Coherent CW The Practical Aspects Part 2: In Part 1, the concept of ccw was described. You'll now see. how you can put the concept into practice. By Charles Woodson,* W6NEY - Coherent cw operation imposes two basic requirements at the transmitting end. First, the keying must be done within the time frames established by a stable frame. reference. These frames must be sufficiently regular to enable the receiving station to determine accurately when they occur. Second. the carrier frequency must be stable within a hertz or- so during the contact, including all keying periods. The . me frames can be established by a fre- quency standard with the reference signal being divided by CMOS or TTL to pro- duce pulses which define the frame. Many ccw stations use standards such as those described by Kelley,' although any corn- '2301 Oak St., Berkeley, CA 94708 'Notes appear on page 23. parable standard would do. To keep the frames accurate within 1/20 of a period for 10 "windows" per second requires a stability factor of 1/720,000 Hz per hour of contact. Since the standard mentioned is accurate and stable to less than I part in 107 over the rc- quircd period, it exceeds the required ac- curacy easily. A station standard suitable for supplying the 10-Hz keying reference and the ccw filter frame reference is shown in Fig. 7. Keying Fig. 8 shows a simple system that may be used for ccw keying. I have adapted both the Heath-1-113-102 and-the Accu- Kcycr' for ccw operation.. The Accu- Kcycr is superior because of its 1-bit memory. At present, I use an AKI3-l keyboard, which is available with a ccw option. I've also used a KIM-1 computer for generation of ccw and ASCII. The computer uses its internal timing clock to generate an interrupt at the beginning of each frame period; The clock frequency must be adjusted precisely for such use. Hand sending of ccw is different from ordinary random-frame cw and takes a while to learn. This is because dots, dashes and spaces can only occur in prc- established frames and we are accustomed to initiating dots, dashes and spaces whenever we wish. With a bit of practice, the initial sending crrors decrease to near that of the error rate of ordinary cw key- ing. You learn to hold the key down until you hear a dot or dash start and then +5V Ui TOL05 O.? Re& .. 04 2N2222 0 +9-15v IN 4 MM& XTAL. 32 OF. ROOM TEMP@ HIGH STASILITY II, yl ::30 3,0 DECIMAL VALUES OF CAPACITANCE NPO Nt5OO_ ARE IN MICROFARADS ().F). OTHERS ARE 450 IN PICOFARADS (PF OR #,F); RESISTANCE S.M. 4.7 k ARE IN OHMS; k - 1000 2 -lo- 7 --- /7-7 /-, -7 2 k IOjF C if. v Ti@ IOPF 16V 16, I MHz (TTL) S 200 OUTPUT S.M. - SILVER MICA IN PICOFARADS (PF OR #,F); R SISTANCES ARE IN OHMS; k - 1000 Fig. 7 - A 1-MH2: frequency Standard 4or ccw station use you're able to send in rhythm with the frames for a word or phrase. A keying monitor is a must! Transmitter Stability The receiving Filter passband requires that the transmitted frequency be stable during the contact period. This is perhaps the most difficult parameter to be met for ccw operation.. For a cw,;ignal time frame of 0.1 second, a 14-MHz signal must be stable to within I or 2 Hz. High@quality crystal oscillators have such stability ex- ccpt when a varying load is placed upon them, as when a transmitter is keyed. Dur- ing keying, the frequency of a typical transmitter crystal oscillator will shift ap- proximately -50 Hz. Under:ordinary cir- cumstances this wouldn't be noticed, but for a ccw signal, this would mean loss of reception because the shift is more than'-- five times the receiving filter passband and would equate to a 20-kHz shift of a regular cw signal. Such shifting produces an amusing situation. When copying with the ccw filter in the presence of strong in- WFfercrice. . the interfering signals sometimes appear to swish up and down the band during keying. Even if-thcy cross the ccw frequency, the time they arc in the filter passband is small. The result is that they have relatively little effect on the ccw signal itself. However, these interfering signals - through cross-modulation, overloading early receiver stages. and their effect on age - can (and often do) cause problems. Transmitter stability has been achieved by using high-quality crystal oscillators which arc not keyed and which are fol- lowed by several stages of amplificrs and buffers to nullify the loading effects of keying. A schematic diagram of such a transmitter-cxciter is shown in Fig. 9. The power output of this exciter is about 0.1 watt and it has been used by itself (with an antenna matching network and keyer in the final stage) and as, a VI-O replacement. Tests have shown that after a 30-minutc %varm-up period the oscillator is ,table %viihin a hertz during keying and rcrimins so for over an hour. The crystal tuning allows VI:0-typc operation over a 20-1 Iz range. To facilitate stability, very little power is drawn from the oscillator and two stages of isolation are used to minimize the load on the oscillator by later stages. In most situations, particular- ly when the rig is left on all the time. the N1500 compensation capacitor and cor- responding trimmer may be omitted and a fixed capacitance value added in parallel with the rest of the units. When the temperature compensation trinimen; are used. (hey are adjusted while measuring the operating frequency at two different temperatures, say, &S and 86' F (20 and 30' C). One trimmer is adjusted t decrease capacitance and the other to in crease capacitance by a like about(. Th frequency is measured at the tw temperature extremes again and this pro ccss continued until the oscillator frequen cy is the same at both temperatures. Another method of transmitter fre (lucricy stabilization is to use 111.1-s to con- trol (he frequency of oscillators and use highly stable oscillator as a reference fo the I'LL. A dircct-convcrsion receiver employing this technique was described by McCaskcy.' Maynard used a 5.0- to 5.5-MHz synthesizer output and a 9-M H/ frequency standard to control an HW-8.' I have used a method which mixes the HFO, 111-0 and VFO frequencies of a doublc-convcrsion transceiver (SI3- 303/Sit-401 combination). locking the result by controlling the Vl:0 frcqticj)cy.'- A simple scheme (shown in Vig. 10) is used for lockine the VI:( 0.MO) of an S13-303 receiver by using the built-in %ariable capacitive diode circuit employed for Rk operation. A high-inipcdance voltmeter connected to point C can be used to monitor the lock condition. During opera- tion, the VI:O is tuned slowly across the frequency of the standard; frequency lock occurs about 250 Hz above and below the reference frequency. Once locked,_ the crystal oscillator controls (he receiver fre- quency and- it can be set more accurately than the VI:O. The crystal oscillator can he replaced by a 5.0- to 5.5-MHz syn- thesizer which is controlled by a suitable reference frequency; Petit has designed such a synthesizer which operates in INW17 steps.' A block diagram of the transmitter cur- rcntly in use at my station is shown in Fig. I 1. The 12.9-Niliz crystal oscillator is designedfor high stability. Similar oscillatorsare used for operation on 21 and 28 MHz.The synthLsizcr is controlled by a I-MHzoscillator similar to that described in Fig. 7. The two oscillators run continuously and are connected to the doubly balanced mixer, but the 14-MHz stage following the mixer is keyed. This allows break-in operation on the same fre- qucncy. Receiver Requirements In addition to the ccw filter, the receiver must exhibit stability on the order of I Hz over the length of a contact and have a tuning resetability which is less than the bandwidth of the filter. Searching for a signal while using a filter bandwidth of only 10 Hz requires almost 200 times as long as it takes to tune a band using a filter with a bandwidth of 2.1 kHz. If the phase and frame size were also unknown, it would take over I(XX) times as long to tune a band searching for a ccw signal as it takes to look for an ordinary cw signal. That is why current practice involves agreeing on a precise frequency and frame length in advance. Adequate stability is easy to obtain with good crystal oscillators in receivers when temperature has been stabilized by a long warm-up period and a stable environment exists. Fig. 12 is a block diagram of the receiver currently in use at my station. Rough tuning is done by adjusting the hi' crystal oscillator and the BFO, which have ranges of about 800 Hz, to the desired fre- quency. The VI:O of the ccw filter center frequency reference (four times the center frequency) is used for fine tuning over a range of about 25 Hz. An i4strip similar to one designed by Hayward' provides performance ,upcnor to others I have used. Best results are obtained when the age is controlled by the agc output of the ccw filter. Kcitaro Sekine, JAIBLV. uses a crystal-controlled FF-901 and also has built a 2980- to 3080-H7 RC VFO for use as the reference for the center frequency of the cc%%, filter. Oscillators in the transceiver have been stabilized bv using temperature compensation methods and high-stability crystals. The Fiker A practical coherent digital Filter may be seen in Figs. 13, 14 and 15. The first CD4060A6 is used as a switching mixer while the second controls the sample and dump functions. An audio signal output may be derived from a digital mixer (such as shown in Fig. 14) driven by the output from the two channels. The signal is the difference between the two and can be made singlc-cndcd by using an op arrip, or both channels may be fed to A/D con- vcrters for computer input. A frarne reference for the ccw filter is shown in Fig. 15. A Microprocessor-Con(rolled FiRer The logic diagram of Fig. 16 is that of the computerized system which has been used at my station. The switching mixers are essentially the same as those used in the filter described previously. A com- puter program controls the A/D convcr- sion and dump functions. Computer con- trol of the mixer has been employed, but use of an opcra(or-controlled VFO is a convenience. The 1-tvlHz internal clock is stabilized and used to define the ccw frames. Phase is adjusted by the computer program. This is done by operator com- niand. The operator indicates an advance- inent or retardation of the framing phase in 10 ms increments by pressing a corn- puter key. I have experimented with a computer program to adjust framing phase automatically. but have not yet found a satisfactory way to maintain framing phase lock during breaks in the QSO caused by QRIM or pauses. Between control of' the sample and dump func- tions. the computer also converts the received Morse signals to ASCII code and transfers the ASCII code to a CRT character display terminal or printer. Weak Signals and Noise The reception of weak ccw signals is quite different from that of ordinary weak cw signals. Under standard conditions, as )e cw signal gets weaker, QRN or QRM -inain as "no signal" output and we eventually end up with a noise level depen- (lent upon the bandwidth of the filter. With ccw, noisc is a series of "dots" in frames and varies randomly in intensity. With the ccw Filter, output is limited by de%ign to one frequency and a weak signal is characterized by missing and extra do randomly mixed with the desired signal Frame phase adjustment is importa because if it is not accurate, a blurring the dots and dashes into adjacent fram occurs. This makes the signal unreadab and it might go unnoticed if it is wea When receiving a series of dots (a sta dard part of a ccw CQ). you can tune maximum contrast between dots an spaces. With a strong signal, even a 10 phase error can be noticed. A slight lea error causes a weak mark just before cac (lot or dash while a lag error results in weak mark just after the dot or dash. Operating Practices Under favorable conditions, it is ofte convenient to operate the ccw filter shorter than optimal frame periods. Wit 0.01-sccond frames, the bandwidth i around 100 Hz and phase adjustnicn makes little difference. Although selective ty is reduced and signal level decreased b 10 dB, this method is used during initia signal detection. Once a signal is located phase adjustment and longer frarn periods may be used to optimize rccep tion. Phase tuning may be used instead o tuning a band of frequencies. This is ac complishcd by using an agreed-on fram length and frequency of operation an tuning for proper phase by adjusting the filter phase. Once phase adjustment is close, the frequency may be fine tuned as well. Present practice calls for sending a 15-second strcam of dots to help in frame acquisition. A steadycarrierof 10scconds duration is an aid when finc tuning to fre- quency. Time-refercrice signals from stations such as WWV may also be used to adjust the keying and reference frames of ccw receiving filters. Such adjustment must take into account the electromagnetic distance of the standards station to the receiving station as well as the clLctro- magnetic distance between coni- municating stations. This procedure allows phase to be fixed and the operating to be the primary parameter which must be considered. Communication between stations located in Japan and California has been successfully accomplished using this technique. It is, however. a more dif- ficult procedure to follow than phase tuning. Conclusions Ccw offers the possibility of employing some interesting operating techniques. Suppose Amateur Radio stations of the world agreed to operate at frequency multiples of 10 Hz. This would provide 20.000channels at the bottom 200 kHz of' a band. Ifoperators further agreed on sending in frames synchronized to 0.1-second UTC time pulses, you could set the framing (about a 0.03 second delay) to correspond to the distance of the stations you wish to contact, say 6200 mi (10.0M kin). Once this is set, a check of the channels may be made for a station at the desired distance. Generally, you could detect signals at distances of 5000 to 7500 mi (8000 to 12,000 km) without further adjustment. Imagine microprocessor con- trol over the entire procedure and the automatic detection of stations a par- ticular distance away! Coherent cw is a useful technique which improves communications effectiveness in excess of 20 dB. This factor can be used to offset poor propagation conditions, small or poorly located antennas, or low-power wideband signals. The transmitter design, which should be well within amateur capability, amounts to taking care in broadbanding the RF stages after modula- tion to maintain amplitude linearity, and in keeping the antenna system SWR verN 10%%. Receivers must not only have wide- band front ends, but must also have good dyna.mic range and linearity to handle both the desired signal and any interference. NVhere an IF is used, the frequency chosen must be higher than for conventional transceivers. In practice, 70 MHz is a com- mon SS IF. Components (such as filters) are available for this frequency to build SS IF modems (modulator/demodulators). ,kmateur SS Experimentation In 1980, the FCC Office of Science and Technology (OST) suggested that radio amateurs experiment with spread-spectrum modulation techniques. The rationale was that (a) the civil radio services could take advantage of the spread-spectrum pio- neering of the military, (b) design of spread-spectrum systems by the private sec- tor was slow because of the high cost of developments vs. return on investment. (c) more experimentation was needed in areas such as designing for low-cost and on-the- air testing in congested frequency bands, and (d) radio amateurs could perform useful experiments without the need for either governmental or industrial research and development money. The Amateur Radio Research and Development Corporation (AMRAD) re- quested, and the FCC granted, a Special Temporary Authority (STA) to permit spread-spectrum tests in the Amateur Radio bands by a small number of amateurs, for one year beginning March 6, 1981. Under the STA, the first Amateur Radio SS tests were conducted by W4RI in McLean, Virginia and K2SZE in Rcchester, New York. Later, WA3ZXN\'in -A.nnapolis, Maryland ran additional on- the-air tests with K2SZE. The equipment was capable of hopping over a frequenc% range up to 100 kHz at rates of 1, 2, ; an 10 hops per second. RF power output level Of 100 and 500 watts were used into dipol antennas. These particular radios functioned best at 5 hops per second. This was subjeci% el% judeed on the basis of least-bothersome in- terterence from the various sienals a@ the different hopping frequencies. It wa@ observed that frequency hopping was more successful in the presence of heavy CW in- terference than it was in the presence of heavy SSB interference. In comparison. conventional SSB usually provided better communications than frequency-hoppe SSE! whenever a single clear channel coui @e found for the conventional 5SB HOII;ever, the conventional SSB coulci 1@ dksrupted by strong interference or. tha charmer. W'hile hampered by, cycl;@ in le',;erence \%hen busy frequencies %%er @'elisited, the frequency-hopped link @oul be maintained despite band congestion. Although the tests were announced beforehand in Amateur Radio publications and on the air from WIAW, no cor- respondence was received indicating that the frequency-hopping tests either in- terfered with, or were heard by, other Amateur Radio stations. The only ex- ception was that several amateurs in the Northern Virginia area could recognize the presence of the frequency-hopped trans- missions on conventional SSI3 receivers after learning what the signal sounded like. All were within 5 miles of W4R1 and were able to hear both ends. AMR.AD has plans for additional FH ex- periments in the VHF amateur bands and has applied to the FCC for a second STA. The coordinator for these experiments is Hal Feinstein, WB3KDU. Others involved are WB4APR, WB4JFI, WB5MNIB and K8MMO. They have developed interface circuits to permit synthesized hand-held transceivers to be computer controlled. I n- itial discussions with the FCC concerning the second STA by Feinstein -,and Perry Williams, WIUED, ARRL Washington Coordinator, revealed that the FCC Field Operations Bureau is interested in con- ducting monitoring and direction-finding exercises against these SS transmissions. The FCC has adopted a Notice of In- quiry and Proposed Rulemaking (Docket 81-414) tha, would amend the rules to per- mit SS operation on the amateur VHF bands. The FCC proposed tha, Aniateu Extra and Advanced class licensees be per mitted to use SS for domestic communi cations provided that only prescribe pseudorandom sequences be employed. A of mid 1984, the Commission has not take final action on this docket. Selected SS Bibliography Reading material on spread spectrum may be difficult to obtain for the average amateur. Below arc references that can be mail ordered. Spread-spectrum papers have also been pubikhed in IEEE Transactions on Communications, on Aerospace and Electronic Systems and on Vehicular Technology. Dixon, Spread Spectrum S@vstems 1976, Wiley Interscience, 605 Third Ave., New York, NY 10016, $29.50. Dixon, Spread Spectrum Techniques, IEEE Ser- vice Center, 445 Hoes La., Piscataway, NJ 08854, IEEE member prices S19.95 cloth- bound, $12.95 paperbound; nonmembers $29.95 clothbound. In addition, the following articles on SS have appeared in Amateur Radio publications: Feinstein, "Spread Spectrum - a report from AMRAD," 73, November 1981. Feinstein, "Spread Spectrum" column, AMRAD Newsletter, April, June, July , August, October, November and December 198 1; January, April, May and October 1982; and June 1983. Back issues are available for $1 each from AMRAD, P.O. Drawer 6148, McLean, VA 22106. Feinstein, "Amateur Spread Spectrum Ex- periments," C`Q, July 1982. Rinaldo, "Spread Spectrum and the Radi Amateur," QST, November 1980. Sabin, "Spread Spectrum Applications in Amateur Radio," QST, July 1983. COHERENT CW While spectrum management has re- ceived much attention in the recent Amateur Radio literature, the problems and possibilities of "more QSOs per kilohertz" were first recognized more than half a century ago. The late Frederick Emmons Terman, 6FT, presented his vision of narrow-band communications in "Some Possibilities of Intelligence Trans- mission When Using a Limited Band of Frequencies," published in Proceedings of the Institute of Radio Engineers. January 1930. As ear ly as 1927, the Bell Telephone Company had reported successful experi- nients with 200-WPM Baudot TTY corn- munications in a 50-Hz bandwidth over undersea cables. The bandwidth reduction resulted from synchronization of the transmitter and receiver. Technology made giant leaps in the next 45 years. In September 1975 QST, Raymond Petit, W6GHM, described the experiments of some radio amateurs with a mode he called "coherent CW.- Petit did no( acknowledge Terman's paper, so we must conclude that he rediscovered the wheel. In any case, CCW is an idea whose time has come. Adrian Weiss, WORSP, disclosed some of the technical details of the CCW system in June and July 1977 CQ. The presentation contains some errors, but the astute reader will be able to recog- nize the significant principles. The bandwidth required for transmitting a radiotelegraph signal is directly propor- tional to the keying rate. For a speed of 12 WPM the unit pulse length is 0.1 second. Since a dot and a space each require 0. I sec- ond, a string of dots at 12 WPM is a square wave having a fundamental frequency of 5 Hz. To preserve the square-wave char- acteristic of the emission, an SSB trans- mission bandwidth of at least 15 Hz is re- quired. A baseband (or dc wire telegraph) receiver needs a similar bandwidth for con- ventional information recovery. Terman reported that with synchronization techni- ques, the receiver bandwidth could be reduced to 1.5 or 2.0 times the keying rate. In conventional (Morse) radiotelegraphy, the intelligence is ultimately received as an audio tone. Even a 15-Hz bandwidth filter centered on, say, 500 Hz, would require a Q of 33, causing intolerable ringing. The ringing problem can be overcome with time-domain processing at both ends of the communications path. The trans- mitter is stabilized to within I Hz of the proper frequency by phase-locking to a reference standard. Precisely timed keying pulses are derived from the same reference standard. A similar reference standard stabilizes the receiver frequency and syn- chronizes the audio output filter. The re- ceiver output is sampled at twice the keying Special Modulation Techniques 21-9 frequency. A block diagram of a CCW communications link is given in Fig. 15. In- creased frequency stability and accuracy can be achieved through phase-locking both reference generators to a standard fre- quency broadcast station. A good signal for this purpose is broadcast on 60 kHz from WWVB. Fig. 16 shows the elements of the audio output Filter in more detail. A combination of digital and analog techniques produces a 3-dB bandwidth of 9 Hz, which is within the range predicted by Terman. When the receiver is properly tuned, the filter input signal frequency is I kHz. Since this fre- quency is "zero beat" with the reference (LO) signal, the mixer output is a dc voltage proportional to the cosine of the phase angle between the input and reference signals. When the signals are phase- coincident the mixer produces a maximum positive voltage. The mixer output voltage swings negative when the input and reference signals are 180' out of phase. A 90' relationship results in zero output voltage. The actual hardware uses square waves for mixer LO injection, so the phase detection transfer characteristic is linear rather than sinusoidal, but the minimum and maxi- mum voltage occur at the same points. The phase of the input signal varies randomly with respect to the reference, even though the reference enerators at each end of the communications link may be locked to the same standard frequency transmission. This variation results from changing propa- gation conditions. Phase variations of the input signal have little effect on the timing of the sampling window because the sampling rate is only one-hundredth of the signal frequency. The sampling window position is adjustable, in any case. To prevent loss of output voltage when the input/reference phase relationship swings through 90', two signal-channels are driven in phase quadrature by the reference generator. Thus, if the input signal is shifted 90, from the reference signal applied to one mixer, that mixer out- put will be zero, but the other mixer will see a O' or 180' relationship. The two chan- nels are summed at the filter output, so the output amplitude is independent of the in- put phase, provided the frequency is zero- beat. The voltage from the mixer is integrated over a 0. I -second period. Near the end of this interval, the timing logic causes the sample-and-hold circuit to acquire the in- tegrated output voltage. One millisecond later, the logic resets the integrator and the cycle repeats. The sample-and-hold voltage controls the amplitude of the reference signal passed by the output modulator. Fig. 16 shows a separate phase-shift net- work at the output modulator for clarity, but the input network can serve both cir- cuits. The square-wave outputs are in pha,;, quadrature. The combined Output wave. form is a staircase that can be filtered into a sine wave with relative ease. The timing signals are derived from the I-kHz reference, which is synthesized from the master frequency standard used to stabilize the receiver LO. A decade counter with a decimal decoder produces 10 out. puts, each having the duration of one in. put pulse and a frequency of one-tenth of the input clock. Output one goes high during the first clock pulse, output two goes high during the second clock Pulse, and so on. Every second pulse of a se- quence of 10 commands the sample-and- hold circuit to sample, and every fourth pulse resets the integrator. A second decade counter/decoder is cascaded with the tim- ing control, and its phase-adjustable out- put gates the timing signals to select a 0. I -second integrating window that is syn- chronized with the incoming keying pulses. Because the signal is sampled at the end of the integration interval, the filter output is delayed 0. I second with respect to the input. Fig. 17 shows the approximate amplitude-versus-frequency response of the filter. Note the symmetry of the skirts. Unlike analog filters composed of linear circuit elements, the sampling filter does not exhibit arithmetic selectivity. The spurious responses on either side of the main passband resemble the infinite re- jection notches characteristic of an elliptic filter and are called aliases. The frequency response is quasi-periodic because signals that are not zero-beat with the reference frequency produce a difference frequency signal from the input mixer. If the input signal is 10 Hz away from the reference, the mixer output will be a IO-Hz ac signal. At the end of the 0.1-second integrating period, the mixer output waveform will have completed one cycle. Assuming the cycle started at zero volts, the sample-and- hold will acquire the integrated mixer voltage at the zero crossing and instruct the output modulator to pass zero reference signal. (This assumption isn't necessarily valid for a single channel, but it holds for the resultant of the quadrature channels.) Any whole number of beat-frequency cycles will cause the sampled voltage to be zero. Since the sampling interval is 0. I second, the response nulls occur every IO Hz away from the peak. If the input and reference signals differ by a multiple of 5 Hz, the mixer voltage is sampled at the peak of a half cycle, causing an alias. The aliases diminish 6 dB every time the beat frequency doubles because the integrator is a first-order low-pass filter having a 6-dB-per-octave rolloff. Noise bursts and strong adjacent-channel signals result in an occasional extra dot or an elongated dash, but are otherwise un- noticed. At the 12-WPM keying speed used by CCW experimenters, a signal-to-noise ratio improvement of about 20 dB can be realized over the bandwidths typically used for CW. Faster speeds are possible, but the bandwidth must be increased at the expense of signal-to-noise ratio. To establish CCW contact, one station sends a preamble of dots to allow the receiving operator to synchronize his filter. Experience thus far indicates that once the filter has been synchronized, it usually won't need adjustment for several hours. Fig. 18 depicts a typical CCW station. The early experimenters built their stations around simple QRP equipment to dramatize the communications advantages offered by the mode and to emphasize the accessibility of the necessary technology. The simple gear requires some add-on cir- cuitry to allow oscillator stabilization. The more modern synthesized tran- sceivers can be outfitted for CCW more easily - replacing the internal reference oscillator with an external standard is all '21'1-2 Chapter 21 Fig. 18 - This is the first complete amateur station to be built for coherent CW operation. Assembled by Andy McCaskey, WA7ZVC, it consists of a modified Ten-Tec PM-2 transceiver and homemade modules that pro- vide for the control and processing of signals as required for coherent CW operation. that's required. To send CCW, the paddle- actuated clock in the keyer must be re- placed by a continuous pulse train from frequency standard. Coordinating on paddle movements with the "metrono requires a different keying technique. buffered keyboard (controlled by the st dard) is the ideal CCW sending instrunte When more stations have CCW ca bility, the mode may prove highly use for emergency communications. Ancit possibility for CCW is in EME work.