Elecraft K1 - Manual (Page 60)

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virtue of an intermediate voltage supplied by op-amp U5A. To turn
both elements of D1 off, pin 4 is made an input.
The states of the DOT and DASH lines are sampled on analog input
RA3 (pin 5). If both DOT and DASH are open, the voltage at RA3
will be about 6 V. If both are closed, the voltage will be 0 V. If only
one paddle is closed, an intermediate voltage results, either 2 V or 4
V. (See RF board, sheet 2.)
Q2 defeats the RIT/XIT offset when its gate voltage is at 6 V,
since its low drain-source resistance forces a fixed 3 V to appear at
the wiper of the offset pot, R3.
U2 and U3 are controlled by the SDA (data) and SCL (clock) lines,
using the industry-standard I2C protocol. U2 is a dual-output D-to-
A converter that controls the power level and generates audio
tones. When tones are not being generated, the tone output is used
to set the crystal filter bandwidth. U3 is a 512-byte EEPROM,
which stores all K1 operating parameters and CW messages.
VFO pot R1 is physically part of the front panel assembly, but its
wiper voltage (0-6 V) is routed to the RF board via J1. R19, located
between the wiper and the high end of the pot, improves tuning
linearity.
Filter Board
Note: This section applies to the 2-band filter module (KFL1-2).
For details on the four-band module, refer to the KFL1-4 manual.
L1-L4 and associated capacitors form two band-pass filters, one for
the premix signal on each band. There are also crystals for each
band (X1, X2). For example, on 40 m (7.000 MHz), the premixer
(U7, RF board) mixes a 3.085-MHz signal from the VFO with a
15.000 MHz crystal oscillator to obtain a difference signal at
11.915 MHz. There is also a sum frequency, 18.085 MHz, which is
rejected by the band-pass filter. The 11.915 MHz signal is routed to
the RF board at P1 pin 6.
L5-L8 and associated capacitors form two RF bandpass filters, one
for each band. T-R switching circuits on the RF board place these
filters in either the receive or transmit path. L9-L12 and associated
capacitors form the two low-pass filters.
All filters and the two crystals are switched by latching relays K1-
K3. Microcontroller U1 pulses the relay coils bidirectionally to
switch them on or off. The reset condition of each relay selects
band 1. Latching relays are only energized when switched between
set and reset, so they consume no power during normal operation.
RF Board
VFO (Sheet 1)
Q8 is used in a varactor-tuned Colpitts VFO. The VFO tunes
"backward." For example, its frequency is about 3.080 MHz when
the operating frequency is 7.000 MHz, and 2.930 MHz when the
operating frequency is 7.150 MHz. This is due to the fact that the
VFO is subtracted from the crystal oscillator in the premixer (U7).
The use of varactor diodes (electronic tuning) results in a
compromise between circuit complexity, tuning range, and stability.
While VFO drift can never be eliminated entirely, steps were taken
to minimize it.
First, the VFO supply voltage is double-regulated (explained in the
next paragraph). This improves stability by guaranteeing a very
stable voltage reference for the varactor diodes. Since the
transmitter circuits run from a different regulator, and since the
VFO is also well-buffered from the transmitter, there is virtually no
VFO frequency shift or chirp on key down over the full power
output range (0.1-7 watts). Second, temperature-compensation
techniques are used in the VFO itself, including careful selection of
capacitor types. The inductor used has a low temperature
coefficient, and its turns are adjusted to select the desired range,
which eliminates drift-prone trimmer capacitors.