Elecraft KX1 Manual - Manual (Page 73)

72
E
LECRAFT
Circuit Details
While reading this section, refer to the Block Diagram (Appendix C)
and schematics (Appendix B).
Receiver: The receiver is a single-conversion superhet, using down-
conversion to a low intermediate frequency (I.F.) of about 4.915 MHz.
Down-conversion minimizes complexity and receive-chain noise,
while the low I.F. allows adequate CW selectivity with a variable-
passband, 3-pole crystal filter. The use of active mixers keeps current
consumption low, compatible with portable operation.
DDS VFO: The VFO is based on a low-power DDS (direct digital
synthesis) IC. A crystal oscillator provides the reference signal for the
DDS, ensuring excellent frequency stability over a wide temperature
range. While a DDS-based VFO does not offer the same signal purity
as an L-C VFO or PLL synthesizer, it is a good choice for a portable
station that will most often be used by a single operator well removed
from strong nearby stations. It is also extremely frequency-agile,
allowing coverage of both ham bands and SWL bands. The unit
chosen (AD9834) requires a minimum of components and draws only
5 to 8 milliamps.
The DDS output is filtered by low-pass filter L4/L5/C50-52. In order
to provide adequate roll-off of non-harmonic spurs about 14 MHz
when operating at 40 meters, the filter's knee is set just above the 20-m
band edge. Because of this, LSB receive mode on 20 meters is less
sensitive; LSB on this band requires a mixer injection frequency of
4.9+14 = 18.9 MHz, which is significantly attenuated by the DDS
filter.
Transmitter: On transmit, the DDS outputs the actual carrier
frequency, so no transmit mixer stage is required. Q1, Q4 and Q5 form
a 3-stage buffer for the DDS signal. Maximum power output from the
final stage (Q6) is about 4 to 5 watts, depending on the supply voltage.
Q7, in the receive band-pass filter, limits the signal voltage that can
reach the receive mixer when the rig is in transmit mode.
Microcontroller: A low-power microcontroller (U1, MCU) is used to
control the transceiver and handle user interface elements, such as the
display and switches. The MCU communicates with the optional
KXAT1 ATU on the VFWD/DATA line at about a 2 kHz data rate.
U1 runs at just below 4 MHz to avoid band-edge spurs.
The KXAT1 provides SWR and power information for the KX1
display in TUNE mode. During normal keying, it provides an accurate
indication of power output (1 bar per 0.5 watts). Without the ATU
installed, the KX1 displays only a qualitative power output indication
(see page 64).