Elecraft K2 Owner's Manual - Manual (Page 113)

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LECRAFT ®
Latching Relays
Latching relays are used for all filter, VCO, and option switching, so there is
no relay current drawn during normal operation. This, combined with careful
power control at all stages in the transceiver, results in receive-mode current
drain as low as 100 mA. The latching relays are all controlled by a single
device, the I/O Controller (see below), which also handles other
miscellaneous I/O tasks on the RF board. DPDT relays are used for all filter
switching, reducing the number of relays needed by a factor of two. 50-ohm
switching is used for all filters, and this combined with careful layout and
guard-banding of the relays results in excellent filter input/output isolation.
Co-Processors and the AuxBus
In keeping with the K2’s modular system architecture, much of the I/O
switching is handled by co-processors. There is only one co-processor in the
basic K2, the I/O Controller (IOC). Some option modules, such as the SSB
adapter, have their own co-processors. This distributed processing technique
allows future modifications to be made to option boards without changing the
transceiver itself. It also reduces cost of the basic K2, since fewer main-
processor control lines are needed.
The IOC, as well as all co-processors on option modules, go into "sleep"
mode with their own 4 MHz clocks suspended during normal operation. For
this reason, there is virtually no digital noise on the RF board to cause
receiver EMI.
When the operator performs an operation that changes relay states, the main
microcontroller (on the Control board) wakes up the co-processors and sends
one of them a configuration command. These commands are transmitted on a
one-wire network called the AuxBus. The AuxBus network line sits at a logic
high during normal operation, and is only activated when needed. The
receiver is muted during commands, so the operator never hears any digital
noise due to AuxBus activity.
Most AuxBus transmissions occur due to operator requests such as a band
change. However, the AuxBus may also be used during transmit to relay
numeric data such as SWR or ALC from a coprocessor to the main
microcontroller. Waking up the coprocessors during transmit has no effect on
the transmitted signal.
Front Panel Board
The front panel PC board plugs into the RF board via a 20-pin single-row
connector, P1. The Front Panel is made up of a number of user-interface
elements as detailed below.
The LCD, DS1, is an 8-digit 7-segment transflective type with three
backplanes (triplexed). Its driver, U1, receives display commands via an I2C
interface.
20 The LCD backlight LEDs, D2 and D3, are used to provide enough
brightness to handle low-lighting situations ("NITE" mode in the menu),
while drawing only a small amount of current (<30 mA). However, they can
be turned off when ambient lighting is sufficient ("DAY" mode) because the
LCD is transflective, i.e. it can either reflect or transmit light. The LCD
displays the operating frequency and status messages, and also has 8
annunciators which indicate the settings of various controls.
A 10-segment LED bargraph, DS2, is used to display received and
transmitted signal strength and ALC level. Using the menu, the operator can
select OFF, DOT or BAR mode for the bargraph, with OFF or DOT modes
typically used to save current during battery operation. U3 and U4 are 8-
output MOSFET driver arrays which control the bargraph, among other
things. Q1 and Q2 form a brightness control. When the NIGHT(low) control
line is pulled to ground by U3, the bargraph supply voltage drops to 2.7 V,
resulting in about 6 mA/LED. The LCD backlight is also turned ON in this
case. When NIGHT(low) is left high for daytime use, each LED draws about
18 mA, and the LCD backlight is OFF.
A high-quality optical shaft encoder, Z1, provides 100 counts per turn. VFO
tuning steps of 10, 50, or 1000 Hz per increment are used, resulting in 1, 5
and 100 kHz per turn, respectively. The encoder is also used to modify
parameters in the menu. The encoder can be turned off by U3 to save current
under certain operating conditions.
S1-S16 are pushbutton switches. Switch data is read by U2, an 8-bit parallel-
to-serial shift register. Each switch has at least two functions: the top label
corresponds to a TAP (short press) and the bottom label corresponds to a
HOLD (long press, ~0.5s). Switch combinations are also supported, although
only two are used (BAND+ and BAND- together enter direct frequency entry
mode, and AGC with PRE/ATTN turns AGC on or off).
20 I2C stands for Inter-IC Communication, an industry standard serial
interface protocol used by Philips and other IC manufacturers.