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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