Theory Of Operation; Rf Board - ELECRAFT K3S Owner's Manual

Theory Of Operation

Please refer to the block diagram of the K3 shown at the end of this section. Schematics and additional details
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can be found on the Elecraft web site. Only basic modules are covered here; for option module circuit details,
refer to the individual instruction manuals (KPA3A, KBPF3A, KRX3A, etc.).

RF BOARD

The RF PCB (Printed Circuit Board) is the heart of the K3 transceiver, both physically and electrically. During
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assembly, it serves as an attachment point for other PCBs as well as chassis panels, acting as the glue that holds
things together. During operation, the RF board provides signal routing to and from all modules.
Over two-thirds of the RF board's components are surface mount devices (SMDs), located on the bottom side of
the board. These are pre-installed so the complete RF board assembly can be aligned and tested at the factory.
The use of SMDs minimizes stray coupling in RF circuits, reduces system cost, and allows the K3 to fit in a
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modest-size enclosure, compatible with home or field operation.
The RF board is divided into several functional areas, which are described below.
Low-Pass Filters (LPFs)
The relay-switched low-pass filters, used during both transmit and receive, are located in the back-right corner
of the RF board. These filters can easily handle 100 watts, and are common to both the K3 /10 and K3 /100.
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Some LPFs cover one band, while others cover two bands that are close in frequency. The input to the LPF
section comes from the KPA3A 100-W amplifier module, if installed; if there's no KPA3A, the input comes
from the 10-W amplifier (see below). The output of the low-pass filters is routed through the forward/reflected
power bridge, then on to either the antenna input module (KANT3), or the KAT3A automatic antenna tuner,
which plugs in at far right.
Low-Power Amplifier (LPA) and T/R Switching
The large hole near the back-middle area of the RF PCB is where the 10-W low-power amplifier module plugs
in. The LPA has three connectors that mate with the RF board, and its power transistors attach to the rear bottom
cover, which serves as a heat sink. This construction method allows the 10-W module to be tested separately
during production. Also in this area is the T/R (transmit/receive) switch, but you'll need to turn the RF board
upside down to see most of the components. The T/R switch uses high-power, high-isolation PIN diodes rather
than relays, resulting in no switching noise during keying.
Low Power Amplifier (LPA)
The low-power amplifier module is capable of up to 12 W power output, and in the case of the K3 /10, is the
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final amplifier stage. In the K3 /100, it provides drive to the KPA3A module. The LPA has three gain stages,
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the last two of which use high-power MOSFET transistors to allow coverage up through 6 meters. At the input
to the first gain stage is a 5-dB attenuator, which is switched in under firmware control at certain power levels to
optimize transmit gain distribution.
Band-Pass Filters (BPFs)
At back-left is the bank of ham-band BPFs. These filters are just wide enough to cover each ham band, so they
provide good rejection of IMD products during both transmit and receive. Hi-Q components, including large
toroids, ensure low loss and high signal-handling capability.
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