beautypg.com

Circuit details, System overview – Elecraft K2 Owner's Manual User Manual

Page 111

background image

110

E

LECRAFT

®

9. Circuit Details

Before reading this section you should become familiar with the Schematics
(Appendix B) and Block Diagram (Appendix C).

System Overview


The K2’s modular design allows flexibility in configuration and provides for
future expansion. At the core of this modular architecture are the three main
circuit boards:

Front Panel

User interface, including display and controls

Control Board

MCU, DC control, AGC, and AF amplifier

RF Board

All RF circuitry, relays, and I/O controller (IOC)


This functional division allows related circuits to be grouped together, but
also provides a high degree of isolation between the analog and digital
sections of the transceiver. The RF board serves as a "mother board," while
the front panel and Control boards plug into the RF board at its front edge.
The front panel and Control boards are mounted back-to-back, with their
ground-plane layers forming a partial enclosure that helps minimize radiated
digital noise.

The K2’s custom enclosure is also modular. It is fabricated in six pieces, with
a unique 2-D fastener used at each joint and also for PCB support. This
design provides a rugged but light-weight enclosure that is ideal for field or
home use.

The top cover, which includes the upper portion of the rear panel, can support
a variety of built-in options such as an internal battery, automatic antenna
tuner, and RS-232 interface. The top cover can be replaced with a 100 W
power amplifier module, converting the K2 into a medium-power station.

Signal Flow


The block diagram (Appendix C) shows overall signal flow in the K2.
Transmit and receive paths are shown for sideband operation. For CW
transmit, the BFO signal is routed directly to the transmit mixer.

The K2 receiver is a single-conversion superhet, utilizing double-tuned band-
pass filters on each band and down-conversion to a low I.F (4.915 MHz).
This approach results in excellent CW and SSB performance. The low I.F. is
compatible with narrow, variable-bandwidth CW crystal filtering and allows
the use of fast I.F.-derived AGC. An I.F. of 4.915 MHz also results in nearly
no birdies across all nine bands. The BFO is microcontroller controlled to
allow upper and lower sideband reception on any band, as well as CW on
either sideband. AM signals can be received in SSB modes thanks to the
stable VFO, although AM transmit is not currently supported.

Individual (per-band) band-pass filters offer improved intermodulation
performance when compared to up-conversion designs that use only a single
low-pass filter to remove image products ahead of the receiver. Up-
conversion also requires the use of a second I.F. to obtain good CW
performance, increasing cost and producing additional spurious signals. (An
alternative is up-conversion followed directly by a product detector and audio
filter. While this results in minimal parts count, it was not considered since
the resulting CW and AGC performance would have been poor.)

On transmit signal flow is reversed, so the BFO is combined with the VCO to
generate an output at the operating frequency, which is filtered by the band-
pass and low-pass filters. A highly stable power amplifier chain up to 10-15
watts on all bands, and the output level can be set in 0.2-W increments (0.1–
W increments below 10 W). The transmit strip is conservatively rated to
provide excellent reliability and immunity to high SWR. High-isolation PIN-
diode T-R switching is used to provide silent, no-relays QSK. (Please refer to
the RF Board section for further details.)