Elecraft K3 Owner's Manual - Manual (Page 74)

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indicators. Mic and headphones can be plugged into the front panel, or optionally at the rear panel (see KIO3
description, pg. 73).
The Front Panel PCB also includes the microcontroller unit (MCU), which manages the operation of the K3. All
inputs, whether from a switch, knob or external PC, are recognized and acted on by the MCU. All control
outputs – such as switching from transmit to receive, sending a CW code element, adjusting the transmitter
power, controlling LED brightness, etc. – are produced by the MCU.
The Front Panel also contains a large amount of EEPROM memory for parameter storage, and FLASH memory
for program storage. This allows the K3 to be re-programmed with the newest firmware by a simple download
from the Internet. It also enables the K3 to remember your favorite settings, particular configuration preferences,
and the last setting of controls when power is removed from the radio.
DSP Board
The K3’s Digital Signal Processing (DSP) capabilities provide a rich set of features to help combat QRM and
QRN while generating some of the cleanest signals to be found in Amateur radio today. A 32-bit floating point
DSP is used for highest performance.
In receive, a 15 kHz IF signal from the RF board is buffered and then digitized by a 24-bit Analog to Digital
Converter (ADC). This provides over 100 dB of dynamic range within the passband of the selected crystal
(roofing) filter. After the ADC, the DSP converts the signal into a floating point value so dynamic range is not
compromised during further processing. Noise blanking and limiting, AGC, amplification, IF and AF filtering
are all done within the DSP. Several noise blanking algorithms (methods) are available in the DSP, and a
sophisticated AGC system is employed. AM, FM, SSB and CW detectors are also implemented by the DSP.
Various audio effects, such as Quasi-Stereo and Binaural, are provided here as well as combining the audio
signals from the KRX3 (if installed).
After processing, the resulting audio signals are generated in a stereo 24-bit Digital to Analog Converter (DAC)
and applied to separate amplifiers for headphones (front and rear) and speaker. A separate 24-bit DAC and
amplifier provide Line Out signals that are not affected by the AF Gain control. This output is typically used by
sound card digital mode software.
In transmit, Line In, rear or front Microphone signals are sent to a 24-bit ADC and then processed by the DSP.
In speech modes (SSB, AM and FM) and soundcard-based data modes, VOX is derived from these signals as
well as receive audio. Microphone equalization, bandpass limiting, conversion to 15 kHz IF, envelope clipping
and filtering (if applicable) are all done in DSP, then the signal is passed to another 24-bit DAC and presented to
the RF board as a 15 kHz IF signal. Direct FSK, direct PSK and CW signals are generated within the DSP for
those modes.
Thus, the DSP is responsible for all signal processing between audio and the 15 kHz IF for both receive and
transmit. Like all other modules in the K3, the DSP is managed by the MCU.
The DSP board is piggybacked onto the Front Panel board as part of the Front Panel assembly. The Auxiliary
DSP (used if the KRX3 Second Receiver Option is installed) and the KDVR3 option plug into the DSP board.
KREF3 (Ref./2 nd LO)
The KREF3 module’s 49.380-MHz temperature-compensated crystal oscillator (TCXO) is the signal source for
the K3’s synthesizers. The high-stability TCXO option is typically better than +/- 0.5 PPM over the 0 to 50 C
temperature range. This signal is also divided by 6 to provide the 8.230-MHz 2
nd -LO signal used by the second
receive and transmit mixers. In addition to the TCXO and dividers, the KREF3 provides the 2 nd transmit I.F.
mixer, which converts the DSP's 15-kHz transmit I.F. output to 8.215 MHz. This signal passes through a wide
crystal filter to ensure rejection of mixer products before being routed to the RF board. The KREF3 obtains its
DC and low-frequency I/O signals via an 8-pin connector on the RF board, but its RF outputs are fed to the RF
board (and sub receiver) via coax cables. The K3EXREF option locks the KREF3 to an external reference.