Elecraft KX3 Owner's Manual - Manual (Page 50)

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MOSFETS allows automatic reduction of power if they become too hot during long transmit periods at high
power levels and high ambient temperature.
The optional KXAT3 automatic antenna tuner (ATU) option connects between the BNC antenna jack and the
RF board’s SWR/power bridge and low pass filters. It uses a latching relay-switched “L network” with eight
inductors and eight capacitors capable of matching a wide range of antenna impedances. The KXAT3 also
serves as a preselector for operating frequencies below 160 meters, significantly improving reception in the AM
broadcast band and below.
Another option, the KXFL3 roofing filter module, provides two additional balanced analog filters in the
baseband AF I/Q amplifier path. These are much narrower than the default bandwidth. The result is a significant
performance improvement in rejecting extremely strong interfering signals that are just a few kHz away from
the received signal.
Control Panel (CP) Board
The CP (Control Panel) circuit board contains all of the KX3’s control circuitry as well as high-level baseband
(AF) stages for the transmit and receive paths. It contains two on-board microcontrollers: one to manage the
radio (MCU), and another to process all transmit and receive signals digitally (DSP).
The DSP is a 32-bit, floating-point device. All modulation, demodulation, AGC, filtering, equalizing, and other
signal processing functions are handled by this IC.
The incoming baseband (audio) signal from the RF board is provided in low-level phase quadrature, or In-phase
and Quadrature (I/Q). These signals are digitized by a very low power, high-performance analog-to-digital
converter (ADC), then passed to the DSP for processing.
The baseband transmit signal is likewise provided in I/Q format to the RF board. A dedicated, high-
performance digital-to-analog converter (DAC) is used to generate a very clean transmitted signal.
DSP program storage is by means of a 2 megabyte FLASH memory device. This IC also provides storage of
operator messages for the DVR function of the KX3 (pg. 21).
Stereo (two-channel) audio from the DSP is converted to analog signals for use with headphones or dual
external speakers (PHONES jack). The use of stereo allows special processing to help reduce operator fatigue
(Audio Effects, pg. 20), and also optimizes dual watch capability (pg. 20), where the signals from VFOs A and
B are routed to the left and right channels. If the internal speaker is used, the audio is monophonic, and is
boosted by a separate speaker amplifier IC. Microphone audio (or low level audio from a computer or or other
source) uses another ADC that runs at all times to enable voice-operated transmit (VOX) operation.
The microcontroller unit (MCU) handles all user interface functions for the KX3, including twenty switches
and five shaft encoders. It also handles timing, sequencing, and overall management of the entire radio. The
MCU uses EEPROM (electrically erasable, non-volatile memory) to store operator preferences, message keyer
(CW/RTTY/PSK) messages, and control settings. A serial port (ACC1) provides an interface between the
MCU and an attached personal computer (PC), with both RS232 and USB cabling options.
The MCU uses two industry-standard serial protocols to control circuits on the RF board. I2C (pronounced I-
squared-C) controls the synthesizer. SPI handles bandswitching, changeover between receive and transmit, etc.
The driver for the liquid-crystal display (LCD) is also controlled via the I2C port.
The KXBC3 battery charger is an optional module that plugs into the CP board. It manages charging of NiMH
AA cells (if applicable). The emphasis on the design is safety, so relatively low charging currents are employed.
The KXBC3 also incorporates a real-time clock (RTC) function, useful for logging and time keeping. The RTC
also allows the KX3 to function like a clock radio, turning itself on at a predetermined time (see the ALARM
menu entry).