Elecraft • www.elecraft.com • 831-763-4211
Circuit Details
The ATU uses three inductors and three capacitors in an L-network, with values optimized for use with moderate-
length wire antennas on 40-20 meters. The capacitance can be placed at the transmitter or antenna end of the network
via K7. Each inductor and capacitor has its own DPDT relay, with the individual sections of each relay placed in
parallel for reliability. The relays are selected under control of the ATU's microcontroller, U1. Latching relays are used
so that they will not consume any power except when the operator is actually tuning.
T1, D1, D2 (etc.) form a directional coupler for SWR and power measurements. The bridge output is buffered by op-
amp U2 and routed to the KX1 control board. The bridge outputs are also connected to A-to-D inputs on the
microcontroller, U1. U1 measures these voltages and converts them to SWR or power readings, using averaging and
linearization techniques to improve accuracy. During transmit, the VFWD line provides an analog power indication to
the KX1’s main microcontroller. At other times this line used to transmit digital data between the ATU and KX1. U1
"sleeps" except during antenna tune-up, so it generates no receiver noise.
KXAT1 Schematic
W. Burdick
E. Swartz
Elecraft
2.6µH
Rev. Sht.
.64µH
1.3µH
Date
MCU
82
By
L1
K4
L2
L3
C1
C2
C3
20 or 22
39
2
4
3
7
5
6
5-30pF
1N5711
1N5711
3.3K
C9
C10
C7
C8
100
.01
.01
200
100K
R3
D1
D2
R4
R2
R1
T1
4
2
3
1
6
5
RF IN
K6
4.0MHz
Z1
P1
32
1
1 of 1
L-NETWORK SELECT
FT37-43
.01
C11
NULL ADJ.
K7
2
4
3
7
5
6
C TX
K5
2
4
3
7
5
6
K6
2
4
3
7
5
6
2
4
3
7
5
6
K1
2
4
3
7
5
6
K2
2
4
3
7
5
6
K3
C ANT
TP1
MCLR
OSC1
OSC2
RA2
RA3
RA4
VSS
RB0
RB1
RB2
RB3
RA1
RA0
VDD
RB7
RB6
RB5
RB4
10
11
12
13
14
15
16
17
18
1
2
3
4
5
6
7
8
9
U1
RF OUT
8
1
K5
K4
K3
K2
8
1
K1
K7
16C716
12
3
4
5
P3
RFC1
.01
C4
.01
C5
NOTES:
1. K1-K7 are single-coil latching relays, shown in the RESET position.
nc
VFWD
6V
/MUTE
L1-L3 use T37-2 cores
12t, 17t, 25t
RF
Control
15 µH
KXAT1 Antenna Tuner
2. indicates component mounted on bottom of PC board.
RYCOM
P2
1
2
Gnd
VREFL
.01
C6
VFWD
XC
9/30/03
