U.S. patent number 3,728,632 [Application Number 05/123,533] was granted by the patent office on 1973-04-17 for transmission and reception system for generating and receiving base-band pulse duration pulse signals without distortion for short base-band communication system.
This patent grant is currently assigned to Sperry Rand Corporation. Invention is credited to Gerald F. Ross.
United States Patent |
3,728,632 |
Ross |
April 17, 1973 |
TRANSMISSION AND RECEPTION SYSTEM FOR GENERATING AND RECEIVING
BASE-BAND PULSE DURATION PULSE SIGNALS WITHOUT DISTORTION FOR SHORT
BASE-BAND COMMUNICATION SYSTEM
Abstract
An electromagnetic signal communication system utilizing short
base-band pulse signals of sub-nanosecond duration employs
dispersionless, broad band antenna transmission line elements for
generating and preserving the character of the short base-band
pulses in respective transmitter and receiver sub-systems.
Inventors: |
Ross; Gerald F. (Lexington,
MA) |
Assignee: |
Sperry Rand Corporation (New
York, NY)
|
Family
ID: |
22409243 |
Appl.
No.: |
05/123,533 |
Filed: |
March 12, 1971 |
Current U.S.
Class: |
375/256; 343/820;
343/908; 455/281; 327/181; 329/312; 343/822; 455/124; 455/41.2 |
Current CPC
Class: |
H04B
7/00 (20130101); H01Q 9/005 (20130101); H01Q
13/08 (20130101) |
Current International
Class: |
H04B
7/00 (20060101); H01Q 9/00 (20060101); H01Q
13/08 (20060101); H04b 001/00 () |
Field of
Search: |
;325/27,38R,43,105-107,129,130,141,325,375,377,386 ;328/59,66-68,78
;329/103,126,161,162,104 ;333/12,13,19,32,34
;343/701,753,773,778,786,822,850,852,904-906,908,912-914,739 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Mayer; Albert J.
Claims
I claim:
1. The combination comprising:
transmitter means for transmitting a base-band signal,
receiver means having substantially non-dispersive TEM-mode
transmission line means for receiving said base-band signal,
pulse forming detector means directly responsive to said
substantially non-dispersive TEM-mode transmission line means for
producing an output signal of substantially greater duration than
said base-band signal, and
utilization means responsive to said greater duration output
signal.
2. Apparatus as described in claim 1 wherein said transmitter means
includes means for transmitting without distortion, a subnanosecond
duration electromagnetic pulse having a base-band frequency range
spectral line content, the energy in any selected one of said
spectral lines being below the ambient noise level at said receiver
means.
3. Apparatus as described in claim 2 wherein said pulse forming
detector means responsive to said substantially non-dispersive
TEM-mode transmission line means comprises semiconductor diode
means having first and second states and coupled in energy
exchanging relation with said substantially non-dispersive TEM-mode
transmission line means.
4. Apparatus as described in claim 3 wherein said pulse forming
detector means responsive to said substantially non-dispersive
TEM-mode transmission line means comprises:
first circuit means biasing said semiconductor diode means in said
first state for permitting said semiconductor diode means to change
from its said first to its said second state instantaneously upon
arrival at said semiconductor diode means of said subnanosecond
duration electromagnetic pulse in substantially undistorted
form,
second circuit means coupled to said first circuit means for
producing said greater duration output signal, and
third circuit means utilizing a version of said extended duration
output signal for returning said semiconductor diode means to its
said first state.
5. Apparatus as described in claim 2 wherein said pulse forming
detector means is biased to respond substantially instantaneously
upon receipt by said receiver means of a base-band signal whose
amplitude exceeds a predetermined amplitude for producing said
greater duration output signal.
6. Communication means comprising:
transmitter means for transmitting a train of subnanosecond
duration base-band electromagnetic pulses,
receiver means having substantially non-dispersive TEM-mode
transmission line means for receiving said train of subnanosecond
duration base-band electromagnetic pulses,
pulse forming detector means directly responsive to said
substantially non-dispersive transmission line means for producing
an output train of non-overlapping pulses each of greater duration
than each of said subnanosecond duration electromagnetic pulses,
and
utilization means responsive to said output pulse train.
7. Apparatus as described in claim 6 wherein said utilization means
responsive to said output pulse train includes means for
abstracting intelligence signals from said output pulse train.
8. Apparatus as described in claim 7 including display means for
displaying said abstracted intelligence signals.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention pertains to the transmission, reception, detection,
and use of very short, base-band electromagnetic pulses and more
particularly relates to means for the transmission and reception of
such base-band pulses of sub-nanosecond duration.
2. Description of the Prior Art
Electromagnetic radio signal communications systems available in
the prior art each require for operation a particular frequency
band of particular width in the total frequency spectrum. As types
of radio communication systems have increased and their
applications have become varied and wide spread, the allocation of
frequency bands so that new transmitters do not overlap in
frequency the wave bands allotted to other communication systems
becomes increasingly difficult. Radio frequency interference in the
receiver corresponding to transmitters in the same and in remote
wave bands has increased the need and cost for providing shielding
and other special design features, for example, in ordinary types
of broadcast receivers. Even radiation of local oscillator energy
must be carefully controlled.
Any spurious radiation, above the extremely low power levels
regarded as permissible, adversely affects conventional radio
reception. On the other hand, if the competing radiation level is
very low and it itself is employed for communication purposes, it
is useful only over extremely short ranges and, even then, may be
seriously disabled in the presence of other legal transmissions or
by ambient electrical noise signals. There is not known in the
prior art a radio energy communication system which may be
successfully operated with substantial distances between the
transmitter and receiver thereof in a wave band already allotted to
other receivers in the same geographical vicinity. More
particularly, there is not known in the prior art a radio energy
communication system of the just described type which can operate
at very low or legal power levels without it itself being the
victim of interference. Furthermore, there is not known in the
prior art a radio energy communication system such as described in
the foregoing and also capable of transmission and reception of
signals having an extremely wide frequency spectrum without
interfering with the transmission of ordinary communication
signals.
SUMMARY OF THE INVENTION
The invention pertains to radio pulse communication systems of a
novel kind so constructed and arranged so as to afford intelligence
communication without interference with conventional types of radio
communication and, in turn, being substantially unaffected in
normal operation by the radiations of other communication systems
or by ambient electrical noise signals.
The transmitter appropriate for employment in the novel
communication system utilizes a non-dispersive transmission line
system for generation of very short base-band or sub-nanosecond
pulses of electromagnetic energy and for their radiation into
space, cyclic energy storage on the transmission line and alternate
cyclic energy radiation therefrom being employed. The transmission
line functions as a non-dispersive radiator radiating the
sub-nanosecond impulses with substantially no distortion. Such
base-band pulses have an extremely wide energy spectrum; while the
total energy content of any given transmitted base-band pulse may
be considerable, the few spectral lines falling within the
relatively narrow pass band of a conventional receiver will have no
effect thereon.
The pulse receiver suitable for detecting such short base-band
electromagnetic pulses also employs a dispersionless, broad band
transmission line antenna, with a circuit cooperating with a biased
semiconductor detector element located within the antenna
transmission line for instantaneously detecting substantially the
total energy of the base-band pulse and for supplying a
corresponding output suitable for application in conventional
utilization circuits. The novel receiver antenna system supplies
substantially the total energy of each undistorted received
base-band pulse directly to the receiver detector; thus, the
receiver is adapted to operate successfully with pulse signals
having a very wide spectral extent. Further, it may operate with
base-band pulse signals having spectral components each of such low
individual energy content as to escape detection by conventional
narrow band receivers. The total energy in each base-band pulse
can, however, be relatively larger than the level of noise or other
interfering pulses or signals in the vicinity of the novel
receiver. Thus, by appropriately adjusting the output level of the
transmitter and the sensitivity or threshold of the receiver
detector, base-line communication signals not affecting other
receivers are readily received and detected by the novel receiver
without it, in turn, being affected in substantial degree by other
radio energy transmissions.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of the novel base-band pulse
communication system.
FIG. 2 is a perspective view, partly in cross section, of a portion
of the receiver of FIG. 1.
FIG. 3 is a circuit diagram showing circuit components of the
receiver of FIG. 1 and their interconnections.
FIGS. 4a, 4b, 4c, 4d, and 4e are graphs of wave forms useful in
explaining the operation of the receiver circuit of FIG. 3.
FIG. 5 is a perspective view, partly in cross section, of the
transmitter of FIG. 1.
FIG. 6 is a circuit diagram showing circuit components of the
transmitter of FIG. 5 and of their interconnections.
FIGS. 7a, 7b, 8a, 8b, 9a, 9b, 10a, and 10b are graphs of wave forms
useful in explaining the operation of the transmitter of FIGS. 5
and 6.
FIG. 11 is an elevation view of a modification of the communication
system of FIG. 1.
FIGS. 12, 13, and 14 are graphs useful in explaining the operation
of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates a short base-band or subnanosecond pulse
communication system 1 employing an impulse or short base-band
pulse transmitter 2 having an antenna 4 operating in conjunction
with an impulse or short base-band receiver 3 having an antenna
5.
Short base-band transmitter 2 may be of the general type disclosed
by G.F.Ross and D.Lamensdorf in the U.S. patent application Ser.
No. 46,079, for a "Balanced Radiation System", filed June 15, 1970,
issued Apr. 25, 1972 as U.S. Pat. No. 3,659,203, and assigned to
the Sperry Rand Corporation. The Ross, Lamensdorf device, for
instance, employs an antenna similar to antenna 4 including an
electrically smooth, constant impedance, transmission line system
for propagating TEM mode electromagnetic waves. The transmission
line system is employed within the transmitter for the cooperative
cyclic storage of energy on the transmission line and for its
cyclic release by propagation along the transmission line for
radiation at the aperture end of an antenna section of the
transmission line formed as a flared or tapered directive antenna
line antenna 4. Thus, cooperative use is made of the transmission
line system for signal generation by cyclically charging the
transmission line at a first rate of charging and also for signal
radiation into space by discharge of the line in a time much
shorter than that required for charging. Discharge of the
transmission line causes a voltage wave to travel toward the open
end or radiating aperture of the antenna structure. The process
operates to produce, by differentiation, a sharp impulse or
base-band pulse of sub-nanosecond duration that is radiated into
space. The Ross, Lamensdorf antenna has, like the present antenna
4, a wide instantaneous band width, so that it may radiate very
sharp impulse-like signals of sub-nanosecond duration, either of
positive or negative excursion, with substantially no distortion.
Further, antennas 4 and 5 may have wide patterns of transmission
and reception or may readily be provided with a focusing
characteristic such that energy radiated or received in a
predetermined direction is maximized. A particular system for use
as transmitter 2 and antenna 4 will be discussed in connection with
FIGS. 5 to 10.
The transmitter 2 and its associated antenna 4 of the communication
system 1 are designed to radiate a short base-band pulse or pulses
which may arrive directly or otherwise at the base-band pulse
receiver antenna 5 to be detected by receiver 3. Receiver 3 is a
base-band or sub-nanosecond pulse receiver for receiving and
detecting such a very short base-band electromagnetic pulse or
pulses, and for supplying an output useful for operating
utilization equipment 3a such a display. The receiver system 3
employs a substantially dispersionless, very wide band transmission
line antenna system 5, which may cover a large or small angular
region in space, cooperating directly with a specially arranged
detector located within the transmission line for detecting the
total energy of the base-band pulse. As will be explained with
reference to FIGS. 2 and 3, a cooperating circuit coupled to the
detector device supplies a corresponding output signal suitable for
application in conventional utilization circuits and also recycles
the receiver system 3 to make it ready for the receipt of a
succeeding short base-band pulse. Since the total energy of the
base-band pulse is instantaneously supplied by the dispersionless
antenna system 5 to the detector device, the receiver 3 may operate
with pulse signals having spectral components the amplitudes of
which are all incapable of detection by conventional relatively
narrow band receivers. One base-band receiver having suitable
characteristics for use as receiver 3 in the novel communication
system is disclosed in the K.W.Robbins U.S. patent application Ser.
No. 123,720 for a "Short Baseband Pulse Receiver," filed
concurrently with the present patent application on Mar. 12, 1971,
issued May 9, 1972 as U.S. Pat. No. 3,662,316 and also assigned to
the Sperry Rand Corporation.
As will be explained, the novel short base-band pulse communication
system 1 of the present invention has unique qualities permitting
its use to convey intelligence through space from one location to
another along an arbitrarily selectable path in a manner which
causes substantially no interference with ordinary radio
transmissions. As will also become apparent, while the novel
communication system 1 may be adjusted to operate at very low peak
and average power densities, it is substantially immune to the
presence of random electromagnetic noise or to the presence of
conventional broadcast signals of usual levels.
As is seen in FIGS. 1, 2, and 3, one receiver antenna 5 suitable
for use in the invention comprises a structure having mirror image
symmetry about a median plane at right angles to the direction of
the vector of the electric field E propagating into the antenna,
for instance, in the direction indicated by arrow 6. The same is
true of the cooperating transmission line 7 (FIG. 2) which
comprises parallel wire transmission line conductors 8 and 9;
conductors 8 and 9 are spaced wire conductors constructed of a
material capable of conducting high frequency currents with
substantially no ohmic loss. Furthermore, conductors 8 and 9 are so
constructed and arranged as to support TEM mode propagation of high
frequency energy, with the major portion of the electric field
lying between conductors 8 and 9. It will be understood by those
skilled in the art that the term TEM mode of wave propagation is
that commonly used in the high frequency literature to specify a
conventional mode of electromagnetic energy propagation. In the TEM
or transverse electromagnetic mode, both the electric and magnetic
field components of the wave are everywhere transverse to the
direction of wave propagation. This is in contrast to the character
of certain other types of conventional electromagnetic waves, such
as the transverse electric (TE) and transverse magnetic (TM) waves.
The TE and TM modes are dispersive modes, while the TEM mode
employed in the present invention is desirably nondispersive, the
velocity of propagation in the TEM mode being a constant rather
than varying with frequency.
The TEM receiver antenna 5 further consists of a pair of flared,
flat, electrically conducting planar members 10 and 11. Members 10
and 11 are, for example, generally triangular in shape, member 10
being bounded by flared edges 12 and 13 and a frontal aperture edge
14. Similarly, member 11 is bounded by flaring edges 16 and 17 and
a frontal aperture edge 18. Frontal aperture edges 14 and 18 may be
straight or arcuate. Each of triangular members 10 and 11 is
slightly truncated at its apex, the truncations 19 and 20 being so
constructed and arranged that conductor 8 is smoothly joined
without overlap at truncation 19 to antenna member 10. Likewise,
conductor 9 is smoothly joined without overlap at truncation 20 to
antenna member 11. It is to be understood that the respective
junctions at truncations 19 and 20 are formed using available
techniques for minimizing any impedance discontinuity corresponding
to the junctions.
It is also to be understood that the flared members 10 and 11 of
antenna 5 are constructed of material highly conductive for high
frequency currents and may be supported within an apertured sheet
15 of low loss dielectric material forming a portion of the
receiver casing. It is further apparent that the interior volume of
antenna 5 may be filled with an air foamed dielectric material
exhibiting low dielectric loss in the presence of high frequency
fields, such material acting to support conductor 10 in fixed
relation to conductor 11. Alternatively, the conductive elements of
antenna 5 may be fixed in spaced relation by dielectric spacers
which cooperate in forming enclosing walls for the configuration,
thereby protecting the interior conducting surfaces of antenna 5
from the effects of precipitation and corrosion.
As noted, the planar collector elements 10 and 11 of receiver
antenna 5 are coupled in impedance matched relation to the two wire
transmission line 7, seen in more detail in FIG. 2. Transmission
line 7 is arranged to have the same impedance as the transmission
line comprising antenna elements 10 and 11. Transmission line 7 may
have its parallel wire conductors 8 and 9 molded into a dielectric
enclosing element 21 for the purpose of accurately determining the
separation of conductors 8 and 9 so that transmission line 7 has a
constant impedance along its length. Dielectric element 21 may be
surrounded, in turn, by a braided or other conductive shield 22
which may be grounded at a convenient location, as by lead 23.
Shield 22 may, in turn, be surrounded by a protective plastic cover
element 24 of the well known type. The two-wire transmission line 7
is thus readily attached to the active input element 25 of receiver
2, as will be discussed in connection with FIG. 3. Generally the
length of transmission line 7 between antenna 5 and active element
25 will be short.
A cooperating antenna 5 and transmission line 7 system of the form
shown in FIGS. 2 and 3 is a preferred antenna, in part, because
desired TEM mode propagation therein is readily established. The
TEM propagation mode is preferred, since it is the substantially
non-dispersive propagation mode and its use therefore minimizes
distortion of the propagating subnanosecond pulse signal to be
received. The simple dual transmission line structure also permits
construction of the antenna-transmission line configuration with
minimum impedance discontinuities. Furthermore, it is a property of
the symmetric type of transmission line forming antenna 5 that its
characteristic impedance is a function of b/h, where b is the width
dimension of the major surfaces of conductors 10 and 11 and h is
the distance between inner faces of the conductors 10 and 11. For
example, the ratio b/h is kept constant in the instance of the
antenna 5 transmission line 10, 11 because the ratio of b to h is
held constant.
According to the invention, the receiver-antenna 5 is made
compatible with transmission line 7 by holding the value of the
ratio b/h constant within antenna 5. In other words, if the ratio
b/h is kept constant along the direction of propagation 6 in
antenna 5, the characteristic impedance of antenna 5 will be
constant along its length and may thus be readily made equal to
that of transmission line 7. By maintaining a continuously constant
characteristic impedance and TEM propagation along the structure
including antenna 5 and line 7, frequency sensitive reflections are
prevented therein and frequency dispersion is eliminated. The
received subnanosecond impulse therefore flows through antenna 5
into transmission line 7 without substantial reflection and without
substantial degradation of its shape or amplitude. Since the full
energy or amplitude of a low-level subnanosecond base-band pulse is
thus delivered to the receiver detector 25 by the
antenna-transmission line system, it is seen that the receiver 3
can be sensitive to extremely short low-level base-band pulses
having an extremely wide spectral content, any component of which
would be incapable of detection using conventional wide pulse
reception techniques. In addition, it will be clear to those
skilled in the art that suitable types of non-dispersive base-band
antennas having different receptivity patterns may be substituted
for the illustrated antenna. For example, the omniazimuthal antenna
disclosed in the G.F.Ross U.S. patent application Ser. No. 832,337
for a "Time Limited Impulse Response Antenna," filed June 11, 1969,
issued June 22, 1971 as U.S. Pat. No. 3,587,107, and assigned to
the Sperry Rand Corporation, may be so employed.
Any subnanosecond pulse collected by antenna 5 is passed with
substantially no degradation within two wire transmission line 7 to
the active detector element 25, which is preferably a tunnel diode
or other high speed diode adapted to serve as an impulse or
base-band pulse detector. A suitable detector or diode 25 has a
negative resistance current-voltage characteristic such that, under
proper bias, the diode response to the arrival of impulse emissions
from transmitter configuration 4 is to move abruptly into its
region of instability, causing it to become less conductive. While
other semiconductor diodes may be used, a suitable diode is the
germanium 1N3717 tunnel diode.
Diode 25 is coupled to conductor 8 through a small capacitor 26 and
through resistor 27 to ground and also to conductor 9 of
transmission line 7. Resistor 27 serves a potential level setting
function, enabling tunnel diode 25 to drive silicon transistor 33,
and aids in providing a proper impedance match to line 7 so that
reflections are avoided. Capacitor 26 also acts as a coupling
capacitor, preventing damage to the receiver if the input is
accidentally shorted. An appropriate bias source (not shown) for
diode 25 is connected to terminal 28 for providing current flow
through adjustable resistor 29 and level setting resistor 30 to
diode 25.
A second series circuit connected to bias terminal 28 comprises
resistor 31, diode 32, which may be a conventional 1N914 diode, and
transistor 33, which may be a conventional 2N3638 transistor. A
third series circuit connected to bias terminal 28 comprises
resistor 34, avalanche transistor 35, which may be a selected
conventional 2N706 transistor, and resistor 36. Resistor 36 is
connected to a tap 39 of a voltage source comprising potentiometer
37 and battery 38. A fourth series circuit connected to bias
terminal 28 comprises resistor 40 and capacitor 41. Capacitor 42,
also connected to terminal 28, forms an alternating current ground
connection.
Diode 43 is coupled between ground and the junction between
resistors 29 and 30. The latter junction is connected through diode
44 to the junction between resistor 31 and diode 32. Diodes 43 and
44, like diode 32, may be conventional 1N914 diodes. The junction
common to resistor 31 and diode 32 is connected to the base of
transistor 35 through resistor 45. The junction 50 between
capacitor 26 and tunnel diode 25 is connected through resistor 46
to the base of transistor 33. The junction between the collector of
transistor 35 and resistor 36 is coupled to one side of capacitor
41 by lead 47, which also serves as an ungrounded output lead for
the circuit. A second ungrounded output lead 48 is coupled at the
junction between resistor 40 and capacitor 41.
While other combinations of parameters may be used in practicing
the invention, representative values of such circuit values include
the following:
Resistor 27 82 ohms 29 1 K ohms 30 390 ohms 31 2.2 K ohms 34 22
ohms 36 270 K ohms 40 56 ohms 45 330 ohms 46 390 ohms Capacitor 26
100 picofarads 41 680 picofarads 42 1 microfarad
For the above values, the power source represented by battery 38
and potentiometer 37 may be adjusted to supply about +150 volts at
tap 39. Also, a -6 volt bias source may be coupled to bias terminal
28.
In operation, tunnel diode 25 is biased very near to its break down
point by manual adjustment of resistor 29. For example, in the
instance of a representative 1N3713 germanium tunnel diode,
somewhat less than 10 milliamperes is required to bias diode 25
near its break down point. In this situation, the voltage levels
seen by transistor 33 are not sufficient to cause it to conduct and
it remains quiescent. Diodes 32, 43, and 44 also remain
non-conductive. The avalanche transistor 35 therefore does not have
any forward bias. Its collector potential, by virtue of the setting
of the tap 39 of potentiometer 37, is such that it cannot break
into spurious saw tooth oscillations. Avalanche transistor 35 is
therefore also in its quiescent state and the receiver is ready for
the receipt of an input short base-band pulse.
Such a short base-band pulse, preserved in shape and amplitude by
antenna 5 and transmission line 7, may be, for example, 0.5
nanoseconds in duration and may produce an instantaneous voltage
pulse across tunnel diode 25, for example, of -0.1 volts peak. Such
a signal is illustrated in FIG. 4a, but exaggerated in duration for
clarity of illustration. Tunnel diode 25 instantly switches from
its quiescent low voltage, positive resistance state through its
unstable negative resistance state to its high voltage, positive
resistance state. Upon this event, voltage and current relations in
the remainder of the circuit are transiently disturbed. Transistor
33 becomes forward biased by virtue of the presence of resistor 46
and the circuit including diode 32, resistor 31, and transistor 33
conducts current after a time delay inherent in transistor 33 and
diode 32, thereby causing the positive going signal of FIG. 4c to
travel through resistor 45 to the base of avalanche transistor 35.
The wave of FIG. 4c begins sharply, for the specific circuit
described in the foregoing, also substantially 20 nanoseconds after
time t.sub.0, which time corresponds to the time of the peak value
of the short base-band pulse of FIG. 4a, and then begins to
experience decay.
The positive wave of FIG. 4c, on the order of +10 volts, at the
base of avalanche transistor 35 causes the series diodes 43 and 44
to draw current heavily through resistor 45. The potential at
terminal 50 of tunnel diode 25 abruptly changes, resetting tunnel
diode 25 and reversing its state. After a time shown in FIG. 4e, it
returns to its normal or quiescent state. Conduction through diode
32 fails, thus protecting the collector of transistor 33 from
experiencing excessive positive bias.
The circuit continues to move toward its original quiescent state.
Capacitor 41 discharges, mainly through the circuit path including
resistor 34, resistor 40, and the avalanche transistor 35, at an
exponential rate of decay in dependence upon the time constant of
that discharge circuit. On the other hand, capacitor 41 recharges
much more slowly through resistor 36.
As noted previously, useful output signals comprising relatively
long duration pulses appear on output leads 47 and 48; these have
the general character shown in FIG. 4d, and appear simultaneously.
The pulse on lead 47 has a substantially -100 volt peak value from
the -6 volt level and a 63 percent of peak amplitude duration d of
substantially 200 nanoseconds. The pulse appearing on lead 48 has a
substantially 30 volt peak value from the -6 volt level and a 63
percent peak amplitude duration of substantially 200 nanoseconds.
The delay e of the sharp rise of the output pulses in FIG. 4b is
again substantially 20 nanoseconds behind the peak at t.sub.0 of
the received short-base pulse of FIG. 4a.
The output signals found on leads 47, 48 may be coupled to any
desired utilization apparatus 51, 52 of the type which functions in
a normal manner upon receipt of pulses of conventional or
non-short-base-band duration normally manipulated by ordinary pulse
handling circuits. Although the actual utilization apparatus is not
a necessary part of the present invention, it will be seen by those
skilled in the art that it may take any of a variety of forms. For
example, a single subnanosecond base-band pulse received by antenna
5 may be considered to be an intelligence transmission and the
consequent output appearing on output leads 47 and 48 may be placed
directly on a conventional cathode ray tube display 52 of the type,
for instance, in which the sweep of the indicator along one
coordinate is triggered by the pulse to be displayed, the pulse
itself, after slight delay, being used to sweep the cathode ray
beam along a second coordinate. Signal processor 51 and display 52
may alternatively, for example, count the number of subnanosecond
pulses received by processor 51 in an arbitrary time period or in a
particular pulse burst and then indicate the total count on a
conventional numeric display 52. A train of subnanosecond pulses
collected by antenna 5 may have a modulation, such as carried by
pulse interval modulation, which may readily be demodulated in a
conventional way by processor 51 and either displayed on indicator
52 or, if the demodulated signal is an audio signal, used to
operate a loud speaker or other audio instrument in a conventional
manner.
It is seen that the receiver of FIGS. 1, 2, and 3 is a wide band or
wide open detector device, a receiver which will respond to any
signal level in excess of the bias level which might be dictated by
the characteristics of a particular tunnel diode 25. The amplitude
of the received impulse or base-band pulse at the receiving antenna
5 may be, for example, about 200 millivolts in a typical operating
circumstance, a value several orders of magnitude greater than the
signals present in an urban environment due to conventional
radiation sources, such interfering signals normally being at a
microvolt level. Accordingly, although the novel receiver of FIG. 3
essentially accepts all signals over a very wide pass band, it is
substantially immune to interference from conventional radiation
sources, including electrical noise signals such as internal
combustion engine ignition noise.
The transmitter-antenna configuration 2, 4 shown in FIG. 1 may be,
for instance, capable of transmitting a regular train or a burst of
extremely short duration, low amplitude impulses. In one typical
situation, these impulselike signals have time durations of
substantially 200 picoseconds and an impulse repetition frequency
in the order of 10 kilohertz. However, the upper bound on the
average power transmitted into all of space may be less than one
microwatt. The spectrum of the transmitted signal is spread over an
extremely wide band width, typically 100 megahertz to 10 gigahertz.
Accordingly, the power radiated in any typical narrow communication
band is far below the thermal noise threshold of a typical receiver
operating in that band. The transmitted impulse is therefore
incapable of interfering with the operation of standard radio
communication equipment, while being remarkably adapted for use
with the receiver of the present invention.
For generating the short base-band pulses required for use in the
present invention, the transmitter apparatus illustrated in FIGS.
1, 5, and 6 may be employed. Referring particularly to FIGS. 1 and
5, the antenna 4 is a structure which may be generally similar to
antenna 5 used with receiver 3. Antenna 4 comprises a structure,
for example, having mirror image symmetry about a median plane at
right angles to the direction of the vector 6 of the electric field
E propagating within the antenna. The same is true of the
cooperating transmission line 60 which comprises parallel plate or
slab transmission line conductors 61 and 61a of similar shape.
Conductors 61 and 61a are spaced planar conductors constructed of a
material capable of conducting high frequency currents with
substantially no ohmic loss. Further, conductors 61 and 61a are so
constructed and arranged as to support TEM mode propagation of high
frequency energy, with the major portion of the electric field
lying between conductors 61 and 61a and with the electric field E
substantially perpendicular to the major interior surfaces
thereof.
The TEM transmitter antenna 4 further consists of a pair of flared,
flat electrically conducting planar members 62 and 62a. Members 62
and 62a are, for example, generally triangular in shape, member 62
being bounded by flared edges 63 and 63a and a frontal aperture
edge 64. Similarly, member 62a is bounded by flaring edges 65 and
65a and a frontal apertured edge 64a. Edges 64 and 64a may be
straight or arcuate. Each of triangular members 62 and 62a is
slightly truncated at its apex, the truncation being so constructed
and arranged that conductor 61 is smoothly joined without overlap
at junction 66 to antenna member 62. Likewise, conductor 61a is
smoothly joined without overlap at junction 66a to antenna member
62a. It is to be understood that the respective junctions 66 and
66a are formed using known techniques for minimizing any impedance
discontinuity.
It is also to be understood that the flared members 62 and 62a of
antenna 4 are constructed of material highly conductive to high
frequency currents. It is further apparent that the interior volume
of antenna 4 may be filled with a a dielectric material exhibiting
low loss in the presence of high frequency fields. The interior of
transmission line 60 may be similarly filled with dielectric
material, such material acting to support conductor 61 in fixed
relation with respect to conductor 61a and, likewise, the flared
antenna member 62 relative to flared member 62a. Alternatively, the
conductive elements of transmission line 60 and antenna 4 may be
fixed in spaced relation by dielectric spacers which cooperate in
forming enclosing walls for the configuration, protecting the
interior conducting surfaces of the antenna-transmitter
configuration from the effects of precipitation and corrosion. For
example, thin vertical walls 68 and 68a of low loss dielectric
sheet material may be used in conjunction with transmission line
conductors 61 and 61a. Side walls for separating the planar
elements 62 and 62a may take the form of triangular low loss
dielectric wall elements 69 and 69a; such side walls, in
cooperation with a thin front of radome wall 59 of low loss
dielectric material, lend mechanical strength to the antenna
configuration 4 and aid in protecting the interior thereof. It will
be understood that the planar elements 62 and 62a forming the
antenna aperture may be exponentially tapered alternative to being
lineally tapered.
A form such as that of the transmission line 60 and the
transmitter-antenna 4 such as illustrated in FIG. 5 is preferred,
in part, because TEM mode propagation therein is readily
established. The TEM propagation mode is again preferred, since it
is the substantially non-dispersive propagation mode and its use
therefore minimizes distortion of the propagating base-band pulse
signal to be transmitted by it. The simple, balanced transmission
line structure permits construction of the configuration 4 with
minimum impedance discontinuities. Furthermore, it is a property of
the symmetric type of transmission line of the antenna
4-transmitter 2 configuration that its characteristic impedance is
a function of b/h, where b is the width dimension of the major
surfaces of conductors 62 and 62a and h is the distance between the
inner faces of the conductors 32 and 32a. As in the instance of
antenna 5, the ratio b/h is kept constant in the transmission line
60 because both b and h are constant.
According to the invention, the transmitter antenna 4 is made
compatible with transmission line 60 by using the same value of the
ratio b/h for both elements. The characteristic impedance of
transmitter antenna 4 will thus be constant along its length and
may readily be made equal to that of transmission line 60. By
maintaining a continuously constant characteristic impedance along
the structure including transmission line 60 and antenna 4,
frequency sensitive reflections are prevented therein. It has been
elected, for the sake of simplicity of explanation, to show in FIG.
5 triangular flaring planar configurations for elements 62 and 62a.
It should be evident, however, that other configurations may
readily be realized which maintain a constant characteristic
impedance according to the above rule, and that such configurations
may also be used within the scope of the present invention.
The system for exciting the transmitter antenna 4 of FIG. 5 has
compatible properties therewith, such as being balanced in nature
and as avoiding the complicating deficiencies of an interface balun
or of other transition elements. The system of FIG. 6 achieves such
objectives and, in addition, makes beneficial use of the balanced
dual element configuration of transmitter antenna 4 as part of the
charging line for the base-band pulse excitation generator. It will
be understood that certain liberties have been taken in the drawing
of FIG. 6 better to explain the structure and operation of the
device disclosed therein. For example, it is seen that FIG. 6 is
intended schematically to indicate conductor elements 62 and 62a of
FIG. 5 as respective single wire transmission lines 82 and 82a
having the same effective electrical characteristics as elements 62
and 62a of FIG. 5 and the same radiating characteristic. As a
further example, junctions 66 and 66a in FIG. 5 are represented by
junctions 86 and 86a in FIG. 6. The symbols 61 and 61a in FIG. 5
are represented in FIG. 6 by symbols 81 and 81a and identify the
opposed conductors of transmission line 60. Dimensions in FIG. 6
are exaggerated, such as the spacing h between conductors 81 and
81a of line 60, as a matter of convenience.
At the left end of line 60, conductors 81 and 81a are joined by a
series circuit comprising battery 90 coupled between charging
resistors 91 and 91a each having a resistance value of R/2 ohms. At
the end of line 60 adjacent junctions 86 and 86a, the conductors 81
and 81a are joined by a series circuit comprising an electrically
actuatable switch 92, which may take the form of an avalanche
transistor or other transistor switch; thus, transistor 92 is
coupled across battery 90 through resistors 91, 91a, 96 and 96a.
Also coupled across battery 90 is an astable multivibrator 94 which
is connected through capacitor 93 to the base of transistor 92 for
the purpose of controlling the state of conduction of transistor
92. Resistors 96 and 96a each have a resistance value of r/2 ohms,
where r is equal to the characteristic impedance of line 60(and of
the transmission line comprising elements 82 and 82a). Transistor
92 is also provided with a base-to-ground resistor 93a.
A stable multivibrator or pulse generator 94 may produce a regular
bipolar wave train such as wave 95, of a predetermined pulse
repetition frequency for actuation of transistor switch 92. In
operation, it will be observed that transistor switch 92 is first
held non-conducting by pulse generator 94 for a time sufficient for
the entire structure including the conductors of line 60 and
conductors 82 and 82a to become charged to a potential difference V
equal to that supplied by battery 90 as if charging an effective
capacitor C.sub.1. On the next cycle of wave 95, transistor switch
92 is rendered conducting, forming a conducting circuit path
through resistors 96 and 96a. The effect is that of putting a
second or effective source B in series with the first source A or
battery 90, but reversed in polarity relative to the polarity of
the first source A.
FIGS. 7a, 8a, 9a, and 10a show the positive voltage V, contributed
by the source A or battery 90, as a positive constant voltage at
successive intervals in the operating cycle. The same set of
figures shows the progress of the negative wave due to the second
or effective source B at the same successive intervals. For
example, FIG. 7a shows the situation at the instant transistor or
switch 92 is rendered conductive; note that the wave due to the
effective second source B has not started to flow.
In FIG. 8a, however, the negative wave of voltage -V/2 from the
effective second source B has begun to flow toward the aperture of
transmitter antenna 4. Upon reaching the aperture ends 64, 64a of
conductors 82 and 82a of FIG. 6, and upon being reflected, the
situation is depicted in FIG. 9a. It is seen that when the -V2 wave
reaches the respective aperture ends 64, 64a of antenna conductors
82 and 82a, it is reflected and begins to flow back toward
junctions 86, 86a. The total contribution of the second or
effective source B, beginning at the instant of reversal, is now -V
volts. It will be seen that the total potential due to the real and
the effective sources A and B between conductors 82 and 82a at the
aperture ends 64, 64a of the antenna at the instant of reversal
suddenly drops from +V volts to zero; this instant of time is one
of primary interest in the operation of the transmitter antenna 4.
The wave due to the effective source B continues to travel back
toward junctions 86, 86a until the antenna conductors 82, 82a which
have served as part of the charging line for the system, are
substantially completely discharged if the value of r is the
characteristic impedance of the line comprising conductors 82, 82a.
The charging cycle is then reestablished when pulse generator 94
again renders switch 92 conductive. The system may be repeatedly
recycled.
It will be readily appreciated that the total potential difference
seen across the frontal aperture edges 64, 64a of the antenna, for
the same successive instants of time as described above, may be
illustrated as in the respective FIGS. 7a, 8b, 9b, and 10b. It is
seen that the potential across the antenna aperture due to the real
source 90 (or A) is progressively eaten away by the travel of the
wave due to the second or effective source B as started toward the
aperture 64, 64a when switch 92 is conductive and is then reflected
at the aperture whereupon radiation occurs, ultimately to effect
substantial discharge of the line formed by conductors 82 and 82a,
the wave having returned to be absorbed in the resistances
96,96a.
As noted previously, it is the instant of reflection of the wave of
the effective source B at the distance L along conductors 82 and
82a (the aperture of transmitter antenna 4) that is of prime
interest. Because of the finite characteristic impedance r of the
transmitter-antenna system, the leading edge of the -V/2 wave
launched into the aperture or mouth 64, 64a of the antenna, which
is in effect an open circuit, reverses in direction of flow while
maintaining its previous polarity. Radiation into space of an
impulse or base-band signal proportional to (dV/dt) must occur at
this instant of time. No further radiation can obtain until after
switch 92 is recycled by pulse generator 94 and conductors 82 and
82a are recharged from battery 90. As noted above, if the
resistance r of the sum of resistors 96 and 96a is made equal to
the characteristic impedance of the transmission line system 62,
62a, the reflected wave front finally terminates in resistors 96,
96a and the potential difference across the entire line drops to
substantially zero. It then begins to recharge to approximately
rv/R volts, recharging requiring 2rC.sub.1 seconds.
The base-band pulse receiver 3, as has been seen, may be employed
in the novel communication system to receive intelligence
communications in a variety of ways, such as by receiving a single
subnanosecond base-band pulse from transmitter 2, then generating
an output pulse of duration for example, of the order of 100
nanoseconds, and displaying same on a conventional indicator 52 of
FIG. 3. In this instance, transmitter 2 of FIG. 6 may be, for
example, operated by manually closing a switch corresponding to
transistor switch 92, at the same time disconnecting battery 90 at
one of its terminals so that transmission line 60 cannot recharge.
Equivalent electronic operation may be readily visualized.
More sophisticated arrangements for conveying intelligence messages
from transmitter 2 to receiver 3 are readily apparent to those
skilled in the art. For example, multivibrator 94 of FIG. 6, or
other known types of pulse generators for forming a square wave
pulse train similar to wave 95, may readily be adjusted to cause
transmitter 2 and antenna 4 to radiate an intelligence
transmission, for instance, comprised of a train of 100 base-line
pulses. Such a train, if detected by receiver 3, may be, as
previously observed, conveyed to a conventional counter circuit for
counting the 100, 100 nanosecond duration pulses, the total count
being displayed on a numeric display 52. Well known pulse
generators, useful in addition to multivibrator 94 for the purpose,
may have their pulse repetition rates readily changed in a
conventional manner from one frequency to another manually or by
electrical command signals. Conventional pulse frequency
demodulation circuits present in processor 51 may then be employed
to process pulse frequency modulated 100 nanosecond pulses to yield
a display of frequency as a message on indicator 52. Similarly,
pulse interval modulation in transmitter 2 and cooperative
demodulation in receiver 3 may be employed for conveying
intelligence messages. It will be understood by those skilled in
the art that a variety of ways is available in the prior art for
impressing intelligence on the carrier-less base-band pulses of
transmitter 2, and for abstracting that intelligence at receiver 3
by well established demodulation techniques operating on the
relatively long pulses generated in receiver 3.
The versatility of application of the novel communication system is
further illustrated by reference to FIG. 11, which figure
represents a relay communication system for sending messages from a
transmitter station A to a receiver station B, where, for example,
an obstacle in the form of a mountain range 99 is interposed
between stations A and B. Transmitter station A utilizes a
transmitter 2 and transmitter antenna 4 which may be like elements
2 and 4 of FIG. 1. Similarly, receiver station B utilizes a
receiver antenna 5 and a receiver 3 which may be like those of FIG.
1.
At station C, located at the top of the mountain or other obstacle
99, is located a repeater system composed of a receiver 3a with an
antenna 5a feeding signals for retransmission to a transmitter 2a
having an associated antenna 4a. It will be understood that the
base-band pulse receiver 3a is similar to the base-band pulse
receiver system 3 of FIGS. 1 and 3 and that the base-band
transmitter 2a is similar to that of FIGS. 5 and 6. It is evident
that the wave forms of FIGS. 4b and 4c output on leads 47 and 48 of
FIG. 3 may be used directly to control a known square wave pulse
generator or shaping circuit for producing an output similar to
that of pulse generator 94 of FIG. 6, thus producing the wave form
95 required for operation of transmitter 2a and for the consequent
reception of base-band pulses by receiver 3 at station B. At
station B, the output of receiver 3 may be employed in signal
processing circuit 51 and display or other utilization apparatus
52, as discussed in connection with FIG. 3.
The novel communication system of the several figures is
characterized by a base-band pulse generator capable of generating
pulses of subnanosecond duration for carrier-less communication and
generally considered, in the time domain, to be triangular in
shape. Typically, the base line durations of such triangular
base-band pulses have values of several hundred picoseconds. Such
pulses have, in the frequency domain, an extremely wide energy
spectrum, running substantially from direct current to infinity,
though with oscillating and diminishing amplitude. While the total
energy content of the transmitted base-band pulse may be
considerable, the few spectral lines falling within the pass band
of a conventional receiver will have no effect thereon. On the
other hand, the receiver of the present invention operates with an
input system having no dispersive elements and no resonant circuits
and is thus, in effect, wide open to receive all of the energy of
substantially all spectral lines of each incoming base-band pulse.
With the exception that there is present a need for time to
generate at the output of the base-band receiver a pulse of energy
content and of duration suitable for operation of the conventional
pulse utilization circuits 51, 52, there is no dependence in the
novel receiver's operation on the past history or future reception
of transmissions i.e., each received base-band pulse is handled as
a distinctly separate event, there being no frequency dispersive
circuits and no resonant circuits requiring a carrier transmission
and generally requiring many cycles of carrier input signal before
sufficient energy is built up in the receiver for an output to be
produced.
The theory of operation of the novel communication system will
readily be understood by those skilled in the art from the
foregoing discussion. However, the following simple analysis of the
invention may be offered as one of several possible analyses which
might alternatively be selected to explain operation of the short
base-band pulse communication system. It will be understood that
there is no limitation solely to use of the following analysis
since other analyses might equally well be employed. The purpose of
the selected analysis is to interrelate time and frequency domain
dimensions in dealing with the carrierless short base-band signals
employed in the present invention and, in turn, to relate such
parameters to the noise level in a conventional narrow band pulse
receiver and its characteristic interference level.
Certain assumptions will be made in order to make the analysis
simple. First, a rectangular rather than a strictly triangular,
base band pulse will be considered of the form:
r(t) = 1 t .ltoreq. (.tau./2)
p(t) = 0 t > (.tau./2) (1)
as illustrated in FIG. 12. As previously noted, the duration of the
normalized one volt amplitude pulse measured at the base of FIG. 12
may be several hundred picoseconds. The pulse described by FIG. 12
has, in the frequency domain, an amplitude spectrum (volts/radian)
as described in FIG. 13, where the first zero crossing in the
amplitude spectrum occurs at
f.sub..infin.= 1/.tau. (2)
If each amplitude of the curve of FIG. 13 is squared, the energy
spectrum of the pulse signal is obtained. By integration of the
squared function from zero frequency (d.c.) to f.sub..infin.=
1/.tau., it is seen that 90 percent of the energy is concentrated
in the region thus defined.
Introducing a second approximation by using Parsevall's theorem,
the spectral energy density function is approximately given by FIG.
14. The power spectrum density function can be found simply by
dividing each ordinate in FIG. 14 by .tau..
Assume that the conventional narrow band receiver with which
interference is to be avoided has a pass band centered at frequency
f.sub.0 and a band width .DELTA.f. Then, the amount of peak signal
power in the band .DELTA.f due to the incident rectangular pulse of
FIG. 12 is given by:
P.sub.peak = (1).sup.2 . .tau. .sup.. .DELTA. f (3)
or, if the pulse of FIG. 14 has an amplitude of V volts:
P.sub.peak = V.sup.2 . .tau. .sup.. .DELTA. f (4)
The average power depends upon the duty cycle or the pulse duration
and the pulse repetition frequency. Average power is given by:
P = V.sup.2 . .tau. .sup.. .DELTA. f .sup.. .tau./T (5)
where T is the time between pulses. Thus
P = (V.sup.2 . .tau..sup.2 . .DELTA. f)/T (6)
The power within the band of an ideal narrow band receiver due to
thermal noise is given by:
P.sub.n = K .sup.. T .sup.. .DELTA. f (7)
where K is Bolzman's constant and T is temperature in degrees
Kelvin. For a non-ideal receiver:
P.sub.n = NF .sup.. K .sup.. T .sup.. .DELTA. f (8)
where NF is the noise figure of the receiver.
What is of interest is the condition for which the potentially
interfering pulse signal is equal to the noise signal; this
condition is called the minimum discernible signal interference
situation. This is now readily found by equating expressions (5)
and (8):
(V.sup.2 . .tau..sup.2 . .DELTA. f)/T = NF .sup.. K .sup.. T .sup..
.DELTA. f (9)
or:
V = (1/.tau.) .sqroot. NF .sup.. K .sup.. T .sup.. T (10)
Equation (10) shows clearly that, as the pulse width .tau. gets
smaller, the pulse signal voltage V must get correspondingly larger
to maintain the same interference level. By way of example,
assume:
.tau. = 200 .times. 10.sup.-.sup.12 seconds
T = 10.sup.-.sup.4 seconds
T = 293.degree. Kelvin
K = 1.38 .times. 10.sup.-.sup.16, and
NF = 10.
Putting these values in equation (10) yields a value of V equal to
30 volts. Thus, under the assumed circumstances, a 30 volt
subnanosecond pulse placed across the input terminals of the
conventional narrow band receiver will just suffice to produce a
minimum discernible interfering signal at the receiver
detector.
It is seen that the invention is an impulse or base-band radio
communication system particularly adapted for using
low-total-energy-level transmitted impulses having a spectral
content spread over a very wide band so as to make no significant
contribution to the background electrical noise level and thus
operating well below levels interfering with government controlled
radio transmissions. The transmitter of the novel system is adapted
to excite a cooperating base-band receiver of such a unique nature
that the latter is substantially unaffected by ambient noise or
ordinary pulse transmissions. Since the transmitter may operate
with very low energy consumption, power supply cost and size are
minimized. Furthermore, with such low power operation, inexpensive
components may find long life use throughout the transmitter. The
receiver is similarly categorized, both the receiver and
transmitter element being of very simple nature and otherwise
inexpensive of installation, maintenance, and operation, adapting
readily to cooperative use with conventional intelligence input and
output equipment.
While the invention has been described in its preferred
embodiments, it is to be understood that the words which have been
used are words of description rather than of limitation and that
changes within the purview of the appended claims may be made
without departing from the true scope and spirit of the invention
in its broader aspects.
* * * * *