October 1966 QST
articles are scanned and OCRed from old editions of the ARRL's QST magazine. Here is a list
of the QST articles I have already posted. All copyrights are hereby acknowledged.
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. A
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 code transmissions.
See also: "What
Causes Clicks?' by W8JI (thanks
to Kevin A. for the link)
See all available
vintage QST articles
Why Key Clicks?
The Necessary Bandwidth for C. W. Signals
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
There 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 keying
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 is shown.
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.
Thus 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.
Any repetitive 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. Keying Speed
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
= 1 code element
between words = 2 code elements
The letter C, for example, consists of
Dash - 4 code elements
Dot - 2 code elements
Dash - 4 code elements
Dot - 2 code
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
This 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.?
It seems 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.
In passing, 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
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. Shaping
What 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 used.
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
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 milliseconds.
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,
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 zero axis.
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?
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. Keying Waveshapes
Most, if 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 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.
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
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
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.
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. Slower Speeds
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.