Elecraft K3 Owner's Manual - Manual (Page 75)

75
KSYN3 (Synthesizer)
Low phase noise is key to both receiver and transmitter performance. In the K3’s synthesizer module (KSYN3),
we start with a clean, wide-range voltage-controlled oscillator (VCO). The VCO frequency is placed near the
desired band of operation using 128 carefully-selected L-C combinations, which keep the ratio of fixed
capacitance to tunable capacitance (varactor diodes) as high as possible.
The VCO is held exactly on frequency by a phase-locked-loop IC (PLL), which samples the VCO output
continuously and compares it to its high-stability reference input. The PLL’s reference input is obtained from a
direct-digital-synthesis (DDS) IC, which is tunable in about 0.2-Hz steps. The reference for the DDS itself is the
49.380-MHz signal from the KREF3 module.
To keep the synthesizer’s output signal virtually spur-free, the DDS is followed by a 4-pole crystal filter. This
eliminates both directly-occurring spurs and the Nyquist sampling spurs that normally accompany a DDS-driven
PLL system.
The combination of all of these noise-minimization techniques results in very low phase noise and negligible
discrete spur content.
Firmware
Overview
The K3 is controlled by a Microchip PIC18F8722 microcontroller (MCU) on the Front Panel PCB module. It
uses a highly-optimized, custom operating system to efficiently handle many complex tasks. In contrast, many
transceivers at the K3’s performance level embed the equivalent of a complete PC (personal computer) running
a conventional operating system. This may simplify software development, but at the cost of higher power
consumption, greater RF interference and noise, and slower power up/down cycles.
At the highest level, the K3’s MCU firmware runs a continuous executive loop. Within this loop, calls are made
to handlers for the all user interface elements (switches, potentiometers, encoders, LEDs, LCD, etc.), I/O
(RS232 command handling, AUXBUS, SPI peripheral control, T/R switching), and process control (timers, state
machines, etc.). In addition, a fast interrupt handler runs every 200 microseconds to service high-speed events
such as optical encoder state changes and incoming serial data. This interrupt handler also provides fine-timing
services for other functions and state machines, such as RTTY or PSK31 text encode/decode.
The main and aux DSPs are peripherals of the MCU. They execute an extensive set of commands, allowing the
MCU to specify operating mode, BFO frequencies, keying waveform start/stop, sidetone pitch, AF and IF gain,
filter bandwidth and position, and many other parameters. In some cases the MCU polls the DSP for information
periodically. For example, if voice VOX is enabled, the MCU polls every few milliseconds to see if the present
mic level is above the user’s specified VOX threshold, in which case the rig will be switched to TX mode.
Transmit ALC and Per-Band Power Control
To eliminate transmit splatter, all ALC in the K3 is applied before the narrow first-I.F. crystal filter. However,
with some vocalizations, speech energy may build up within a narrow crystal filter to produce a slight peaking
effect. Because of this, the user should set the K3's power output level such that it peaks at or below the safe
level for any external amplifier under all speech conditions.
To facilitate this, optional per-band power control can be used (see CONFIG:PWR SET). This compensates for
per-band gain variation in the external amp or the K3 itself, reducing the need for ALC. For example, all crystal
filters have a small amount of ripple (typ. +/- 0.5 to 1.0 dB) that can result in a slight difference in average
power output between LSB and USB with some voice characteristics. But since you typically use only one of
the two sidebands on a given ham band, per-band power control can conveniently compensate for small
variations.