Harris Platinum HT EL 2000LS, Technical Manual

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Section IV
Theory of Operation
4.1
Introduction
This section provides theory of operation for the HT EL 2000
TV Transmitter. For purposes of discussion, the circuitry is
divided into functional subassemblies in the following text. Refer
to the separately packaged diagrams as required.
4.2
System Overview
The transmitter is composed of subassemblies mounted in a
common cabinet. The 1 watt visual and aural signals are gener-
ated in a exciter chassis with exciter power metering and control.
The exciter front panel and the Interface Logic Module imple-
ment transmitter metering and transmitter control.
Modulated visual RF signal is amplified in a Driver Module and
divided four ways. Three outputs drive visual Power Amplifiers
(PA’s), the other is terminated in a load. The three PA’s are
combined in a three way Gysel combiner. The Gysel combiner
features a constant 50 ohm input and a high degree of isolation
between modules. This allows removal of one module without
impacting the others. The output forward and reflected power is
sampled by the transmitter for meter reading and VSWR protec-
tion.
Aural drive from the exciter is amplified by a driver module. This
aural RF signal is also sampled for metering and VSWR protec-
tion.
The power supplies consist of two high power switching supplies
connected in parallel for the RF modules and a small supply for
the interface logic.
The optional Exciter Switcher contain level sensing circuitry,
logic and transfer relays for automatic exciter switching.
4.3
Exciter, Meter and Control Board
The meter board is mounted to the back side of the exciter front
panel and provides the metering, control and status indications
for the transmitter.
4.3.1
A/D & Displays
Schematic drawing 839-7994-125 shows the meter board for the
transmitter with sheet one being the analog metering section. The
meter is a digital 4-1/2 digit A/D convertor with multiplexed
binary coded decimal (BCD) outputs. This is followed by a BCD
to seven segment display decoder and LED displays.
The A/D U1 can display 0000 to 9999 on the digital displays DS1
and DS2 and is updated about 5 times a second. The display is
normally calibrated for 2000 when the Bar Graph is at full scale
or 1.25 volts at pin 5 of U2 and U3 Bar Graph display ICs.
The A/D reference adjust is set for 1.000 volts between pin 2 and
ground. Each metering input has a calibration control.
4.3.2
Bar Graph Display
The Bar Graph displays consisting of U2 and U3 are designed
using LM3914s cascaded to make a 20 bar magnitude display
which lights all 20 bars with 1.25 volts applied to pin 5. Each
meter position also has a corresponding calibration adjustment
for the analog bar display.
4.3.3
Analog Multiplexers
The analog multiplexer consisting of U4 and U6 provides the
analog selection switches to apply the various voltage monitor
points the A/D and Bar Graph displays. IC U4 is an 8 channel
multiplexer and is used to route the power sample points to buffer
U7 and on to U5 an analog squaring circuit. The squared sample
is then passed through U6-1 to 2 and on to the display circuits.
The two linear voltage samples, 50V and supply current, are
routed through U6-4 to 3 or U6-8 to 9 to the displays.
4.3.4
Meter Selector
The multiplexers are controlled from the meter selection PAL
U17 on sheet 4 of the schematic. This PAL contains a three bit
up/down counter plus decoders to enable the multiplexers for the
ENABLE
ADR
U4 U6 DESCRIPTION
0 4-8 1-2 VISUAL FORWARD POWER
1 5-8 1-2 AURAL FORWARD POWER
2 6-8 1-2 VISUAL REFLECTED POWER
3 7-8 1-2 AURAL REFLECTED POWER
4 12-8 1-2 EXCITER VISUAL OUTPUT POWER
5 1-8 1-2 EXCITER AURAL OUTPUT POWER
6 x 4-3 50 VOLTS SUPPLY
7 x 8-9 MODULE CURRENTS
Table 4-1. Meter Counter/Display Function
Rev. B: 7/15/02 888-2457-001 4-1
WARNING: Disconnect primary power prior to servicing.