Try Googling 'cyclodos' and 'cyclophone' and see what you come up
with. I found that
now a German company which makes apparel from recycled inner tubes
and tents (among other things), and
cyclophone is a weird bicycle-mounted contraption for blasting
sound while peddling down the street. In 1946, the terms cyclodos
and cyclophone referred to modulator and demodulator tubes, respectively,
for pulse-time modulation applications. Fortunately, the science
of pulse modulation quickly evolved past such devices. This article
goes into quite a lot of detail on the beginnings of pulse modulation
techniques developed for radar systems during World War II. It
is very informative without going into the gory details of equations
that govern the theory.
Microwave Pulse Modulation for Ham Communications
By Robert Endall
A simple analysis of a wartime development which will have
widespread ham application in the u.h.f. and s.h.f. bands.
Fig. 1. Signal Corps 4500 mc, eight-channel
pulse transmission equipment, showing all spare and operating
components and antenna, set up for field operation.
Fig. 2. Block diagrams of conventional
type of transmitter and receiver that may be used for any
type of modulation. (A) transmitter. (B) receiver.
In the past, radio amateurs have always been in the forefront
of technical progress in experimental communication on the higher
frequencies, and many of the advances in high-frequency communication
are traceable directly to amateur activity. Before the war many
hams were spending much of their time on the 56- and 112-mc. bands
and experimenting with higher frequencies. Wartime progress has
so greatly extended the radio frequency spectrum that communication
frequencies are now regularly assigned and may be expected to carry
useful traffic up to 30,000 mc. Many radio amateurs, especially
those who have for the past five years been active in wartime u.h.f.
and microwave research, will now want to go on the air at carrier
frequencies well up in the microwave region.
The cost of ultra-high frequency equipment, the expense of tubes
for generating microwave power, and the difficulty of modulating
at u.h.f., have up to now been a handicap to widespread amateur
experimentation on these frequencies. The intensive military program
of development and production of high frequency equipment has done
much to eliminate these difficulties. The application of mass production
techniques has greatly decreased the expense of equipment.
The development of disc-seal tubes, magnetrons and klystrons,
has greatly simplified the problem of generating microwaves. Now
the new technique of pulse-time modulation, which has recently been
removed from the secret list by military authorities, introduces
a considerable simplification in the problem of modulating microwave
A pulse-time-modulated carrier consists of signals of the simplest
type - short pulses of r.f. having constant shape and amplitude
with variable timing. Because of bandwidth considerations, this
type of modulation has its maximum usefulness at microwave frequencies.
Although developed primarily for commercial multiplex telephone
transmission, pulse-time modulation has distinct advantages which
suit it particularly well to amateur communications:
1. Since the carrier is modulated only by being switched on or
off to create the pulses, the circuit can be of an extreme simplicity
of design not possible with other systems.
2. By making full use of the much wider bands per channel which
are not only available, but definitely preferable, at the higher
frequencies, this system makes it possible to reduce considerably
the influence of parasites of artificial origin and to increase
considerably signal-to-noise ratio.
3. Higher peak power and much higher frequencies are attainable
than would otherwise be possible because of limitations due to heating
of the transmitter tubes.
Thus, at the same time that it effects economy in transmission
and simplifies and improves the efficiency of reception, pulse-time
modulation provides also a greater degree of static reduction in
In addition, the multiplex properties of pulse-time modulation
open up still another application for amateur communication in helping
to overcome line-of-sight difficulties to some extent The transmitted
pulses are of very short duration and separated by comparatively
large spaces, therefore more than one telephone channel may be transmitted
on the same carrier provided that the pulses are suitably displaced
with respect to one another. It is quite conceivable that by making
use of this factor, amateurs may cooperate in setting up long nets
and relay systems by means of which each operator will be conducting
his own conversation and acting as a relay point for as many as
eight or ten other conversations taking place over distances far
beyond the limitations of ordinary line-of-sight communication.
Theory of Pulse-time Modulation
Fig. 3. Methods of pulse-time modulation.
Intelligence is transmitted by r.f. pulses having constant
amplitude and duration.
The basic theory of pulse-time modulation is extremely simple.
It consists of transmitting intelligence by pulses of r.f. having
constant amplitude and duration, the instantaneous amplitude of
the voice being translated into variation of time intervals between
successive pulses. The rate of this variation corresponds to the
instantaneous frequency of the signal. The pulses themselves are
of very short duration, separated by comparatively large spaces.
The spacing between the successive pulses can be varied in a
number of different ways. Two of these possible methods of position-variation
which have been used in practice are of interest to amateurs, and
will be described.
The unmodulated carrier, consisting of a series of pulses having
a constant repetition rate, is shown in (A) of Fig. 3. The individual
pulses may be designated by numbers 1, 2, 3, 4, 5, etc., and the
pulse repetition rate R per second. Then the time interval between
successive pulses is T = 1/R seconds. When modulation is applied,
the timing between the pulses is varied by an amount proportional
to the modulating voltage, and at a rate corresponding to the frequency
of the modulating signal.
In the first type of time modulation, one set of pulses is kept
fixed in its time position and serves as a reference set for the
time modulation of the other pulses. For instance, in Fig. 3A and
3B it can be seen that the odd-numbered pulses 1, 3, 5, etc., always
remain in the same position under one another, while the position
of the even-numbered pulses 2, 4, 6, etc., varies. The fixed pulses
are the reference (or marker) pulses, and the others are modulated
in position with reference to them. Fig. 3B illustrates the pulse
position relationships for a single-channel system having a peak
modulation of T/5. As shown in the upper part of the diagram, the
distance between the modulated pulse and the marker is increased
for positive modulation; the lower figure in diagram B shows the
corresponding decrease in distance between modulated and marker
pulse for peak negative modulation.
Fig. 4. Antenna system used in conjunction
with equipment illustrated in Fig. 5.
The process of modulation may be better understood by reference
to Fig. 3E, which shows the pulse-position relationships when the
single-channel system is modulated by one complete cycle of a 1000-cycle
sine wave. During the positive half-cycle the pulse spacing is increased,
as shown, by amounts varying from zero to T/5 according to the modulating
voltage; while during the negative half-cycle of modulating voltage
the spacing is decreased by amounts varying from zero to T/5. A
comparison between pulse positions for the modulated and unmodulated
condition can be seen in the middle diagram of Fig.3E, which shows
the pulse sequence through a complete position-modulated cycle.
The position variations of the modulated pulse for the 1000-cycle
modulation are shown in summarized form in the lower figure, which
shows the various positions of the pulse at different times in the
modulation cycle. A system of the type shown, having a frame period
of 125 microseconds (i.e., repetition frequency of 8000 cycles),
is capable of transmitting a telephone channel of 3000 cycles.
(This system may be used for multiplex transmission in the manner
shown in Fig. 3C. In this case the same marker pulses and the same
frame period are used as in the single-channel system just described,
but for each marker pulse there are now three modulated pulses each
capable of carrying its own independent modulation. Thus, when the
B pulses are modulated, the distance between the B pulses and the
marker is varied within its limits regardless of the instantaneous
position of the A and C pulses. Likewise the positions of the A
and of the C pulses are varied regardless of the other channels.
In the system illustrated, the A pulses constitute channel I, the
B pulses channel II, and the C pulses channel III. When multiplex
transmission is used, the marker pulse is made wider than the others,
as illustrated, in order to distinguish it from the channel pulses.)
The second type of pulse-time modulation does not make use of
a fixed series of pulses. Instead, the pulses are divided into pairs,
and the timing between the two pulses of each set varied in accordance
with the modulating voltage. Thus, pulses 1 and 2 would be one pair,
3 and 4 another, 5 and 6, etc. For the condition of peak positive
modulation as shown in the upper part of Fig. 3D, these pairs are
moved closer together while the pairs of pulses 2 and 3, 4 and 5,
6 and 7, etc., are moved farther apart by an equal amount. For the
condition of peak negative modulation as shown in the lower part
of the diagram, the situation is exactly reversed. Pulses 1 and
2, 3 and 4, etc., are moved farther apart while 2 and 3, 4 and 5,
etc., are brought closer together. There is no change in the average
Fig. 5. Signal Corps 1400 mc. pulse-time
modulation equipment for transmitting and receiving eight
independent telephone channels simultaneously. The photograph
shows all operating and spare components in place.
Fig. 6. Tubes for generating receiver beat oscillations
and microwave power. (Left) Klystron. (Center) Disc-seal
(lighthouse) tube. (Right) Magnetron.
In pulse-time modulation systems the exact shape, duration or
amplitude of the pulse has no fundamental importance, although it
does affect the signal-to-noise ratio of the system. As in the case
of a frequency-modulated system, the amplitudes are made uniform
in the receiver by an amplitude-limiting circuit, thus making possible
a considerable reduction in noise on the sole condition that the
maximum potential due to noise is lower by a certain amount than
the maximum amplitude of the received pulses. (In fact, even if
the noise amplitude is greater than the signal pulse amplitude,
there is still the possibility of time modulating during part of
the pulse interval only, and of eliminating the majority of the
interference by blocking the receiver except during the extremely
short interval when the pulses are actually transmitted.)
Any noise small enough in amplitude to be eliminated by the limiters
will nevertheless generate an audible noise at the output of the
demodulator, because it also has the effect of advancing or retarding
the time position of the leading edge of the desired pulse. This
effect is decreased as the steepness of the wave front of the transmitted
pulse is increased.
Since both the required bandwidth and the signal-to-noise ratio
are determined by the steepness of rise of the transmitted pulse,
it is thus possible to strike the best compromise between bandwidth,
noise, and receiver-oscillator stability to make most effective
use of the high-frequency bands.
A number of the circuits which have been developed for time-modulated
pulse transmission and reception are of interest to radio amateurs
from the viewpoint of experimental communication on the microwave
frequencies. Some of these circuits can be used with very little
modification, while others can easily be adapted to make them reasonably
inexpensive and suitable for ham use. A brief consideration of the
existing circuits will serve to illustrate how amateurs can use
them for experimental microwave communication, and how they may
go about designing their own circuits for high-frequency pulse communication.
The principles of pulse-modulation transmitters and receivers can
best be understood by considering them from the viewpoint of the
basic principles of radio transmitter and receiver design. Fig.
2 shows block diagrams giving the essential details of the most
general types of transmitter and receiver. These block diagrams
apply to all three types of modulation now in general use - amplitude
modulation, frequency modulation, and pulse-time modulation.
For any type of modulation, the transmitter consists essentially
of the following sections: (a) an audio amplifier, (b) the modulator,
(c) a high-frequency oscillator for generating the carrier, (d)
a radio-frequency power amplifier. The receiver can be divided into
the following sections: (a) a radio-frequency amplifier, (b) a high-frequency
local oscillator and converter, (c) an intermediate-frequency amplifier
and a series of limiters, (d) a converter or demodulator to restore
the audio characteristics of the original signal, (e) audio filters
and an audio amplifier to bring the signal to the desired level.
In u.h.f. receivers, the r.f. amplifier (a) is generally omitted
and the signal from the antenna fed directly to the high-frequency
Fig. 7. Block diagram of a pulse-time
modulation transmitter (A) and pulse time modulation receiver
(B), using only conventional type tubes.
Fig. 8. (A) Schematic diagram of pulse-time
transmitter modulating circuit. (B) Diagram of pulse-time
receiver demodulating circuit.
The circuits for the three types of modulation differ from one
another primarily in the modulating and demodulating circuits, and
in the manner in which the modulation is applied to the output stage
of the transmitter. In an AM transmitter, the modulation is generally
applied to the power amplifier or to an intermediate power amplifier.
In an FM transmitter, the modulation is applied before the power
amplifier to the r.f. oscillator, whose frequency is caused to vary
by an amount proportional to the instantaneous audio amplitude.
A pulse-time modulated transmitter uses no power amplifier, the
oscillator supplying the carrier power directly to the antenna.
The modulator in this system serves the function of converting the
audio amplitude into time-modulated pulses, which are applied to
either the grid or plate circuit of the power oscillator so that
the carrier is generated in short position-modulated pulses.
The essential difference between AM receivers and receivers for
FM and pulse-time modulation is in the use of limiters. The AM receiver
is a linear system as regards both the audio signal and the noise
input, whereas the use of limiters in FM and pulse-time modulation
receivers makes possible a considerable reduction in the noise output.
Pulse-time modulation possesses the further advantage that oscillator
tuning and stability are much less critical than in FM reception.
Modulation and demodulation, which is where pulse-time communication
differs basically from AM and FM, may be accomplished by many different
(a) One pulse-time system of the first type - ie., using a fixed
series of marker pulses - makes use of two tubes, known as the "cyclodos"
and the "cyclophone," developed specifically for this purpose. The
essential features of these tubes are shown schematically in Fig.
10. Both tubes make use of an electron beam which, by means of an
ordinary circular sweep circuit, is made to strike the aperture
plate in a circular path. (For ordinary telephone communication,
8000 revolutions per second is a suitable frequency of rotation
for the electron beam.) The aperture plate contains radial slits,
so that during the time the beam is passing each slit the electrons
go through and strike the collector segments, while at all other
times they are intercepted by the aperture plate. Thus, once in
each complete rotation of the electron beam a short pulse of current
is passed to each collector segment. The tubes shown in the diagram,
having aperture plates with four slits, would be suitable for a
multiplex system having three channels, one slit serving for the
marker pulse and the others for the three time-modulated signals.
Fig. 9. Transmitter (A) and receiver
(B) for time-modulated pairs of pulses.
Fig. 10. Artist's illustration of the
cyclodos tube (A) and the cyclophone (B).
In the modulating tube (the cyclodos) the slits are placed at
an angle to the radius of the circular plate, as shown in Fig. 10A.
It can be seen from the diagram that because the slits are tilted
at an angle, the time when the beam crosses the open slit changes
either forward or backward as the radius of rotation of the beam
changes. Thus, audio-frequency amplitude variations may be converted
into pulse-time variations by varying the instantaneous voltage
on the deflecting plates and thereby changing the radius of rotation
of the electron beam. The dotted circle in Fig. 10A represents the
circular pattern obtained when all three channels are unmodulated.
The solid path represents modulation in all three channels - instantaneous
peak positive modulation in channels 1 and 3, negative modulation
in channel 2. (The marker pulse, of course, is always unmodulated.)
In the demodulator tube (the cyclophone) shown in Fig. 10B, the
slits are placed along the radius of the circular aperture plate
instead of being tilted. To convert the time-modulated pulses into
amplitude variations, the electron beam is made to rotate in a circular
path, but the control grid is kept sufficiently, negative so that
the beam is normally cut off except when one of the time-modulated
pulses arrives. The pulses are applied to the grid and, depending
upon where the electron beam is directed with respect to the slit
at the time when the unblocking pulse arrives, there will be a variation
in the amount of beam current passing through the slit and reaching
the collector segment.
At the present time the cyclodos and cyclophone tubes are not
commercially available. When they do become available the cost of
the tubes, although expected to be moderate, will be a factor in
determining their suitability for amateur use. For the time being,
a reasonable substitute can possibly be improvised for amateur service
by making use of an ordinary cathode-ray tube and phototube arrangement.
This can be accomplished by cutting slits in a piece of black cardboard
which fits over the face of the cathode-ray tube. Light shields
and phototubes are placed in front of the slits so that each phototube
records the light coming through one slit. In this manner, the slotted
piece of black cardboard is made to act as an aperture plate and
the phototubes take the place of the electron-collecting segments.
(b) A method of accomplishing pulse-time transmission and reception
without requiring the use of the cyclodos and cyclophone is shown
in Figs. 7 and 8. A block diagram explaining the operation of the
transmitter is shown in Fig. 7A with the basic details of the modulating
circuit - the oscillator clipper, the marker pulse generator, and
the pulse position modulator - given in greater detail in Fig. 8A.
The recurrence frequency is determined by the 8000-cycle oscillator,
which at the same time provides a waveform suitable for starting
the marker pulse generator and the pulse position modulator circuits
for each channel. The marker pulse is obtained from the 8000-cycle
sine wave by means of the oscillator clipper and the marker pulse
generator. The pulse position modulator consists of a double triode
connected as a biased multivibrator, and a single triode used as
a pulse generator. Its operation may be explained briefly as follows:
In the normal state (represented by the upper set of voltage
values in the schematic) the second section of the multivibrator
conducts current. The application of the marker pulse causes the
first section to become conducting and the second section non-conducting,
even after the initiating pulse has ceased. This condition persists
until current flowing through the 3.3 megohm resistor charges the
coupling condenser sufficiently to permit the plate current to flow
in this section of the tube, at which point a rapid reversal takes
place. Modulation is produced by variation of the potential to which
the 3.3 megohm grid resistor is connected, by connecting it to the
output of an audio amplifier tube. The mean length of the square
wave generated by the multivibrator is adjusted to place the channel
pulse in its nor-mal unmodulated position by means of the variable
load resistor in the plate circuit, which adjusts the time constant
of the multivibrator. The transient in the pulse generator tube
when the reversal takes place causes the position-modulated pulse,
by pulsing into one-half cycle of oscillation the tuned circuit
formed by the inductance L, and the input capacity of the following
tube. The remaining features of the transmitting circuit are quite
conventional, as can be seen from the block diagram.
The operation of the receiver circuit can be understood from
the block diagram in Fig. 7B and the demodulator schematic in Fig.
8B. The pulse position modulation is converted to amplitude variation
by first converting it to pulse length modulation, then filtering
out the voice frequencies below 3000 cycles from the pulse length-modulated
signal. This conversion is accomplished by means of a multivibrator
circuit. For each multiplexed channel there is a gate pulse which
is generated in a fixed relationship to the marker pulse, as shown.
When the voltage of the modulation pulse is superimposed on the
gate pulse, the multivibrator is triggered and remains operative
until the end of the gate pulse interval. This results in an output
square wave whose leading edge is varied at the voice frequency.
The remaining features of the receiver circuit are quite conventional,
as can be seen from Fig. 7B.
The circuits for pulse-time modulation and demodulation employing
conventional tubes are seen to be somewhat more complicated than
those making use of the cyclodos and cyclophone tubes, especially
for multiplex communication. However, for single-channel operation
the circuit is not too complex, and it has the advantage of using
standard tubes which are readily available. At the same time, it
affords the radio amateur an interesting opportunity to experiment
with extremely useful pulse and non-standard waveform techniques.
(c) Circuits for pulse transmission and reception by variation
of the timing between pairs of pulses without the use of a reference
pulse are shown in Fig. 9. In the transmitting circuit the pulses
are generated by connecting the grids of the pulsing tubes in push-pull,
and the plates in parallel. The manner in which the pulsing and
pulse timing is accomplished can be seen from the wave-shapes shown
in the diagram. With no modulation applied, one or the other of
the pulsing tubes passes current at all times except when the input
from the constant-frequency source is near the zero value. For near
zero values, both tubes are simultaneously cut off and a short pulse
of relatively high potential is passed into the transmitter. With
no modulation these pulses are uniformly spaced. When modulation
is applied the bias potential of both grids is varied in opposite
directions, thus drawing alternate pulses together. Reversing the
polarity of the modula-tion potential causes the pulses to be pushed
The demodulator in the receiver makes use of two pulsing oscillators
of the type used to produce sawtooth potentials in television receivers,
each adjusted to the same frequency as the oscillator in the transmitter,
with a common plate resistor to make them tend to operate 180°
out of phase. The received pulses synchronize the operation of the
oscillators, alternate pulses controlling each oscillator. Thus,
when a received pulse advances the pulse of one oscillator, it retards
the pulse of the other. Since the received pulses modulate the phase
or timing of the oscillations of each pulse oscillator, but not
the frequency, they vary the average current through the tubes.
Therefore once the oscillators have dropped into synchronism with
the received pulses, the average plate current will be modulated
by the pulse modulation. This provides the original audio signal
which was transmitted.
From the above discussion of the theory of pulse-time modulation
and from the circuits that have been described, the amateur who
desires to experiment in microwave communication should be able
to design and construct a pulse-time modulation rig to use on these
Posted March 16, 2015