Harris DX 25U, Техническое руководство

Страница: 53 / 274

on a PA Combiner/Motherboard. All RF drive cables are the
same length, so that all PA Module inputs are in phase.
4.7.3
Power Amplifier Description
In the Power Amplifier stage, the digital information constructed
by the Analog to Digital (A/D) converter is used to switch 64
RF amplifiers ON and OFF. The output combiner sums the
individual units of RF voltage developed by each amplifier.
The Power Amplifier stage may be thought of as a Digital to
Analog (D/A) converter, where the output is a high power,
amplitude modulated, RF signal.
The action of the RF combiner and RF amplifier modules
produce RF voltage “steps” at the combiner output. The power
output of each RF amplifier depends on the total number of
modules switched ON at any time. Switching on twice as many
RF amplifiers will produce TWICE THE VOLTAGE output and
FOUR TIMES THE POWER output. If a small number of
modules are switched on, each module has a small power output.
If a large number of modules are switched on, each module has
a larger power output.
The DX-25U uses a 12-bit digital “word” to control the RF
amplifiers. If the Power Amplifier stage consisted only of binary
“weighted” amplifiers, each bit would control one amplifier and
it would require 12 RF amplifiers to represent the digital word.
However, the RF voltage delivered by the amplifier representing
the most significant bit would have to be equal to one-half of the
peak RF voltage created with all 12 modules ON. In an RF
voltage combiner, all RF voltages are added in series. The same
current flows through all outputs as through the load, and
one-half the peak voltage is also one-half the peak power. The
largest “step”, then, would have to be able to deliver over 75
kilowatts, the next over 37.5 kilowatts, and so on. It is more
practical to use a larger number of smaller power amplifiers.
The DX-25U uses 58 equal RF voltage “Big Step” amplifiers
and six “Binary Step” amplifiers. The binary amplifier outputs
equal 1/2, 1/4, 1/8, 1/16, 1/32 and 1/64 of a “Big Step” amplifier
output and are controlled by the six least significant bits of the
digital word. The six most significant bits control the total
number of “Big Step” amplifiers ON at any time. Typically, 23
RF amplifiers are ON for a 25kW carrier.
However, AM transmitter PEAK output power requirements are
much greater than the transmitter CARRIER power. Also,
broadcasters may require additional transmitter power to over-
come antenna system power losses. For this reason, the carrier
power output of the DX-25U is rated at up to 30kW. The positive
peak modulation capability of an AM broadcast transmitter
depends on the maximum peak power output available from the
transmitter. For example, a +100% modulation peak represents
a peak output power of four times the carrier power, or 100kW
for a 25kW transmitter. This requires that double the number of
amplifiers be ON for the peak as compared with the carrier level.
Therefore, at the 100% positive peak modulation of a 25kW
carrier, 47 RF amplifiers are ON. With a carrier power of 30kW,
a 125% positive peak requires a peak output power of 151.9
303.8kW and additional RF amplifiers are turned ON.
SUMMARY: The more “steps” or RF amplifiers that are turned
on, the more power will be transmitted. The DX-25U uses a
combination of 58 equal RF voltage “Big Step” amplifiers and
six binary weighted RF voltage “Binary Step” amplifiers to
develop the modulated RF envelope. The modulated RF output
is made up of equal VOLTAGE steps, not equal power steps.
Assuming that the supply voltage remains the same, the RF
output VOLTAGE from each module remains the same no matter
how many other modules are on. Since the combiner secondar-
ies are in series, the TOTAL RF VOLTAGE induced on the
combiner rod will increase by the number of STEPS turned on.
With a constant combiner impedance of approximately 8 Ohms,
an increase in RF voltage will increase the RF current in the
combiner and increase the power. The power output from each
amplifier module changes, however, depending on the total
number of amplifier “steps” that are switched on.
4.7.3.1 Switching RF amplifiers On or Off
RF amplifiers are switched ON or OFF by applying or removing
RF drive to the module with a solid state switching circuit.
Because low voltage, low current circuits are used in the switch-
ing process, very little power is consumed.
4.7.4
Combiner Description
Sheet 4 of the DX-25U Overall Schematic, 839-7855-151,
identifies motherboards and module numbers as viewed from
the rear of the transmitter.
The Power Amplifier stage is made up of 64 plug-in RF ampli-
fier modules. These include 58 “Big Step” modules RF33-RF90
and six “Binary Step” modules RF91-RF96, which plug into
three Main Combiner/Motherboards, A5-A7, and one Binary
Combiner/Motherboard, A8.
Sixteen ferrite core toroid transformers on each motherboard
combine the RF voltage outputs of the modules through a solid
copper rod which which passes through the center of the trans-
formers. The four Combiner/Motherboards are assembled as a
single column.
The PA Combiner RF ground point is at the bottom of the
column. When the A/D Converter sample frequency is one-half
carrier frequency, a parallel resonant circuit consisting of L4 and
C4 present a high impedance to the half-carrier frequency.
One Modulation Encoder (A37) takes the digital output from
the Analog to Digital Converter and converts it into control
signals for the RF amplifier modules. Due to the way the system
would be configured as a DX-50, the RF Amplifier steps con-
trolled by the A37 Modulation Encoder are RF33 through RF96
with the last 6 steps being the Binary Steps (RF91 through
RF96).
The PA Modules are also identified by step number on the
interlocked module access doors in the Center Control Compart-
ment and Left Control Compartment. Each module has two LED
“fault” indicators, visible through openings in the access door.
If a shorted MOSFET causes a fuse to open, the “fault” indicator
for that fuse illuminates. When a module is switched ON, a green
LED indicator illuminates.
Section IV - Overall System Theory
Rev. AA: 8-9-00 888-2297-002 4-7
WARNING: Disconnect primary power prior to servicing.