Elecraft K1 - Manual (Page 59)

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E
LECRAFT
12. Circuit Details
While reading this section, refer to the Block Diagram (Appendix
B) and schematics (Appendix C).
Overview
The K1 is a compact, high-efficiency CW transceiver capable of
operating over a wide range of frequencies. The two (or four) bands
of operation are determined by a single plug-in filter module,
allowing the builder to swap in a different set of bands if desired.
The firmware will recognize whether a two- or four-band board is
installed.
The filter module includes the band-pass filters and crystals for the
K1's pre-mix conversion scheme, in which a low-frequency (~3
MHz) VFO is subtracted from a high-frequency crystal oscillator to
produce a "pre-mixed" injection frequency for the transmit and
receive mixers. Because of the VFO's low operating frequency,
stability is excellent. An analog VFO is used, resulting in a lower
noise floor than most synthesized transceivers. The filter board
also has RF band-pass and low-pass filters for each band.
The receiver is a single-conversion superhet, using down-
conversion to a low intermediate frequency (I.F.) of about 4.915
MHz. Down-conversion minimizes complexity and receive-chain
noise, while the low I.F. allows good CW selectivity with a 4-pole
crystal filter. The use of active mixers keeps current consumption
low, compatible with portable operation.
There are three stages of transmitter amplification. Maximum
power output from the final stage (class C) is about 5 to 7 watts,
depending on the band and supply voltage. The T-R circuitry is all
solid-state (no relays), resulting in smooth, fast QSK.
A low-power microcontroller (MCU) on the front-panel board is
used to control the transceiver and handle user interface elements,
such as the display and switches. A second, smaller microcontroller,
the I/O controller (IOC), acts as a driver for the filter and
attenuator relays. The IOC is controlled by the main MCU via a
one-line network, the auxBus. Option microcontrollers (i.e., on the
noise blanker and antenna tuner) also communicate via the auxBus.
Front Panel Board
Microcontroller (MCU) U1 is used to control all K1 functions, and
doubles as the driver for the LCD, DS1. U1 runs at just below 4
MHz. The crystal, X1, is intentionally kept under 4 MHz (by using
large oscillator capacitors), so that any internally-generated band-
edge signals will be below the bottom edge of the band.
The LCD segments are driven with 50% duty cycle square waves to
ensure that the average DC voltage on each segment is 0 volts. The
LCD backplane (pin 24) is also driven by a low-frequency square
wave. To turn on a segment, it must be driven 180-degrees out of
phase with the backplane. In-phase segments remain off.
The MCU has a number of miscellaneous functions. It reads the
states of the switches (S1-S6) by periodically making the associated
LCD drive lines into inputs. The VFO signal is amplified by Q1,
then counted at MCU pin 6 (RA4). Pin 3 (RA1) is made an A-to-D
(analog-to-digital) input to read the battery voltage, or an output to
turn the ATTN LED on or off. Pin 4 (RA2) drives to 6 V or 0 V to
turn on the RIT or XIT LEDs, respectively (both part of D1), by