approximately 9 dB of gain.
The RF input to the assembly connects
to SMA jack J1. The amplified RF output
of the assembly is at SMA jack J2.
Typically, with an input signal of +21
dBm at J1 of the assembly, an output
of +30 dBm can be expected at J2.
The +28-VDC bias voltage connects
through FL1, an RF-bypass, feed-
through capacitor, that is mounted to
E1 on the amplifier board.
4.8.3 (A3-A1) 1-Watt Amplifier
Board (1227-1303; Appendix D)
The 1-watt UHF amplifier board is part
of the 1-watt UHF amplifier assembly
(1227-1319) and provides
approximately +10 dB of gain.
The UHF signal enters the board at J1,
an SMA connector, and is applied
through coupling capacitor C1 and a
stripline circuit to Q1, a UTV040F UHF
wideband-amplifier device. The base
and collector voltages needed to
operate the transistor are obtained
from the +26.5 VDC line that connects
to the board at E1. The collector
voltages are fed through VR1, VR2, R3,
and R4 and the base voltages are fed
through R2 and R1. The amplified UHF
output of Q1 is coupled through a
stripline circuit and C14 to J2, the
output SMA jack of the board.
The board is powered by the +26.5
VDC that is produced by an external
power supply in the tray. The +26.5
VDC enters through E1 and is fed
across R4 and R5, which drops
approximately 6 volts, to the collector
(+20 VDC). The voltage is filtered by
RF decoupling components L4, C11,
C12, and C15 before it is connected to
the collector. The bias voltage is
connected across R3, VR1, VR2, R1,
and R2, which sets the base bias
voltage at +.8 VDC and forward biases
Q1. C6, L1, and L2 provide RF
decoupling of the bias voltage before it
is connected to the base.
The board has a self-bias protection
circuit that uses zener diodes VR1 and
VR2. If the current draw of the device
increases, the voltage drop across R4
and R5 increases; this decreases the
voltage that is applied to VR1 and VR2.
These two diodes drop a fixed voltage
of 20 VDC across them. As a result, if
the voltage drop across R4 and R5
increases, the voltage available to the
base of Q1 decreases and the device
will eventually be shut off.
4.8.4 (A4-A1) Single Stage Amplifier
Assembly (1265-1418, low band
/1265-1416, mid band/1265-1417,
high band; Appendix D)
4.8.4.1 Low Band
The single stage UHF amplifier
assembly, Class A, low band, is made
from the generic single stage amplifier
board, Class A (1265-1415). The
assembly uses a single PTB20101
Ericsson device that is made up of two
transistors in parallel and operating
Class A, which amplify the signal by
approximately +11 dB. Bias adjust pot
R6 sets the operating current for Q1.
Q1 and Associated Circuitry
The RF input signal connects to SMA
jack J1 on the board. The RF input is
applied through AC coupling and DC
blocking capacitor C1 to L1 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.
The two outputs, applied to the bases
of Q1, are 180° out of phase with each
other. C3, C4, C6, and C5, which are
adjusted for peak output, are for
impedance matching to the input of the