Elecraft K2 Owner's Manual - Manual (Page 115)

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LECRAFT ®
U7 provides 2 kbytes of non-volatile configuration data storage. This memory
is used for VCO lookup tables, CW messages, frequency memories and other
variables that must be permanently saved. The EEPROM can be written
millions of times without loss of data. During normal operation on a single
frequency (such as when in a QSO), the EEPROM is not accessed at all.
However, whenever the VFO is moved, a 30-second timer is triggered. Once
the VFO has stopped moving for 30 seconds, the EEPROM is updated with
the latest VFO frequency. In this way, the K2 always saves the most recent
"important" frequency. (The EEPROM update also takes place any time you
change bands or operating modes, etc., so you don’t have to wait for 30
seconds to record an important configuration change.) An alternative strategy
used by many rigs is to use battery-backed-up RAM, continuously recording
the operating frequency. We preferred to eliminate the backup battery, which
often has a high failure rate and must be periodically replaced.
The Control board provides a built-in voltmeter and ammeter. By jumpering
P7 appropriately, the operator can monitor either the internal 12 V supply
voltage or the voltage from a test probe plugged into P5. U3B buffers the DC
signal from the probe, and also is used in conjunction with Q11 to provide
supply current monitoring. The current sense resistor, which has a value of 50
milliohms, is located on the RF board (R115).
U4 is a low-dropout 8 V regulator, which is stable with a K2 input DC
voltage as low as 8.2 V. Since all signal-generating and signal monitoring
stages in the K2 run from this 8 V supply, the transceiver will function
normally even when running from very depleted batteries; most transceivers
use a higher regulated voltage for these stages and in some cases will not
operate reliably even at a battery voltage of 11 V. (Transmit power will be
scaled back and a warning message displayed if the battery voltage drops
below a critical value or if current drain is excessive.) U5 provides 5 V for
logic circuits on the front panel and Control board, but this signal does not
appear on the RF board, so noise is minimized.
8 V Switching: Q1 and Q2 provide stable +8 V sources on transmit (8T) and
receive (8R). (Q23 on the RF board is used to guarantee that 8R goes to 0 V
on receive to maintain proper reverse voltage on T-R switch diodes.)
An optional audio filter module (KAF2 or KDSP2) can be mounted on the
bottom of the Control board. These options provide analog or digital filtering
functions. The audio filter module has its own co-processor.
Q6 and Q7 disconnect the AF amplifier from the product detector on transmit, which is necessary for clean QSK. U9 is an LM380 audio amp IC, supplying
approximately 1W of audio drive to a 4-ohm speaker in the cover of the K2.
Sidetone is injected post-volume control so that sidetone and receiver audio
volume can be controlled independently.
The AGC circuit is the only RF stage located on the Control board.
Mixer/oscillator U1 generates a low-level signal at about 5.068 MHz, then
mixes it with the 4.915 MHz I.F. signal from the RF board to produce a new
auxiliary I.F. of about 150 kHz. This auxiliary I.F. signal is then amplified by
U2B and detected by D1 to create a positive-going AGC voltage, which is
then routed back to the RF board to control the I.F. amp (U12). While it is
possible to generate the same AGC voltage by simply amplifying and
detecting the 4.915 MHz signal itself, this technique often necessitates
shielding of the AGC RF amplifier stages to prevent radiation of the I.F. or
BFO signals back into the receiver I.F. strip. We obtain all of the gain at 150
kHz instead, so the 4.915 MHz signal is not re-radiated. 150 kHz is high
enough to obtain fast AGC response—two orders of magnitude faster than is
possible when audio-derived AGC is employed.
RF Board
The RF board is the largest of the three K2 boards, and serves as a structural
element that the chassis and the other boards attach to. This board contains all
of the RF circuits (amplifiers, oscillators, filters, etc.). Refer to the RF board
schematic (Appendix B).
Sheet 1: Synthesizer
The K2 uses a PLL (phase-locked-loop) synthesizer IC (U4) in conjunction
with a wide-range, band-switched VCO (Q18). The synthesizer provides
approximately +7 dBm output from 6 to 24 MHz, which is then injected at the
transmit and receive mixers (sheet 2). Phase noise performance of the
synthesizer is very good despite its low parts count and absence of shielding.
The reference oscillator for the PLL IC is temperature-compensated by the
components on the thermistor PC board. This circuit works by applying a
variable offset voltage to varactor diodes D16 and D17 to compensate for
drift As temperature increases, the uncompensated oscillator would drift
down in frequency. The thermistor causes a slight increase in the bias voltage
to these diodes as the temperature increases. The relative values of RA-RD
and the thermistor, Rt, (see below) set the rate of gain change with
temperature.