ELECRAFT 107
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. However, 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 can be mounted on the bottom of the
Control board. This filter will provide analog and/or digital signal processing
functions. The audio filter module has its own co-processor so that it can be
enhanced in the future.
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.
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