RF Technology PA220
Page 7
2
CIRCUIT DESCRIPTION
2.2 Power Splitter/Combiners
The dc supply is fed to the amplifiers through resistors R27 and R28. This allows the
collector current of each amplifier to be measured at the test socket.
2.2 Power Splitter/Combiners
In the 100W models, zero-degree hybrid power splitter/combiners are used to parallel
the two amplifier stages. The hybrids consist of quarter-wave 70
Ω
transmission lines
and 100
Ω
RF resistors CX1-4, R9 and R10. This configuration provides wider
bandwidth and better balance than lower cost 90-degree hybrids.
2.3 Directional Coupler
The forward and reverse power components are measured through a coupled line
directional coupler. The output of the coupled line is frequency compensated by R13,
R14, C29 and C30 before being detected by D1 and D2.
The output of the detectors is proportional to the forward and reflected voltage
components.
2.4 Low Pass Filter
A low pass filter consisting of L12 - 14 and C39 - 42 reduces the harmonic components
to less than -80dBc. The filter uses a combination of lumped elements and printed
microstrips to obtain the required harmonic attenuation.
2.5 Power Control Circuits
The forward and reverse voltages from the directional coupler are amplified and
inverted by U1a and U1b. The amplified voltages are combined before connecting to
the input of error amplifier U1d.
Error amplifier U1d compares the detected voltage with the dc reference voltage from
output power trimpot RV1. The amplified difference at the output of U1d is supplied to
the rear panel system connector for connection to the T220 ALC input.
Equipment Type
Key Specifications
Power Supply
13.8Vdc, 15A
RF Source
25 Watt (eg. T220 exciter)
RF Load / Attenuator
50
Ω
, 50/100W, SWR<1.2:1
RF Power Meter
eg. HP437B or calibrated detector and voltmeter
Table 2: Standard test equipment for the PA220 Power Amplifier
