Section G
RF Combiners:
Binary Combiner/Motherboard (A1)
and Main Combiner/Motherboards (A2 through A4)
G.1
Introduction
This section includes a description of the Main and Binary
Combiner/Motherboards.
G.1.1
Principles of Operation
Each RF amplifier output is connected to a ferrite toroid trans-
former. A basic principle of toroid cores is that each time the
conductor passes through the center or inside of the toroid, it is
counted as a turn. All of the secondaries of the RF transformers
are connected in series by one continuous conductor. Because of
the basic principle of a “winding” or “turn” passing through the
center of the toroid, a solid copper pipe or rod can be used as the
secondary conductor.
The combiner secondary (copper rod) is made in sections, which
are bolted together to facilitate removing any combiner/mother-
board if necessary. Each motherboard holds 16 toroid transform-
ers with the RF amplifiers mounted in two rows of eight.
All of the Big Step RF amplifiers RF33-RF90 have transformers
with a turns ratio of 16:1; that is, sixteen turns for the primary
winding and one turn for the secondary. Amplifiers RF91-RF96
(B7-B12) use a combination of different supply voltages and
transformer turns ratios to achieve binary weighting.
Each RF amplifier module induces an RF voltage in the com-
biner’s secondary “winding” (rod). The RF voltages from all RF
amplifiers which are turned “ON” add in the secondary (the
copper rod). The total Power Amplifier stage output appears at
the end of the combiner, at about an 8 Ohm impedance point. At
the transmitter’s nominal 25 kW power, RF current in the com-
biner secondary (pipe) is 40 Amperes, and the large copper rod
used for the secondary is required to keep IR losses low.
Because the modules are effectively connected in series by the
transformers, the same current flows in all modules. This is true
whether a module is in the “on” or the “off” state.
At any instant in time, some of the modules will be “OFF”
(except at a very high positive modulation peak, when all mod-
ules will be ON). RF amplifier modules used as Binary Steps will
deliver less RF voltage than those used as Big Steps. In the
transmitter Power Amplifier stage, individual RF amplifier out-
put voltages add to produce the total RF voltage. At any instant,
the RF voltage observed on an oscilloscope (or producing an
instantaneous reading on a modulation monitor) is the sum of the
incremental voltages from the contributing modules. The current
at that instant is this total voltage divided by the combiner load
impedance (approximately 8 Ohms).
At 25 kW carrier power, the RF current in the combiner is nominally
40 Amperes. At a 100% positive modulation peak, this current will
double to 80 Amperes (the RF voltage at the output will also double).
Typically 23 modules are “ON” at carrier power (25 kW, and 47
modules will be “ON” at a 100% positive modulation peak. This
will provide twice the VOLTAGE across the combiner and there-
fore twice the RF output voltage.
Refer to Section IV, Overall System Theory, for more information.
Refer to Section V, Maintenance, for general pc board maintenance
procedures. There are jumper settings on the Binary Com-
biner/Motherboard. There are no adjustments on the boards.
G.2
Circuit Description
Refer to the Main Combiner/Motherboard schematic, 839-7855-
094, Binary Combiner/Motherboard schematic, 839-7855-093,
and Sheet 4 of the Overall schematic, 839-7855-151, in the
Drawing Package.
G.2.1
Main Combiner/Motherboards (A2 through A4)
Each Main Combiner/Motherboard contains combiner trans-
former toroids T1 through T16 and printed circuit board sockets
for 16 RF amplifier modules. A tapped air-core “efficiency coil,”
is paralleled with each transformer winding. Tap position de-
pends on operating frequency and is listed on Sheet 1 of the
frequency determined components chart, 839-7855-140, in the
Drawing Package.
The motherboard also contains connectors for DC supply volt-
age, RF drive inputs, and encoded audio inputs from the modu-
lation encoder board.
G.2.1.1
DC Supply
On the Main Combiner boards, all modules operate from the
+230 VDC supply input at E1 and E2.
G.2.1.2
RF Drive Inputs
RF drive for the sixteen modules on each motherboard enters at
J17, J18, J19 and J20. Each module receives two RF drive cable
inputs from the RF Drive Splitter A15. All RF drive cables are
the same length, so all RF drive signals are in phase.
G.2.1.3
Encoded Audio (Module On/Off Control Signals)
The encoded audio inputs, control signals, from the Modulation
Encoder board enter at J21, J22, J23 and J24. These encoded
digital signals turn on the number of modules needed for the RF
output at each instant in time.
G.2.2
Binary Combiner/Motherboard (A1)
Refer to the Combiner Motherboard/Binary Schematic, 839-
7855-093, in the Drawing Package.
The RF Input and Control Signal Input signals enter the Binary
Combiner/Motherboard on the same input jacks as the Main
Combiner/Motherboards.
On the Main Combiner/Motherboards, all active amplifiers de-
liver the same RF voltage. On the binary combiner/motherboard
12-12-94
888-2297-002
G-1
WARNING: Disconnect primary power prior to servicing.
