Elecraft K2 Owner's Manual - Manual (Page 116)

E
LECRAFT ® 115
U4 provides coarse tuning (5 kHz steps). Fine steps are achieved using a 12-
bit DAC (U5) to tune a voltage-controlled crystal oscillator (Q19), which is
the PLL reference oscillator. The reference oscillator range needed on each
band varies in proportion to the VCO output frequency. To cover exactly 5
kHz in 10 Hz steps on each band, an automatic calibration routine is provided
in firmware. The DAC is swept from its highest output voltage down, and the
DAC word needed to select each 100 Hz step is recorded in EEPROM on a
per-band basis. 10 Hz steps are then interpolated based on the 100 Hz table
data. Crystal X1 in the PLL reference oscillator can be tuned by varactor
diodes D16 and D17 over a range of about 10 kHz, which is required in order
to tune the full 5 kHz on the lowest band (160 m), but still provides better
than 10 Hz resolution on the highest bands.
The synthesizer design is unique in that three inexpensive DPDT latching
relays are used to select one of eight VCO ranges, thus requiring only a single
high-Q VCO inductor (T5). The relays are optimally interconnected to allow
for maximum coverage of the nine HF bands, plus a large out-of-band tuning
range. Computer simulation was used to find a relay topology that allowed for
the use of standard 5% fixed capacitors along with the smallest practical
varactor diode capacitance. As a result, the VCO exhibits low noise on all
bands and has a low max/min tuning ratio on each band.
In order to provide some allowance for unit-to-unit variance in T5, a much
higher value slug-tuned inductor (L30) is placed across T5’s high-impedance
winding. L30 has only a small effect on the Q of T5, but provides about a
20% tuning range. The combined parallel inductance is very small (only 1
µH), resulting in a very large C/L ratio on the lowest bands.
U3 buffers the VCO signal. Q16/Q17 provide stable ALC to keep the VCO
voltage fairly constant over the entire frequency range despite variations in
the VCO transistor, Q18.
Also shown on sheet 1 is the DC input circuitry (bottom right-hand corner),
which is designed to protect the K2 and its power supply from almost any
conceivable mis-connection or short. D10 protects the K2 from reverse
polarity at the DC input, while dropping only 0.1-0.2 volt. F1 is a thermal
self-resetting fuse that goes into a high-resistance state if a short or other
high-current situation exists anywhere inside the K2. F1 resets quickly once
the source of the short is removed. D12 provides reverse-polarity protection
for the internal battery, if applicable.
Sheet 2: Receiver and Low-Level Transmitter Circuits
The receiver is a single-conversion superhet with an I.F. (intermediate
frequency) of 4.915 MHz. The preamp and attenuator are switched in using
latching relays so that no current is required except when switching them on
or off. The mixer is a diode ring type, providing good dynamic range (Z6),
and is followed by a strong post-mixer amplifier, Q22. The current drain in
Q22 can be reduced by the operator using a menu option that turns off Q12.
A 5-pole variable-bandwidth crystal filter is used on CW (X7-X11). This
filter is optimized for use at low bandwidths (~200 to 500 Hz), but can be set
both narrower and wider as needed with only a small additional loss. The
shape factor and passband ripple content are optimized at around 300 Hz. (On
SSB, a separate fixed filter is switched in; this filter is located on the SSB
adapter.)
AGC is derived from the output of the I.F. amp by using an auxiliary, low-
frequency I.F. of about 150 kHz (see Control Board). The AGC signal is then
applied to pin 5 of the I.F. amp (U12).
A second crystal filter (X6/X5) follows the I.F. amp to reduce wideband
noise. This filter is also tunable. Varactor diode D39’s capacitance is
increased during CW use, but on SSB is reduced, making the response quite
broad. Q25 turns on only if the optional SSB adapter is installed and its fixed-
bandwidth "OP1" filter is selected. This pulls capacitor C179 to ground,
which interacts with L34 to shift the second crystal filter's center frequency so
that it matches that of the OP1 filter.
The product detector is a Gilbert-cell mixer/oscillator (U11). Due to the loss
in the second crystal filter, the input voltage to U11 never exceeds the range
that the device can handle.
U11 also provides the BFO signal, which is tunable over about a 4 to 5 kHz
range by varactor diodes D37 and D38. X3 and X4 have carefully-controlled
characteristics and are well matched. As in the PLL VCXO (Q19, sheet 1),
the two crystals de-Q each other to increase the tuning range of the BFO.
On transmit, the BFO buffer/attenuator (Q24) is turned on. Q24’s drain
voltage is controlled by the microcontroller, providing BFO amplitude
control. Precision PIN diode D36 provides additional reduction in low-level
signal leakage when Q24 is turned off. U10 mixes the VCO with the BFO on
transmit, and video amplifier U9 increases the signal level while providing a
low-impedance output to drive the bandpass filters (sheet 3).