User's Manual

300-Watt Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT830A, Rev. 1 4-20
to U3 and U4; U3 and U4 are voltage-
regulator ICs for the +12 VDC. The
voltage-regulated and current-limited
+12-VDC output of U3 connects to J4,
pins 1, 2, 3, and 4, which is one of the
+12-VDC outputs of the board. The
green LED DS3 will be lit if +12 VDC is
present to J4; C7 and C8 are bypass
capacitors. The +12 VDC output of U4
connects to J5, pins 1, 2, 3, and 4, which
is another one of the +12-VDC outputs of
the board. The green LED DS4 will be lit
if +12 VDC is present to J5; C9 and C10
are bypass capacitors.
CR5, CR6, CR7, and CR8 rectify the AC
that is then filtered by C1 and connected
to U1 and U2; U1 and U2 are voltage-
regulator ICs for the +12 VDC. The
voltage-regulated and current-limited
+12-VDC output of U1 connects to J6,
pins 1, 2, 3, and 4, which is one of the
+12-VDC outputs of the board. The
green LED DS1 will be lit if +12 VDC is
present to J6; C3 and C4 are bypass
capacitors. The +12-VDC output of U2
connects to J3, pins 1, 2, 3, and 4, which
is another one of the +12-VDC outputs of
the board. The green LED DS2 will be lit
if +12 VDC is present to J3; C5 and C6
are bypass capacitors.
4.1.11.2 -12 VDC Power Supply
The 18 VAC from an external step-down
toroid connects to J1, pins 1 and 4, of the
board and is wired to CR9, CR10, CR11,
and CR12 which form a full-wave bridge
rectifier network for the –12-VDC power
supply. F1 is a 3-amp fuse that provides
over-current protection. The rectified
output is filtered by C11 and fed to U5,
the voltage-regulator IC for the -12 VDC.
The voltage-regulated and current-
limited -12 VDC is connected to the
-12-VDC output jacks J7, pins 5 through
10, and J8, pins 5 through 10. The green
LED DS5 will be lit if -12 VDC is present
to J7 and J8; C12 and C13 are bypass
capacitors.
4.1.12 (A21) 5-Section Delay
Equalizer Board, 44 MHz (1072090;
Appendix D)
The 5-section delay equalizer board, 44
MHz, is designed to correct for any group
delay errors that have been created by
the digital mask filter. The board consists
of five separate delay/attenuation
equalizer sections. Each section is tuned
across the 41 to 47 MHz band depending
on where the correction is needed. J9 on
W3 determines the input impedance for
the IF source:
• J19-1, J19-2=50 Ω
• J19-2, J19-3=75 Ω
J23, J27 determines 6 dB attenuation;
J23-1, J23-2 and J27-1, J27-2 means
that there is no attenuation; and J23-2,
J23-3 and J27-2, J27-3 means that there
is 6-dB attenuation.
Delay equalizers 1 through 5 are each
tuned to a different frequency to equalize
the delay characteristics of that
frequency. Attenuation equalizers 1
through 5 are tuned to the same
frequency as their matched delay
equalizer section and are isolated from
them by a 3-dB pad. Each
equalizer/attenuation pair is followed by
a 12-dB gain stage with 6 dB of
attenuation between the sections.
Depending on where the jumpers are
positioned, each equalizer section can be
jumpered into the circuit or bypassed by
using a 2-dB pad.
4.2 (A19) 8-VSB Digital Modulator
(1075164; Appendix C)
4.2.1 (A6) Vector Modulator Board
(1520-1107; Appendix D)
The vector modulator board receives the
I and Q outputs from the VSB modulator
board (1561-1201) and modulates and
mixes the signals for a 44-MHz IF output
signal. The board allows for the
adjustment of the offset and gain of the
I (in-phase) and Q (90° out-of-phase)