User's Manual

300-Watt Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT830A, Rev. 1 4-21
signals. The board also provides for the
frequency response and gain adjustment
of the IF signal. The circuitry for the I
and Q signals to the mixers is identical.
The I signal enters the board at J3-2.
Diodes CR1 to CR3 provide surge
protection to buffer U4. The output of
U4 is fed through variable resistor R32,
the gain pot for the unmodulated signal.
The circuit consisting of U1 and its
associated components provides the DC-
offset adjustment at the input of U5.
Variable resistor R5 provides the shift in
DC level to the input of U1, causing a
corresponding shift in the output. The
resulting output from U1 is fed to the
input of U5; the output of U5 is fed to
mixer Z1.
The 46.49056-MHz signal enters the
board at J7 and is amplified through two
MAV-11 op-amps. The signal is then fed
to phase/splitter U8 and the outputs of
U8 provide the in-phase and out-of-
phase local oscillator signals for the I
and Q mixers, respectively. The outputs
from the I and Q mixers can be phase
and gain adjusted by C29, R58, C32,
and R79. These two signals are then
mixed together at Z2 with a resulting
output of 44 MHz. The frequency
response of the IF signal is adjusted
using variable resistors R29 and R30
and capacitors C19 and C20. The gain is
adjusted using variable resistor R3.
The ±12 VDC required to operate the
board enters at J8. The ±10 VDC for the
offset circuits is supplied by U12 and
U13A and their associated components.
U12 is a regulator with a +12-VDC input
and a +10-VDC output. U13A inverts
this voltage for the –10-VDC source.
4.2.2 (A2) Switch Board (1561-
1202; Appendix D)
The switch board provides three front-
panel momentary contact switches for
user control and interface with the front-
panel LCD menu selections. The
switches, SW1 to SW3, complete the
circuit through connector J1 to
connector J17 on the VSB modulator
board (1561-1201). When they are
closed, the switches place a logic low at
U14 on the VSB modulator board.
Jumper J2 provides a logic low to U14
on the VSB modulator board to enable
the Test mode.
4.2.3 (A7) Local Oscillator Board
(1561-1203; Appendix D)
The local oscillator board provides the
46.690560-MHz phase-locked output for
the vector modulator board (1520-
1107). The on-board oscillator, U2,
feeds the IF output at J6 and the IF
sample at J8. The oscillator also
provides a sample to phase-lock control
IC U4. When switches SW1 to SW3 are
set to a predetermined state, and the
10-MHz reference is present on the
board, U4 will output a correction
voltage to maintain the phase lock of
oscillator U2. The correction voltage is
fed through buffers U1A, U1B, and U5B
to pin 2 of the oscillator. The switch
settings are determined by the following
equation (the resulting value is
converted to binary with the least
significant bit at SW3-1):
Switch setting = (LO frequency/2) –3
Once the switch settings are calculated,
and there is a 10-MHz reference present
at the rear of the tray, the board will
begin the process of phase locking the
VCXO. There are three amber LEDs on
the board indicating the progress of the
PLL circuit; these LEDs are designated
step 1, step 2, and step 3.
When the circuit begins the PLL process,
step 1 will illuminate. As the process
continues, step 1 extinguishes and step 2
illuminates. Finally, step 2 extinguishes
and step 3 illuminates. Once step 3
extinguishes, the front panel PLL Locked
LED will illuminate within 30 seconds.
The board drives two LEDs on the front
panel LED board. The PLL Locked