Elecraft K2 Owner's Manual - Manual (Page 112)

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LECRAFT ® 111
Coverage of 160-10 meters is provided by a single wide-range VCO (voltage-
controlled oscillator). High-side and low-side injection are both used,
depending on the band, so the overall VCO range is limited to about 6 to 24
MHz. Only one VCO is needed, with a single high-Q inductor and three small
DPDT relays configured to select one or more fixed capacitors. The VCO is
driven by a PLL synthesizer. 5 kHz frequency steps are used at the PLL,
while 10 Hz increments are provided by a 12-bit DAC driving an 11 MHz
VCXO (PLL reference oscillator).
Crystal Filters and BFO Settings
The signals you tune in on the K2's receiver are "shaped" by the crystal filter,
which passes only a narrow range of frequencies. The pitch of these signals is
determined by the BFO (beat-frequency oscillator). Figure 9-1 shows an
example of how these signals are related. The BFO frequency is below the
filter passband; this is the case for the CW "normal" and LSB modes on the
K2. Two different filters are shown: FL2 (narrow, for CW), and FL1 (wide,
for LSB voice). Frequencies in the 4915 kHz range are shown because this is
the K2’s intermediate frequency, or I.F.
FL2
4913.0 4913.5 4914.0 4914.5 4915.0
BFO Signal 1 Signal 2
FL1
Figure 9-1 CW Normal or LSB.
In this example, filter FL2’s bandwidth is set for about 1 kHz, and it is
centered at 4914.0 kHz. The BFO is set for 4913.0 kHz.
Signal 1 (4914.0 kHz) will be passed by FL2, and you’ll hear it at an audio
pitch of 1 kHz (4914-4913). Signal 2 (4915.0 kHz) will be rejected by FL2,
but passed by FL1, and heard at 2 kHz. The same BFO setting can be used for
both filters, because the lower boundary of the K2’s variable-bandwidth
crystal filter stays fixed as it is made wider. Only the upper edge moves
significantly.
Figure 9-2 shows the BFO positioned above the same two filters, which will
allow the K2 to receive USB and CW Reverse (opposite-sideband CW).
Since the upper boundary of the filter moves as the filter is widened, the BFO
frequency must move the same amount. BFO2 is used with FL2, and BFO1 is
used with FL1.
FL2
4913.5 4914.0 4914.5 4915.0 4915.5
BFO1
FL1
BFO2
Figure 9-2. CW Reverse or USB.
The CAL FIL menu function provides the means to control how wide the
filters are, and where the BFOs are located in relation to them. (The numeric
parameters you select using
CAL F IL are translated into voltages that
control the filter and BFO by means of voltage-variable-capacitance diodes,
or varactors.)
Microcontroller (MCU)
The K2’s microcontroller is an integral part of all transceiver operations.
Firmware is used to advantage to provide many functions traditionally
provided by discrete control logic. For example, the VCXO (PLL reference
oscillator) is linearized on each band by a firmware auto-calibration routine,
with resulting tables stored in EEPROM. Another example is firmware ALC,
which is used on CW to maintain the user-specified power level across all
bands. The SSB adapter, when installed, provides its own optimized hardware
ALC.
Extensive use of firmware also results in many useful operating features not
usually found on transceivers in this price class. These features include built-
in test equipment (frequency counter and digital voltmeter), auto-calibration,
dual VFOs, memories, split operation, RIT/XIT, and a versatile keyer.
Provisions have also been made in firmware to support a wide range of option
modules. (See full feature list elsewhere on the web site.)