(Revised Suptembor 1980)
CC',ViN 77: 125



Coherent cw: Practical Aspects (Part II of a
              two-part series)



           Charles Woodson, W6NEY
                2301 Oak St.
             Berkeley, CA 94708



     In part one I described the nature of coherent digital communication. Coherent cw
requires transmission to be sufficiently coherent that a synchronization of the frames of the
received signal and the receiver filter integration frames can be achieved. This paper will
describe how this has been achieved in practice.



The Transmitter Requirements

     The transmitter requirements are basically two. First, the keying must be done in a regu-
lar coherent fashion within time frames established by a stable frame reference. Namely, all
dots, dashes, and spaces of the transmission must coincide with frames which are highly regu-
lar. Second. the frequency and the stability of the carrier frequency must be within a cycle or
so during the QSO, including during all portions of the keying.



Keying

     First, let's consider the keying. Dots and dashes must occur in established regular frames
which are sufficiently regular that the frame phase can be acquired by the receiver. Such
frames can be determined by a stable frequency standard, with the source signal being divided
by CMOS or TTL logic to produce pulses which define the frame. Many of us use have used
K4EEG standards (1974) although any comparable standard would do. Petit (CCVrN,1976)
describes the TTL methods needed. To keep the frames accurate within 1/20 period, for 10
windows per second, requires an accuracy of 1/20 1/10 1/60 1/60 cycles per hour of QSO.
Since these oscillators are accurate and stable over a QSO period to less than 10-7, they exceed the

accuracy of 10-5 needed for the keying framing. Figure I shows a station standard suitable for supplying the 10 HL keying reference and the ccw filter frame
reference



     Figure 2 shows a simple keyer for coherent keying. Basically, the idea is that a dot can
only occur during a frame, and a dash only during 'three frames, not across parts of frames. If
the key is pressed before a frame beginning, a dot or dash can not begin until the frame begins
and must continue until the corresponding frame ends.

     The Health HD-10 (Woodson, 1975) has been adapted for coherent keying as well as the
accukeyer (Woodson, 1975). The accukeyer is quite superior in this situation because of its
one-bit memory. A keyer monitor is a must. I am using a AKB-I keyboard which is available
with a ccw option. I have also used a KIM-I computer to generate coherent Morse and ASCII.
The computer system uses an interrupt generated by the internal timing clock to determine the
beginning of each frame period. The timing clock frequency must be adjusted precisely.

     In practice, hand sending of Coherent cw is different from ordinary random-frame cw and
takes a while to learn. This is because dots and spaces can only occur in the pre-established
frames and we are accustomed to starting dots and dashes whenever we wish. I found that with
a bit of practice the resulting sending errors go down to near the error rate of ordinary CW.
Apparently the hand sender learns to hold the key down until he hears a dot or dash start, and