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RF Technology PA220
2 CIRCUIT DESCRIPTION
Pins
Function
1, 2, 14, 15
Positive supply
12, 13, 24, 25
Ground (negative supply)
8
ALC output
Table 1: Pin connections for the 25 pin “D” connector on the rear panel
Note: The amplifier is capable of delivering as much as 120 Watts continuously.
In certain conditions
1
an RF power of 130 Watts or more can be available via this
connector. This corresponds to peak voltages in excess of 100 while currents in
excess of 2 Amperes may flow. Appropriate care should be taken when working
on the PA220 to avoid making or breaking connections when the amplifier is
operating, and to avoid RF burns through close proximity to live connections, etc.
1.4.3
25 Pin Connector
The 25-pin ``D'' connector provides connection to ground and dc power, and from the
automatic level control (ALC) circuit. The pin connections are given in table 1. The
ALC line floats high to approximately 7Vdc. Pulling this line low
will reduce output power. Voltages below 0.5V will reduce the output by more than
20dB. A pull-down current of approximately 1mA is required.
2
Circuit Description
The following descriptions should be read as an aid to understanding the block and
schematic diagrams shown in figures 1 - 4.
2.1
Amplifiers
The RF power amplification is provided by two single transistor amplifiers, Q1 and Q2.
Each amplifier is rated at 50 watts output. The input and output impedances of the
transistors is matched to 50
Ω
by broad band microstrip matching networks.
Trimmer capacitors C10 and C24 are used on the input networks to optimize the input
match at center of the desired frequency range. Similarly C58 and C59 on the output
networks are used to optimize the output efficiency.
Since the design of the amplifiers allows them to be very broad band, they will not
usually require re-adjustment unless changing frequency from one end of the band to the
other.
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1
Excessive powers may be available, for example, in the event of a failure of the ALC loop,
such as may arise if the ALC feedback connection is broken.
