AMATEUR RADIO
By GARRY CRATT, VK2YBX
Coherent CW - a new low
power transmission technique
C.Oherent CW is a new transmission technique capable
of providing a tenfold increase in signal readability. It
has been developed over the last 10 years by amateur
experimenters world wide. It is basically a refinement
of the oldest electronic signalling system, Morse code.
To understand how coherent
CW, or CCW (so named by Ray Petit
W6GHM in a 1975 QST article)
works, it is important to first
understand the basic limitations of
cw.
As we know, CW has an advantage over other modes of transmission in that it occupies a narrower
bandwidth. This means that, for a
given transmitter power, the effective radiated power is increased
when using CW as compared to
other transmission modes.
Just as SSB is often quoted as
providing an eightfold increase in
efficiency over AM due to its narrower bandwidth, the whole concept of CCW relies upon the fact
that the narrower the system bandwidth used, the more efficient the
use of the available power. However, this is only half the story. The
timing of the signals is also very
important.
In general, receivers with IF
filters wider than the desired
signals are ineffective because they
allow undesired signals and noise
to be received. At 12 words per
minute, a CW signal occupies about
10Hz, so that an IF filter with a
bandwidth of even a few hundred
hertz does nothing to improve
reception.
Analog high-Q filters at such narrow bandwidths are unobtainable,
and a PLL filter capable of 10Hz
bandwidth would take tens of
seconds to lock. The ideal filter
would provide a bandwidth of 10Hz
and would improve receiver signalto-noise dramatically. In effect, a
1-watt signal received through a
10Hz filter is comparable to a
230-watt signal received through a
2300Hz filter.
Basically, the answer to this
dilemma is to design the receiver to
respond to certain characteristics
so that it can differentiate a signal
from noise. There are three critical
CW
CCW
Fig.1: CCW signals are sent as multiples of a basic time unit.
66
SILICON CHTP
(and interrelated) characteristics:
(a) precise operating frequency; (b)
precise pulse length; and (c)
predetermined pulse sequence .
If some kind of transmitter/
receiver synchronisation was
available between path "ends", it
would be much easier to achieve
such bandwidths (by telling the
receiver filter when to expect a CW
character) . Such a technique could
be equally applied to RTTY, FSK
and other data signalling formats.
Time stations
Locking the receiver and transmitter stations precisely to the
same frequency can be achieved by
making use of HF time and frequency standard broadcasts. There are
a number of these stations around
the globe such as WWV, WWVH
and WWVB in the USA, JJY in
Japan, CHU in Canada, and VNG in
Australia.
A station "master standard" can
be derived from these broadcasts.
The transmitter can use these
signals to gener.ate a pulse of
known width and phase, exactly
synchronised to the standard derived from the HF time signal station.
The receiver samples the received
signal in intervals determined by
its master oscillator (which is phase
locked to the HF time standard) and
is therefore "coherent" with the
transmitter.
By analysing a CW signal, it can
be seen that it is comprised of a
series of units of time which we call
a "frame" (see Fig.1). By using
digital techniques, it is possible to
determine the exact instant that a
frame begins and ends.
z
PADDLE OR
COMPUTER IN
CCW
KEVER
PULSE
PHASE
TRANSMITTER
PULSE
LENGTH
REFERENCE
FREQUENCY
MASTER
FREQUENCY
STANDARD
CCW
FILTER
RECEIVER
REFERENCE
FREQUENCY
MASTER
FREQUENCY
STANDARD
PULSE
PHASE
AUDIO
OUT
PULSE
LENGTH
FILTER
DRIVER
Fig.2: the basic CCW system. The receiving and transmitting stations are
synchronised by signals received from a time and frequency station (eg,
WWVB in the USA or VNG in Australia).
Normally, CW dots, dashes and
spaces begin at random times
depending upon the operator, and
thus the frame length varies to an
unpredictable degree. By sending
CCW so that all dots, dashes and
spaces are multiples of the basic
time unit, the signal will be received within predictable time frames.
Now although this may sound difficult, users report that this technique is usually mastered within a
short time. Naturally, the use of a
keyer or computer to generate
characters makes the job even
easier.
Fig.2 shows the basic CCW
system. In order for CCW stations
to communicate, it is necessary for
both stations to agree in advance on
the operating frequency and the
frame length (normally 0.1 second)
for an operating speed of 12 words
per minute. Since a dot and a space
each require 0.1 seconds, a string
of dots at 12 WPM is a square wave
having a basic frequency of 5Hz.
To establish communications, a
CCW station sends a sequence of
dots , allowing the receiving station
to acquire the signal and synchronise the IF " filter" . Once locked, the filter should remain stable
for hours.
There is great deal more to CCW
than this short article indicates but
it is clear that the technique shows
a lot of potential for amateur experimentation, particularly for long
distance transmissions and EME
(Earth-Moon-Earth) work.
Further reading on CCW
(1). "Coherent CW" ; QST magazine,
May & June 1981.
(2). "What Is Coherent CW? ";
Japanese Ham Radio Journal ,
January 1976.
(3). "Coherent CW : Amateur
Radio's New State of the Art" ; QST
magazine, September 1975 (Ray
Petit).
(4). "Universal Frequency Standard"; Ham Radio magazine ,
February 1974.
(5). The 1989 ARRL Handbook for
Radio Amateurs (page 21-16).
Sangean ATS-803A shortwave receiver
There are a number of receivers available to the
shortwave listener today but most are useless if
you want to listen to CW or SSB transmissions on
the HF bands. This receiver offers continuous
coverage from 150kHz to 30MHz and as a bonus
will pick up the FM broadcast band.
The ATS803A is made in Taiwan
and looks similar to the Sony
ICF-2001 (which is now superseded)
but with a number of refinements
including FM stereo reception via
headphones, variable RF gain control, a tuning knob and a much
larger battery compartment.
The cabinet is well finished in
black plastic, with gold trim on the
tuning knob and brushed aluminium
around the LCD readout. The LCD
readout has 12mm digits and is
very easy to read. Below the
readout is a list of the frequency
ranges for the various shortwave
bands.
Below that again is the control
panel which has 22 buttons. Five of
these select the band: FM (88 to
108MHz), AM, LW (beginning at
150kHz), MW (beginning at 520kHz)
and SW (beginning at 2300kHz).
Tuning can be done in several
ways: (1) you can punch the station
frequency in directly via the
pushbuttons and then hit the "execute" button; (2) you can use the
Start/Stop buttons for scanning upwards from .any frequency; (3) you
can use the up/down buttons or the
tuning knob for manual tuning; or
(4) you can call up one of 14 stored
station frequencies.
RF gain control
For dedicated HF enthusiasts,
the unit has an adjustable RF gain
control, which allows the user to
adjust the receiver sensitivity in
cases where signal overload is a
problem. The provision of a BFO
(beat frequency oscillator) allows
reception of single sideband (SSB)
and CW signals. This feature is of
particular importance for those interested in amateur, aircraft or
marine HF transmissions.
There is also provision for the
connection of a number of accessories, including external antenna, external DC supply (9V at up to
400mA), headphones (stereo for
stereo FM reception, mono for
JU NE 1989
67
