40
SWR Bridge and Antenna Tuner Unit (ATU)
RF from the SWR Bridge is sent to the ATU which converts the impedance seen at the antenna connector to a
50 Ω non
-reactive load as measured by the standing wave ratio (SWR) bridge.
The ATU uses an “L” matching network with series inductance and shunt capacitance. Combinations of eight
capacitors in the Capacitor Bank and seven inductors in the Inductor Bank are switched in by relays to provide
up to 2624 pF of capacitance and up to 8370 nH of inductance. An algorithm operates relays to select the
inductors and capacitors required and chooses whether the capacitors are placed on the transmit side or antenna
side of the inductors to arrive at a tuning solution for the current operating frequency. This is called a Full
Search Tune.
When the ATU relays are changing capacitors or inductors, the amplifier circuit is kept in receive mode (RF
routed through the TR switch but not the PA module), using only the RF power from the driving transceiver to
monitor the SWR.
The antenna tuner is available when the amplifier is on, in both
OPER
and in
STBY
, for use by the barefoot
transceiver. The tuner is relay-bypassed when the KPA1500 is off.
Control Logic and Fault Conditions
A microcontroller unit (MCU) controls and monitors critical operating conditions. The MCU also controls the
ATU functions.
•
When a key line low is sensed, the amplifier is switched into transmit mode if no fault conditions are
present, the input signal is not in the 26 to 28 MHz range and an ATU tune operation is not underway.
•
When RF drive is applied, the frequency is measured and the correct low pass filter is switched into the
signal path.
•
The KPA1500 exchanges band data with the Elecraft K3
S
or K3 and automatically selects the proper
low pass filter, enabling rapid band changes. The MCU does not need to detect the frequency input for
filter selection.
•
Band data changes can also occur as a result of a frequency message from other transceivers using the
XCVR SERIAL
port (Flex, Icom CI-V, Kenwood serial or Elecraft KX3/KX2 serial data).
However, RF frequency sensing does take priority if a tuning solution does not agree with the band data to avoid
transmitting into an incorrect bandpass filter. The optional KPAK3AUX cable kit is needed to enable this
feature, as shown in Figure 4, page 15 or Figure 5, page 16.
Under fault conditions, the MCU protects the amplifier to prevent damage or improper operation. There are two
fault conditions (soft and hard), and are triggered by the following conditions:
•
Soft:
A 3 dB attenuator is automatically switched in if excessive the RF drive power is applied or if
other conditions occur such as excessive RF output, reflected power (high SWR) or current is drawn by
the finals.
•
Hard:
If an unauthorized frequency is transmitted, the KPA1500 switches to
STBY
until RF drive is
removed. This fault will automatically reset if a valid frequency is applied.
When a fault condition is detected, the TR switch is inhibited so no RF power reaches the PA module. If RF
drive is high enough to possibly damage the TR switch, the amplifier is switched to bypass, protecting the TR
switch.
The amplifier is always switched to
STBY
during hard faults. A hard fault requires the operator to remove or
correct the condition and press the
RESET
key to return the amplifier to normal service.
