Operating instructions

RF Technology PA501 Page 7
2 CIRCUIT DESCRIPTION 2.2 Power Splitter / Combiners
amplifier stage, and is followed by a lumped/distributed network that performs both
impedance matching and power combining ahead of the output filter.
The PA501 design is very broad band and will not usually require adjustment once it
has left the factory unless components are changed or the equipment is required to move
frequency from one extreme end of the band to the other.
While it is often possible to achieve an adjustment that provides higher efficiency or
higher output power at one particular frequency, such adjustment will almost certainly
compromise operation in other parts of the band. Such adjustment is not recommended
and voids the warranty. Owing to the risk of severe RF burns the PA501 should not be
operated with its protective covers removed except by qualified personnel.
2.2 RF Amplifiers (Rev 05/9151)
The input to the PA501 is connected to the first RF amplifier stage, based around Q3,
by a matching network consisting of C43/44, etc. Q3 has a gain of 7-8dB, raising the
power level to about 35 Watts.
The main RF power amplification is provided by two single, class-C, transistor
amplifiers centered around Q1 and Q2. Each amplifier contributes half the output
power. The input and output impedances of the transistors are matched, and the power
split before the second stage, and combined after it, by broad band matching networks
employing both lumped and distributed (microstrip) components.
Variable capacitors are used in the input network to optimize the input match across the
desired frequency range, between the two RF stages, and in the output network to
optimize the output efficiency.
The dc supply is fed to the three RF power transistors through resistors to allow the
collector current of each to be measured at the front panel test socket.
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 and
impedence matched by R100, R101, R102 and R103 before being detected by D101 and
D101.
The dc output of the detectors is proportional to the forward and reflected RF power.
2.4 Low Pass Filter
A low pass filter consisting of L1 - 4 reduces the harmonic components to less than
-70dBc. The filter uses a combination of lumped elements and printed components to
obtain the required harmonic attenuation.