Midnight  Coherent CW Transceiver

CCW paper delivered
   at FDIM 2023

CCW slide deck presented
   at FDIM 2023

Schematic 1: Processor & I/O

Schematic 2: RF Deck

CCW Interview with N2APB

Coherent CW Archives

“How Low Can You Go?”
The Design of a Modern Coherent CW Transceiver

For Weak Signal Communications

Figure 1:  Prototype CCW 'engine' on red pcb shield mounted atop K1SWL PicoMite Memory Keyer

Overview

Imagine how exciting it would be to copy a solid 599 signal when you hear nothing but the noise floor. This project chronicles the design of such a transceiver using an inexpensive Raspberry Pi Pico controller to achieve 20- 30 dB signal path improvement over conventional CW rigs. Implementing techniques born almost 50 years ago one can turn a 5-watt QRP signal into the equivalent of more than 300 watts on the receive side with properly equipped stations.

Coherent CW: A Quick Technical Backgrounder

An age-old axiom is popularized in detective stories: “The more we know about something we seek the easier it is to find.” This can also apply very well to weak signal reception of Morse code. The things we need to know for schedules of course include the date and time, but equally important are three factors enabling coherent CW reception: the frequency to be used, the timing (wpm) of the CW signals being received, and the phasing of the signals or knowing precisely when to start looking for a dot or a dash. To paraphrase an aspect of the Shannon-Hartley1 communications theorem, when we fit the narrowest bandwidth filter to the given information rate of a channel, the signal-to-noise ratio (SNR) skyrockets, which is a good thing because then only the ‘marks’ in the channel (i.e., the dits and dahs) are coming through while both the man-made band noise (QRM) and nature-made band noise (QRN) are vastly suppressed.

In pseudo-technical terms, by knowing where to look, when to look and specifically how long to be looking - all with great stability and accuracy - we can create an extremely narrow (10 Hz!) bandpass filter for the Morse signals to yield a high SNR allowing the transmission to be easily decoded by human ear. Heretofore, if one were to try creating this extremely tight bandpass filter by using traditional techniques of analog filters or even digital/DSP filters, severe ringing and other artifacts would result, thus creating a very unpleasant and totally unusable listening experience.

The narrow 10 Hz CCW filter shown in the center is comparatively miniscule compared to traditional CW and SSB filters.
One can envision the amount of QRM and QRN eliminated when using the CCW filter,
and the correspondingly high SNR for the precisely tuned 800 Hz CCW signal being passed.

For additional detail ...

Please read the paper "Modern CCW" paper, slide deck and interview that I provided at the FDIM 2023 QRP Conference.

Research for CCW encoding and detection algorithm improvement is continuing with David Kazdan AD8Y, professor at Case Western Reserve University, Dave Benson K1SWL, and Peter Eaton WB9FLW.

It is my hope and desire that our work may result in a low-cost Coherent CW transceiver design that can be productized to serve a useful role in the weak signal CW communications field for Amateur Radio Operators worldwide.

George Heron N2APB