Note: Descriptions are shown in the official language in which they were submitted.
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The invention relates to an information trans-
mission system for wireless transmission, comprising an
inquiry station adapted to transmit a high fre~uency
signal as an inquiry signal and a reply unit receiving
the inquiry signal and adapted to re-transmit a modified
high frequency signal comprising the information to be
transmitted to the inquiry station in which it is detected.
Such a system may for example be used for railway
carriage identification, in which case the inquiry station
is fixed and the reply unit placed on the waggons to be
identified, and thus movalbe past the inquiry station.
The reply unit then should be passive in the sense that
no supply of power shall be necessary ror its function.
The reply unit does not have to be repro~rammable.
In another application the system can be used for
supplying traffic information to drivers of trains and
cars, in which case the inquiry station is placed upon the
movable vehicle and the reply units are made fixed. Also
in this case it is required to have a passivé reply unit.
It may be necessary in this case to make the reply unit
re-programmable, for example in order to indicate a
maximal speed which varies in dependence upon the weather,
the slippery state of the roads etc. In case Or a
re-programmable reply unit it can be necessary to have a
power source in the reply unit, for example in the form
of a small battery, or to connect an external power source
during the re-programming operation.
In a known device of this kind an inquiry signal
is transmitted, the frequency Or which is varied linearly
with time and which thus during each such sweep contains
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a lot of different frequencies. The reply unit consists
Or a number of cavities tuned to certain ones of the
frequencies o~ the inquiry signal, which cavities are
passing these frequencies to an antenna for re-transmission
towards the inquiry station. The function is thus that
power only on those frequencies, which correspond to the
resonance frequencies of the cavities, are re-transmitted
while all the remaining power is dissipated in the reply
unit. The information resides in the frequencies which
are re-transmitted.
SUch a reply unit is entirely passive and robust
in its construction, but the equipment as a whole will be
relatively complicated and expensive due to the varying
frequency on the transmitter side and the shape of the
reply units involving for each reply unit a specific
combination of a large number of accurately tuned resonant
cavities. ~urthermore, it cannot be avoided that the
transmitted power is reflected against objects in the
vicinity Or the reply units, which can disturb the detection
of the reply code, and the efficiency, i.e. the ratio Or
re-transmitted to transmitted power, is very low. The
reply unit in this known equipment is not re-programmable
but must be replaced in its whole when the reply code is
; to be changed.
It is also known to use a continuous or pulsed
single frequency signal as inquiry signal. In such case
the reply unit comprises at least one oscillator. The
oscillator is used to produce a reply signal having a
frequency different from that of the inquiry signal.
Oscillators may also be used to transmit a code message,
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the code being composed of difrerent frequencies similarly
to the reply signal produced by the abovementioned resonant
cavities.
It is an object of the invention to provide an
information transmission system Or this kind employing a
continuous or pulsed single frequency signal as in~uiry
signal and producing a reply signal having a frequency which
is a harmonic Or the frequency Or the inquiry signal, in
which no oscillator is required in the reply device but
the reply device is entirely passive in the sense that it
does not require any individual power source, and in which
the amo.unt Or information to be transmitted can be made
arbitrarily large within the limits set by the available
transmission time.
- . According to the invention the reply device comprises
a nonlinear element to which the signal transmitted from
the inquiry station and received by the reply device is
ap~lied and from which at least one harmonic is derived
and re-transmitted as reply signal to the inquiry station
after modulation in a modulator, which is controlled by an
; active code unit in such a way that the re-transmitted
reply signal is given the form of a modulated pulse train,
the driving power for the code unit being obtained by
rectification of the inquiry signal so that part of the
. energy of the inquiry signal is used for driving the code
unit and the modulator and another part being used for
direct re-transmission in pulse code modulated form after
a passive frequency transformation in said non-linear
element.
: 30 Thus, in the system according to the invention
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the necessary dirference between the frequency of the
inquiry signal and that of the reply signal is achieved
by a passive frequency transformation, for instance
frequency doubling, by means of a nonlinear element.
The coded message is not represented by a multifrequency
code but by a time-distribution code produced by modulation
of the reply signal, e.g. by blocking and deblocking the
reply signal in accordance with the desired code. The
power for driving the code unit is obtained by rectifying
part of the inquiry signal.
Since the coding involves a time distribution a clock
generator is needed to govern the feed-outrhythm of the code.
In a preferred embodiment of the invention the inquiry
signal is pulsed and the clock signal for the code unit is
derived by detection Or the inquiry signal. The clock
generator may also be located in the reply device, e.g.
when the inquiry signal is continuous. In the latter case
some sort Or oscillator is needed in the reply device.
However, this oscillator is not to accomplish any frequency
separation between the inquiry signal and the reply signal
and nor shall it control any frequency distribution in a
multifrequency code which would mean very exacting require-
ments as regards frequency stability.
The principle of the invention, involving a division
of the energy of the inquiry signal into two parts, viz.
one part which is re-transmitted to the inquiry station
after a frequency transformation in the nonlinear element
and a second part which is rectified to supply driving
power for the code unit, may be illustrated by the following
example:
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It is assumed that the nonlinear element has a
square-law characteristic which means that the current
through the element is proportional to the square of the
voltage applied to it, i.e.
i = const. v2
in which i denotes the current through the element, V
denotes the voltage ap~lied to the element and "const"
denotes a constant. If a signal V = Asin~t, in which A =
amplitude, ~ = angular frequency and t = time, is applied
to the element, the current i will be:
v2 = const. A2 ~ cnSt 2
The current contains a D.C. component which can be
used as driving power, and anA.C. component whose frequency
is twice as large as the frequency of the applied signal.
By suitable design of the nonlinear element and the filter
circuits connected thereto any harmonic can be selected
and used as carrier frequency for the retransmission.
. . .
The invention is illustrated in the accompanying
drawing, in which ~ig. 1 shows a block diagram of an
information transmission system according to the invention
with wave forms showing the time ~function for the signals
to and from the reply unit and ~ig. 2 shows a block diagram
of thè reply unit.
In Fig. 1 reference numeral 10 designates an
inquiry station comprising a microwave generator 11 with
the following pulse modulator 12. The generator 10
delivers continuously a carrier frequency fO, which is
pulse modulated in the modulator 12 with a pulse modula-
tion frequency fc. Alternatively, a pulsed microwave
generator can be used. The pulsed microwave s;gnal is
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transmitted through an antenna 13.
When a reply unit 14 comes within the coverage Or
the transmitted signal this signal is received by a
receiver antenna 15. After processing in the reply unit
a modified microwave signal is retransmitted through an
antenna 16. The antennae 15 and 16 can in practice be two
microwave gates connected to an external common antenna.
The retransmitted signal is also a pulsed signal but in
the retransmitted pulse train the individual pulses appear
10 ` according to a given code, which contains the information.
The retransmitted signal has a carrier frequency which is
a harmonic Or the transmitted frequency, for example the
second harmonic. The signal is decoded and the information
evaluated in a decoder unit 17,which at its input has a
rilter 18 tuned to the selected harmonic.
In Fig. 1 is shown in the uppermost diagram the
shape of the transmitted signal and in the lower diagram
an example on a reply signal having the double frequency.
Fig. 2 shows a simplified block diagram of a reply
unit according to the invention. Immediately afte~ the
said receiving antenna 15 there is a nonlinear element 20,
to which the received signal is fed. To the output of
; the other element 20 there is connected on the one hand a
first filter 21 and on the~other hand a second filter 22.
The first filter 21 has a large time constant and delivers
a D.C. voltage, which is applied as driving voltage to a
code unit 23. The second filter 22 is tuned to twice the
transmitted carrier frequency and delivers its output
signal to a signal input of a modulator 24, At a control
input the modulator receives control pulse~ from the code
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unit 23 in accordance with a program memQry included in
the code unit. Clock pulse information ror the code unit
is derived from the incoming pulse signal and the code unit
- may for this purpose have a control input, which received
the incoming signal before it is filtered. ~or each
received pulse the code unit delivers or does not deliver
a control pulse for the modulator in accordance with the
given code as counted from a start moment. The modulator
is in principle an electronic switch adapted to pass or
suppress, respectively, the microwave pulses from the filter
22. The pulses passed by the modulator are transmitted
through the antenna 16.
In order that the clock pulse information shall not
get lost and/or power supply for the code unit shall not
be reduced by.interruptions in the circuit Or the nonlinear
element, the switch may cut Orr the application Or micro-
wave pulses to the antenna Or the reply device by absorbing
them in an energy dissipating circuit, e.g. by reducing
the quality factor of the filter 22.
The code unit starts to operate and deliver control
pulses to the modulator 24 as soon as a surficiently high
driving voltage is received from the rilter 21. Preferably
the transmission Or the actual reply code is preceded by
a start code for defining a start moment. After each trans-
mission Or the reply code transmission Or a new start
sequence with ~ollowing reply code is effected, possibly
arter a small pause. This procedure is repeated as long
as the reply unit receives an inquiry signal.
The nonlinear element can be a diode. The logic unit
is preferably made Or MOS-circuits in inte~rated shape,
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which have a small current consumption so that it will be
possible to utilize the received power in described manner
both to drive the code unit and to retransmit a portion of
the power as a reply signal of the double frequency. The
code unit &S well as the other components are Or a
construction known per se and will not be described in
detail.
Instead Or transmitting a pulsed inquiry signal
from the inquiry station it is also possible to transmit
a continuous microwave signal as inquiry signal. However,
in such case the reply unit must itself contain a clock
for the code unit, which will complicate the reply unit in
some degree and will increase the need for driving power.
It is not necessary to utilize the second harmonic
as carrier frequency in the reply signal but any desired
harmonic can be retransmitted as reply signal. The filter
22 in the reply unit can also be omitted.
The nonlinear element may consist of two diodes
instead of a single diode. In that case one diode is used
for producing the harmonic and the other diode for producing
the rectified voltage to be used as drive voltage for the
reply unit.
For the modulation of the reply signal any method
can be used which affords discrimination between binary
ones and binary zeros. However, preferably amplitude
modulatian is used, in which case one binary element may
have the same form as the inquiry pulses (non-modulated
pulses) while the other binary element has another amplitude,
which may be brought about e.g. by suppressing part of the
reply pulses or reducing their amplitude.
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In the above description the information trans-
mission system has throughout been described as operatin~
at frequencies within the microwave range. As the limits
of the microwave range are not well defined and as the
principle of the information system can be used in
frequency ranges lying close to the microwave range, these
frequency ranges are also included in the invent;on. The
usable frequency range is only limited by the fact that the
information transmission shall be effected as wireless
transmission by means of radiation.
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