User's Manual

Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT835A, Rev. 1 4-23
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.
4.3.11.2 -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.
4.3.12 (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.
4.4 (A4 and A5) Variable
Phase/Gain Trays (1245-1200;
Appendix C)
The Variable Phase/Gain Tray sets the
phase and gain adjustments of the RF
to provide maximum output when the
outputs of the two Amplifier Arrays are
combined. There are two Variable
Phase/Gain Trays in the 835A UHF
Transmitter, each controlling the phase
and gain of one of the two Amplifier
Arrays.
The Tray is made up of (A4) a Variable
Phase/Gain Assembly (1245-1204),
which contains (A4-A1) a Variable
Phase/Gain Board (1245-1201), (A5) a
Metering Board (1245-1202) and (A3)
a +15V/-12V Power Supply Board
(1245-1203).
4.4.1 (A4-A1) Variable Gain/Phase
Board (1245-1201; Appendix D)
The RF Input signal connects to the
rear panel of the Tray at J1 and is
cabled to J1 on (A4-A1) the Variable
Phase/Gain Board (1245-1201) that is
mounted inside (A4) the Variable
Phase/Gain Assembly (1245-1204) for
RFI and EMI protection. The Variable
Phase/Gain Board contains two main
circuits, one for ALC and one for Phase
adjustment of the RF signal. A front