shaping is essential in today's crowded communications spectrum.
Spectral masks are precisely defined in order to prevent "spreading"
beyond the allocated frequency ranges at defined power levels. Whenever
anything other than a continuous sinewave is being broadcast, there
is spectral content generated in addition to the fundamental frequency.
Fourier transform of the waveform reveals which frequencies
at what power levels comprise the waveform. The CW (continuous wave)
signal used by Morse code operators is a pure sinewave (or nearly
so), but there is a spectral problem with it every time the signal
turns on or off because of the square-ish edges involved during
switching. RC networks are used in the transmitter circuits to tame
the edges so that they do not turn on and off so quickly and in
doing so reduce the extraneous frequency content. Author George
Grammer argues that even though the signal could theoretically be
made 'clickless' (aka "chirpless'), there is an auditory benefit
to the clicks or chirps that aids operators listening to high speed
October 1966 QST
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present. All copyrights hereby acknowledged.
See also: "What
Causes Clicks?' by W8JI (thanks
to Kevin A. for the link)
Why Key Clicks?
The Necessary Bandwidth for C. W. Signals
George Grammer,* W1DF
clicks necessary? There are those who contend that they are, the
argument being that at high code speeds "soft" dots and dashes become
unreadable. The issue is clouded by personal preferences as to how
a keyed signal should sound, just as there are personal preferences
about voice "quality". Putting aside such subjective factors, the
question" Are key clicks necessary?" can be rephrased: "How much
bandwidth is necessary for good code transmission?"
is a long-standing answer to this last question. It is to be found
in the international regulations, where the necessary bandwidth
is specified as the keying speed in bauds multiplied by a factor
which is 3 for circuits where the signals are steady, and 5 for
circuits where fading is bad. To see how this specification affects
amateur practice it is necessary first to review a few fundamental
Fig. 1 - Upper: A code element is the length of the shortest
pulse-a dot or space in International Morse Code. Lower: A succession
of alternating dots and spaces considered as an a.c. square
wave superimposed on the average value of current or voltage.
The fundamental sine-wave frequency for such a square wove also
Fig. 2 - A d.c. circuit which would generate the square waves
shown in Fig. 1.
Fig. 3 - A shaped dot and its relationship to closing and opening
The building block of
telegraph transmission is the code element, the time duration of
the shortest keying pulse. In International Morse code the shortest
pulse is one dot. Since, by definition, the space length is equal
to one dot length, a space is also a code element. This is shown
in Fig. 1, where the top drawing could represent a d.c. circuit
being keyed in a string of on-off dots and spaces. Such a circuit
is shown in Fig. 2. If the string of dots is continuous and fast
enough to let the meter's pointer settle down at an average value
of current, the meter will read just half what it would with the
key closed. This is because the current is off just the same length
of time as it is on. We can look at this continuously keyed circuit,
therefore, as one in which the keyed signal is alternating about
an average direct current equal to the meter reading.
we have an a.c. square wave superimposed on the average d.c. One
cycle of this square keying wave runs from the beginning of a dot
through the following space to the beginning of the next dot. This
is shown in the lower drawing in Fig. 1. Obviously, one cycle of
the keying wave is equal to two code elements.
waveform, of whatever shape, can be reproduced by a collection of
sine waves in harmonic relationship to a lowest frequency which
is the same as the basic repetition rate of the waveform under consideration.
This" fundamental" sine wave is also sketched in Fig. 1. If we are
sending 25 dots per second, for example, the fundamental keying
frequency is 25 cycles per second. By adding the proper harmonics
to the fundamental, the actual square-wave shape can be approached
as closely as we like. Getting those square corners, though, takes
very high-order harmonics - harmonics whose frequencies may be many
times the fundamental frequency. This means that the circuit bandwidth
has to be large compared with the fundamental keying frequency if
square-wave keying is to be closely approximated.
it isn't necessary to use anything like a real square wave for good
keying. It has long been recognized that a keying waveshape which
contains only the third harmonic of the fundamental is quite sufficient
for good copy. This is the reason for the factor 3 in the regulations.
On this basis, a 25-cycle fundamental would take only a 75-cycle
bandwidth. It is also recognized that when the signal-to-noise ratio
is poor a somewhat sharper keying wave is needed; this explains
the factor 5, meaning that the fifth harmonic of the fundamental
keying frequency is transmitted.
C.W. keying can be clickless - without signal deterioration
at any sending speed an amateur will use.
Transmission speed is ordinarily expressed in bands rather than
in cycles per second. A baud is one keying element per second; therefore
one cycle per second is equal to two bauds. In International Morse
a dash is three code elements long, but since a dot or dash has
to be followed by at least one space, a dot is considered to consist
of two code elements and a dash to have a total of four. Thus
One dot = 2 code elements
One dash =·4 code elements
between letters = 1 code element
between words = 2 code elements
The letter C,
for example, consists of
Dash - 4 code elements
Dot - 2
Dash - 4 code elements
Dot - 2 code elements
Space - 1 code element
making a total of 13 code elements.
If it is sent in exactly one second, the speed is 13 bauds, and
the fundamental keying frequency is therefore 6.5 cycles per second.
method of measuring keying speed is exact, while "words per minute"
is rather nebulous. The w.p.m. figure is dependent on the selection
of words of average length; several such selections have been made,
and the resultant w.p.m.-per-baud factor varies from a shade over
1 to about 1.2. Thus a keying speed of 25 bauds can be interpreted
as something between 25 and 30 w.p.m, More to the point, a speed
of 50 bauds is about as fast as any amateur will go with hand keying,
so our opening question boils down to this: What bandwidth is necessary
for a speed of 50 bauds - that is, 50 to 60 w.p.m.?
reasonable to assume that no one would attempt such a speed, unless
signal were good. Under such conditions the international regulations
say that the necessary bandwidth is 3 X 50, or 150 cycle s. This
is small enough to be contained easily within the passband of the
narrowest c.w. filters used in today's receivers.
it should be noted that the fundamental frequency is 25 cycles when
the speed is 50 bands, so transmitting the third harmonic along
with the fundamental calls for a keying bandwidth of only 75 cycles.
The extra factor of 2, above, comes in because when the keying wave,
which is modulation just as much as voice, is applied to a radio-frequency
carrier two sets of sidebands are generated. Thus the radio-frequency
bandwidth is twice the keying bandwidth.
Fig. 4-A (left) - Shaped dot generated
at a 46-baud rate with approximately 5·millisecond rise and decay
times. Vertical lines are from a 1000-cycle signal applied to the
Z axis for timing.
Fig. 4-B (right) - The corresponding
frequency spectrum as shown by a Panoramic analyzer. Distance between
vertical lines is 50 cycles, for a total bandwidth of 500 cycles
for the entire picture. Decibel scale at the left is with reference
to the key-down signal amplitude which was set at 0 db. in this
and the spectrum plots of Fig. 6. The fundamental frequency components
are 23 cycles on either side of the carrier frequency, which appears
slightly to the left of the vertical zero axis. Note that the odd
harmonics of 23 cycles are predominant, the even harmonics being
relatively small. The 3rd harmonics are 20 db. down and the 5th
harmonics are about 28 db. down. Higher-order harmonics are practically
negligible. With 7-ms. rise and decay times the 5th harmonics are
down 30 db.
we have been discussing so far is the necessary bandwidth for a
very special case - an interminable string of dots and spaces of
equal length. Actual code transmission consists of dots, dashes,
and spaces - the latter of various lengths - and since whatever
shaping is used will be applied to the beginnings and ends of dots
and dashes alike, it is more appropriate to talk about the rise
time at the beginning of each pulse and the decay time at the end.
Ideally these two times would be equal. Practically, they are seldom
so, although they can be made approximately the same by careful
adjustment of the shaping circuits. Also, the shapes of the rise
and fall of amplitude differ when practical shaping methods are
There is a useful approximate formula which
states that the bandwidth of a pulse is equal to 1 divided by twice
its rise or decay time, whichever is smaller.1 The rise
(or decay) time is defined as the time required for the pulse to
go from 10 percent to 90 percent of its maximum amplitude. For a
75-cycle bandwidth this formula gives 6.7 milliseconds as the rise
or decay time. Alternatively, we may consider that we have a 200-cycle
i.f. passband available in the sharpest receiver, and for such a
bandwidth find that the formula gives a rise or decay time of 5
In other words, a rise or decay time of 5
to 7 milliseconds is short enough for the fastest hand keying speeds
and a signal so shaped occupies no more bandwidth than can be handled
by the sharpest receiving filter. Furthermore, careful listening
tests show that a keyed signal using these rise and decay times
has no clicks. The transition from key open to key closed, while
difficult to describe accurately in words, is a moderately firm
thud which does not have any resemblance to the sharp sound that
distinguishes an unmistakable click.
1 Ref Reference Data for Radio
Engineers, International Telephone & Telegraph Co., New York;
fourth edition, p. 542.
Fig. 5 - Setup for obtaining the scope patterns shown in Figs.
4A and 6A. The pickup unit and tuned scope coupler can be made
as described in QST for October 1964 (also in Single Sideband
for the Radio Amateur, Fourth Edition, p. 196).
Fig. 6-A - Dot with no intentional shaping; conditions otherwise
the same as in Fig. 4. There is a finite decay time inherent
in the keying system, but the rise time is quite short.
Fig. 6-B - Corresponding frequency spectrum over a 500-cycle
bandwidth; carrier frequency slightly to the right of the vertical
Fig. 6-C - Same as B, but with the carrier set at -0.4 to show
outlying components not visible in B. Bandwidth to the right
of the carrier is 450 cycles. Note that the odd-harmonic components
have not dropped to -40 db. in this range.
Fig. 7 -Scope photograph of a received signal having essentially
no shaping. The spike at the leading edge is typical of poor
power-supply regulation, as is also the immediately-following
dip and rise in amplitude. The clicks were quite pronounced.
This pattern is typical of many observed signals, although not
by any means a worst case. The signal was taken from the receiver's
amplifier (before detection) using a hand-operated sweep circuit
to reduce the sweep time to the order of one second.
At a speed of 50 bauds one code
element occupies 20 milliseconds (1 sec. divided by 50). Fig. 3
shows, in a somewhat idealized way, the effect of shaping with 5-ms.
rise and decay times. In this drawing it is assumed that the output
rises to 10 percent of its maximum amplitude 1 ms. after closing
the key, and decays to 90 percent 1 ms. after opening the key. The
effect of shaping is to lengthen the dot duration, overall, but
to shorten the time during which the amplitude is maximum. This
immediately poses another question: What is the effective length
of such a dot?
An ultraconservative viewpoint would be that
the dot length is the time during which the amplitude is within
1 decibel of maximum. This is approximately the time between the
90-percent amplitude points. The keying shape shown in the drawing
would have a dot length of 15 ms. (A to B) and a space length of
25 ms. (B to C) on this basis. A more realistic assumption would
be that a 3-db. drop would establish the dot and space times, in
which case the dot length is 18 ms. and the space length 22 ms.
In this drawing the dot and space lengths reach equality when the
amplitude is down 6 db.
Since reception is by ear and not
by machine, the question of the effective dot length cannot be resolved
with complete objectivity. There appears to be no actual problem
in recognizing the dots as separate entities with shaping of this
general order. If they seem light to some and heavy to others, it
is easy to change the keying weight slightly so the dwell time differs
somewhat from the space time. Or the receiving operator can readily
apply audio clipping to a dot that seems short; 6 db. of clipping
would make the dot and space times equal in this example. Clipping
also shortens the rise and decay times and makes the keyed signal
sound "harder" - which some like.
Neither of these measures
increases the keying bandwidth. The operators at both ends of the
circuit have a great deal of control - control that does not increase
the interference to stations trying to operate on nearby frequencies.
not all, shaping systems in amateur c.w, transmitters use the discharge
of a capacitor to slow down the break end of a code character. The
waveshape of the decay is superficially exponential, resembling
the discharge of a capacitor through a simple resistance but is
considerably modified by the circuit conditions. However, the general
effect is that the transmitter output decay rapidly at first and
then tails off more and more slowly.
This curve is inverted
on the make end of the character, rising rapidly at first and then
slowly approaching the maximum amplitude. The critical points in
both shapes are the starting points, where the change from off to
on, or from on to off, begins. With truly exponential curves this
sudden transition from "nothing " to "something" on make would result
in a long string of harmonics - i.e., a wide band would be generated.
Fortunately, tube characteristics tend to eliminate the sharp corners
on both make and break.
A typical dot waveshape with blocked-grid
keying is shown in Fig. 4A, where the rise and decay times have
been adjusted for approximately 5 ms. at a keying rate of 46 bauds,
the highest speed of the electronic keyer used. The corresponding
frequency spectrum is shown in Fig. 4B. (If anyone doubts that a
keyed signal consists of a carrier and sidebands this picture should
settle the question.)
The vertical lines in the scope pattern,
A, are the peaks of a 1000-cycle liming wave applied to the intensity
or Z axis of the scope. The setup for making patterns of this type
is shown in Fig. 5, and can easily be duplicated by anyone having
an electronic keyer, a general-purpose oscilloscope, and a 1000-cycle
oscillator having a reasonably pure waveform. The vertical lines
mark 1-millisecond intervals. Timing is essential with oscilloscopes
of the type ordinarily found in amateur stations, since the "linear"
sweep is usually not very linear at the 20- to 25-cycle sweep rate
required for showing just one dot and its accompanying space at
a 40- to 50-baud rate.
Fig. 6A shows a dot at the same speed
as Fig. 4, but with no shaping, and Fig. 6B is the corresponding
frequency spectrum. This is a "hard" signal on both make and break,
although it should be noted that because it is a good square wave,
particularly on the make side, it is less clicky than many signals
that can be heard at almost any time on any band where c.w. operation
is going on. Fig. 7 is a typical example of a clicky signal recorded
off the air. Power-supply regulation accounts for the large spike
on make. The immediately-following undulation in amplitude is caused
by the power-supply choke; an appreciable length of time is required
for the output current to build up through it after the initial
"bump" has been supplied by stored-up energy in the filter capacitor.
Checking With a Receiver
a setup such as Fig. 5 is useful and instructive, it takes no elaborate
monitoring equipment to arrive at a satisfactory adjustment of keying
waveshape. Your receiver will tell you everything you need to know,
provided you use it properly.
The transmitter's output should
be fed into a dummy antenna - a reasonably good one, not just an
incandescent lamp or two. Lamp resistance varies too much with current,
and the thermal lag may cause the results to be misleading. Good
dummy antennas are not expensive, and every amateur station needs
one for all types of transmitter testing.
The antenna should
be taken off the receiver so there will be no overloading. Set the
audio gain control to maximum, tune in your key-down signal, turn
on the b.f.o., and decrease the r.f. gain until the signal is about
S9. Make sure that this setting of the r.f. gain is within the linear
control range - that is, the signal should not sound the least bit
mushy or thin, and an increase or decrease in gain should change
the audio output in proportion. Setting the audio and r.f. gains
in this way will effectively eliminate any automatic gain control
action in most receivers, but if there is a separate a.g.c. switch
turn it to "off"; you can learn nothing about your keying if the
receiver gain varies while the amplitude of the shaped character
is building up and decaying.
After getting these receiver
settings right, turn off the b.f.o. and switch to a.m. reception
with the widest bandwidth available in the receiver. Now key your
transmitter. There will be an increase in background noise when
the key is down, but this is normal. (If you have hum on your signal
it will also show up, but a properly filtered power supply will
show none.) Listen carefully when the key is closed, and equally
carefully when it is opened. If there is the slightest trace of
a hard click, the shaping is poor and the signal will be taking
up a wider band than it should. The most you should get is the previously
mentioned fairly soft thudding sound when the key is closed. This
may not even be present on break, because of the nature of the rise
and decay curves.
After adjusting the shaping to eliminate
clicks completely, switch on the b.f.o. again. This will tell you
how your signal will sound to others. If it seems unnecessarily
soft you may have gone too far in slowing down the rise and decay
times. A few back and forth trials should result in clean keying
with no trace of click. If you are using a bug or electronic keyer,
adjust the keying weight so the dots and spaces sound about equal.
That's all there is to it, except for one thing: If shaping
adjustments don't get rid of clicks you've got other troubles. Sparking
at the key and contact bounce in a bug or keying relay are the most
likely prospects.2 They have to be cured before you can
begin to control your keying characteristics.
If the shaping
job has been done properly, the final test is to switch in the receiver's
narrowest filter and detune until the beat note just drops into
the noise. Then switch off the b.f.o. At this point you should hear
nothing when you key the transmitter, even if the filter is as narrow
as 200 cycles. If anything at all is heard, the keying is too hard
- provided, that is, that the receiver isn't overloading. Overloading
will show up as a change in background, possibly accompanied by
clicks that actually aren't on the signal. Once again, let us emphasize
that the receiver has to be operating linearly and with constant
gain. If the gain rises 40 or 50 db. when you tune your signal out
of the passband (as it can do very easily if the a.g.c. is operating)
you haven't proved anything. The same statement goes for any checks
you may attempt to make on another fellow's signal.
Most c.w. work is at speeds
ranging from 15 to 35 w.p.m, - that is, at a rate of about 12 to
30 bauds. Since the required bandwidth is directly proportional
to the baud rate, most amateurs can use rise and decay times considerably
longer than 5 to 7 milliseconds. On the other hand, shaping of this
order does not produce key clicks, as we have said, and confines
the transmitted bandwidth to a figure that is compatible with the
highest c. w. selectivity ordinarily available in current receivers.
There seems to be no need, therefore, to change the shaping every
time the sending speed is changed. Once set for no clicks at the
highest speed at which the operator will send it may be left alone
- provided it can be maintained under the variable conditions thrust
on the keying system by changing frequency within a band, on going
from one band to another, or by different transmitter loading adjustments.
Maintaining the keying waveshape under such conditions is no mean
feat. Some of the problems that come up in this connection will
be discussed in a subsequent article. Shaping circuits themselves
are well covered in the keying chapter in the Handbook.
2 Sparking at the key
contacts usually gives rise to clicks only within the station; although
these clicks do not actually go out on the air with the signal they
can obscure the real state of the shaping when the station receiver
is used as a monitor. See Handbook chapter on keying.