7: Antenna Tuner Board
PRC1099A-MS
7-5
Comparator U1B compares the current detector voltage to the reference level
and sends the value back to the processor. The processor uses that value to
determine the current peak that corresponds to the lowest level VSWR.
7.1.12
D/A Decoder
The processor can only recognize a high or low output, so it is necessary to
provide a reference level to comparator U1B to measure the analog output
from the current detector. The output from the decoder is controlled by the
processor and is quantised into 64 steps. The binary steps are selected by
switching R12 through R17. Switching R7 and R8 controls the gain of
amplifier U1A.
7.1.13
Serial Decoder
Serial decoders U3A and U4A translate the serial data from the processor to
16 different outputs controlling the relay drivers that switch the various
inductors or capacitors in or out of the RF path to or from the antenna.
7.1.14
Relay Drivers
Relay drivers U5, and U6 are 16 NPN transistors contained in two 16-pin DIP
packages. There are 14 latching relays in the tuner that have separate coils for
latching and unlatching the relay. The relays require short current pulses for
latching or unlatching and draw no current in the static condition. Separate
drivers latch each relay, while transistor Q1, driven by output Q8 of U3A,
unlatches all relays. The processor always unlatches all relays before each
switching operation and resets those latched relays that require no change of
state, as well as those relays requiring switching. No separate drivers are
required to unlatch each relay.
The Antenna Tuner switches between transmit and receive operation using the
T/R relay that switches the antenna input between the receiver input and the
transmitter output. A second pole on this relay switches the regulated 8V line
to provide T8 and R8 voltage outputs. These control lines switch the
transceiver circuitry. Transistor and diode gates are used extensively for the
switching function. The input to the double-balanced mixer is switched by a
relay to minimize high level intermodulation distortion.