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Circuit Details
Refer to the schematic, Appendix A.
RF Board
The ATU uses eight series inductors and eight parallel capacitors,
configured as an L-network. Both the inductors and capacitors have
very high voltage and current ratings to prevent malfunction in the
presence of high SWR and high power. For this reason, the KAT100
can easily handle 150 watts, even with SWRs in excess of 10:1.
The capacitance can be placed at the transmitter or antenna end of the
network by relay K17. Each inductor and capacitor has its own relay,
and a final relay (K18) controls the antenna switch. The relays can
handle about 10 amps. The inductor relays short their respective
inductors when not needed. Based on lab tests, this technique does not
degrade Q or cause unwanted magnetic coupling.
The relays are selected under control of the ATU's microcontroller.
Open-drain shift-registers U2-U4 actually drive the relays. Data is sent
to U2-U4 by the microcontroller via a serial interface. L, C, and
network in/out values are stored in the microcontroller's on-chip
EEPROM. An external EEPROM (U7) is provided for possible future
expansion.
T1 and associated circuitry form a Bruene-style bridge used for SWR
and power measurement. The output from the bridge is buffered by op-
amp U5. The /FSCL and /RSCL lines are pulled low by two open-drain
outputs on U3 when high power is used (K2/100, 11 watts or greater).
This scales the bridge outputs by a factor of 3.16 (square root of 10) to
keep SWR and power measurements in the appropriate range. The
scaling is turned off at lower power levels to improve SWR-
measurement accuracy.
If the KAT100 needs to provide a closed-loop power control signal
back to the K2 via the VRFDET line, an additional high-current buffer,
U6, is turned on (VRFEN = 5V). This is only required if there is no
KPA100 connected (i.e., if a basic K2 is connected to the KAT100 via
a KIO2 module).
The KAT100 can be powered from either J1 (external) or P3
(internal), providing flexibility for builders who wish to add accessory
modules into the top cover (this applies to the KAT100-2 only).
Q1 and Q2 provide automatic turn-on capability for the KAT100,
eliminating the need for an on-off switch. Whenever a K2 is
connected to the KAT100 and is turned on, the 12CTRL line from
either J3 (external) or P4 (internal--KAT100-2 only) turns on Q2,
which in turn pulls Q1's base low. Q1 has a very low C-E saturation
voltage, so it can easily provide the needed operating current for the
KAT100 even though it is a TO-92 plastic device.
U8 provides regulated 5 volts to the logic circuits and front-panel
LEDs.
The microcontroller sleeps during receive mode, with its oscillator
disabled. This completely eliminates any RFI. The microcontroller will
wake up on transmit (8R line going low) or when an auxBus message is
received.
Jumpers W1-W7 and connectors P1, P7, and J7 are provided for future
firmware/hardware expansion.
Front Panel Board
LED drivers U101-U102 are shift register/driver ICs of the same type
used to control the relays. Individual resistors are used to establish the
current for the LEDs, which would allow the builder to adjust individual
LED intensity if desired. However, this should not be necessary since
the LEDs are generally well-matched.
Q101 and Q102 provide one of two fixed voltages to LED array. If
the /NIGHT line is pulled low, the voltage drops to reduce brightness
for nighttime use. The transistors are wired in parallel to increase
current-handling capability.
