Elecraft KX3 Owner's Manual - Manual (Page 49)

49
Theory Of Operation
This section includes:
§ a functional description of the KX3’s RF, control panel, and option PC boards
§ block diagram of the KX3 (pg. 51)
§ glossary of selected technical terms (beginning on pg. 52)
Additional information including an FAQ (answers to frequently asked questions) can be found on the Elecraft
web site.
RF Board
The RF PCB (Printed Circuit Board) contains all of the KX3’s RF circuitry as well as low-level baseband (AF)
stages in the receive path. The Control Panel (CP) board (following page) generates all digital control signals
for band switching, T/R, signal path routing, etc.
The relay-switched low pass filters are used during both transmit and receive. A few of the filters are dedicated
to one band but most cover two bands. The signal on the antenna side of the filters pass through a
forward/reflected power and SWR bridge to the HF-6 meter antenna jack (BNC), or to the optional KXAT3
automatic antenna tuner. Latching relays are used to minimize power consumption.
Transmit/receive (T/R) switching splits the common signal path from the antenna and low pass filters to either
the transmitter power amp or receiver input. It also allows sharing the band pass filters between receive and
transmit. All T/R switching is done with high-power PIN diodes and CMOS switches to facilitate high speed
transitions between receive and transmit.
The synthesized, digitally controlled local oscillator (LO) provides quadrature signals to the transmit and
receive mixers, as well as a reference signal to the optional KX3-2M/4M module. The LO has very good phase
noise performance and can cover a very wide frequency range. Since the KX3 uses a “zero IF” architecture, the
KX3’s LO frequency is always very close to the operating frequency.
The RF band-pass filters are used for both transmit and receive. Filters are selected with low-loss CMOS RF
switches. The band pass filters significantly attenuate receive signals at harmonics of the RX frequency,
particularly the odd harmonics.
Following the band pass filters and T/R switching are two RF preamplifiers and an attenuator. These provide
various tradeoffs between RF gain and noise figure (or MDS), as well as local oscillator (LO) isolation. The
latter would be useful in situations where another receiver in close proximity could be tuned to the same
frequency. A problem inherent in most direct conversion (zero IF) receivers is that some LO energy leaks to the
antenna and is radiated. This can be a problem when another receiver & antenna is in very close proximity and
is tuned to the same frequency. The isolation preamp in the KX3 (RX ISO menu entry) virtually eliminates this
signal leakage.
The RX mixer converts the RF signal to quadrature baseband (AF I and Q signals), which are low-pass filtered
and amplified before being passed to the CP board for analog to digital conversion (ADC). I and Q baseband
signals (In-phase and Quadrature) from the mixer are also buffered and sent to the RX I/Q output. This allows
attaching the KX3 to a PX3 Panadapter, or to a PC or other device running SDR software (pg. 26).
The main AF amplifier section also interfaces to the optional KXFL3 filter module that provides narrower
analog filter bandwidths for improved dynamic range performance with nearby strong interfering signals.
The TX AF Amp and TX mixer block converts baseband (AF I and Q) modulating signals to an RF signal
which is then routed to the T/R switching and band pass filters. This signal provides excitation to the 10W
power amplifier. The 10W amplifier (PA) uses a pair of RF power MOSFETs. Temperature monitoring of the