Output Assembly - Harris ZX2500 Technical Manual

Section 4 Theory of Operation
The output of each half of the IPA module feeds a Wilkinson power splitter which in
turn supplies drive power to the RF input connectors on each PA backplane. The input
splitter features isolation between its outputs to keep the drive to each PA module
constant as other PA modules are plugged in and pulled out. A ninety degree offset in
every other output cable ensures that reflections are absorbed by the splitter ballast
loads as PA modules are unplugged.
The RF signal is amplified in the PA modules with a nominal gain of 14 dB to 20 dB,
depending on frequency and operating mode. Each PA module utilizes a pair of VHF
MOSFETs operating in a push-pull configuration. Consult schematic 843-5569-071 for
more details.
The output from the PA modules is passed through the PA backplanes to the RF output
assembly and the transmitter output. The RF output assembly is a combination
Wilkinson power combiner, harmonic filter, and directional coupler line section. RF
samples from the output assembly are used for the internal power metering and for
monitoring use by the customer.
4.3

Output Assembly

Consult output assembly schematic based on amplifier chassis type.
ZX5000: 839-8464-080
ZX3750: 839-8464-087
ZX2500: 839-8464-088
The output from the PA modules is passed through RG-142 BNC cables from the PA
backplanes to the output assembly. The output assembly is a combination Wilkinson
power combiner and harmonic filter. The harmonic filter provides greater than 50 dB
attenuation at the second harmonic and greater than 60dB attenuation at the third and
higher harmonics to 1 GHz. A ballast load is provided for each input with coupling
provided by a quarter-wave balun. Since all ballast loads have a ground reference, no
static drain choke is required. The ballast loads are visible from outside of the output
assembly with the amplifier rear door open, thereby allowing easy inspection and
verification of load integrity. A temperature sensing thermistor on each ballast load
signals a fault when the flange temperature of a load exceeds 125 degrees centigrade.
Whenever a combiner imbalance exists, some power will be directed to the ballast
loads. This imbalance could be due to a PA module being shut down or removed from
the transmitter. When a module is removed, the transmitter continues to produce (n/m)
of the original power, where "n" is the number of modules still transmitting and "m" is
the total number of modules in the parallel combined stage. Of this power, (n/m)^2
leaves via the transmitter output, while the remainder is directed to the ballast loads.
4-2
WARNING: Disconnect primary power prior to servicing.
888-2595-001
ZX Series
12/20/11