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Claims  |
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We claim:
1. A signal broadcast system comprising a broadcast transmitter and at
least one broadcast receiver, the system comprising:
means for generating at the transmitter a first carrier of a first
reference frequency N and for broadcasting that first carrier;
means for generating at the transmitter a second carrier of second
frequency N/n derived from the first reference carrier where the divisor n
is greater than 1, for modulating the second carrier with a digital
modulating signal in accordance with the digital information to be
transmitted thereby, and for broadcasting the digitally modulated second
carrier;
means at the receiver for receiving the first carrier and for dividing it
by the divisor n to produce a corresponding demodulating signal of
frequency N/n; and
a demodulator at the receiver receiving the second digitally modulated
carrier and the demodulating signal and demodulating the second digitally
modulated carrier with the second demodulating signal to generate a
resulting digital information signal.
2. A system as claimed in claim 1 wherein the said first frequency N is in
the range 10 kHz-500 kHz.
3. A system as claimed in claim 1, wherein the divisor is an even whole
number integer.
4. A system as claimed in claim 1, wherein the said first frequency N is in
the range 10 kHz-500 kHz and the divisor is 2.
5. A system as claimed in claim 1, wherein the said second carrier is phase
modulated by the digital modulating signal.
6. A system as claimed in claim 1, wherein the digital modulating signal is
a coded binary signal, and the said second carrier is phase modulated by
the coded binary digital modulating signal.
7. A system as claimed in claim 1, wherein the said receiver also comprises
means for generating a second carrier of reference frequency N/n and for
transmitting said carrier; and the said transmitter also comprises
receiving means for receiving the said second carrier from the receiver,
and wherein the receiver in transmitting mode employs the said second
carrier of frequency N/n to module the transmitted signal.
8. A signal communication system as claimed in claim 7, and comprising with
each receiving means a visible display unit receiving the digital
information signal and displaying information thereof.
9. A system as claimed in claim 1, wherein the transmitter is a base
transmitter and also comprises receiving means for receiving said second
carrier, and the receiver is a module receiver and also comprises means
for generating a second carrier of frequency N/n and for transmitting said
carrier wherein the said first carrier is generated at the base
transmitter and transmitted therefrom in the form of sequential discrete
envelopes;
the base transmitter when transmitting generates a base data word to be
transmitted, modulates the second carrier therewith and transmits the
resultant modulated second carrier within the respective envelope;
the receiving means at the module receiver detects the base data word and
in response to its termination generates a timing period interposed
between the received base data word and an associated module data word to
be transmitted by the module receiver;
the transmitting means at the module receiver modulates the second carrier
with the module data word and transmits the resultant modulated second
carrier at the termination of the timing period.
10. A system as claimed in claim 9, wherein the module receiver receives
the said first carrier in the form of the said sequential discrete
envelopes, and wherein the lengths of each base and respective module data
word and the interposed timing period are such that the module data word
terminates with a respective received first carrier envelope so that both
the base and module data words are framed thereby.
11. A system as claimed in claim 10, wherein the receiving means of the
transmitter detects in a received second carrier modulated by a module
data word the leading edge of each module data word at the end of the
respective timing period following the termination of the corresponding
transmitter data word by reference to the transmission within the time
period of the respective discrete envelope of the respective base data
word and the respective timing period.
12. A signal communication system as claimed in claim 9, and comprising
with each receiving means a visible display unit receiving the digital
information signal and displaying information thereof.
13. A signal communication system as claimed in claim 1, and comprising
with each receiving means a visible display unit receiving the digital
information signal and displaying information thereof.
14. A signal communication system comprising a base transmitter including
transmitting means and also including receiving means, and comprising a
plurality of module receivers, each of which includes a respective
receiving means and also a respective transmitting means;
the transmitting means of the base transmitter including means for
generating a first carrier of a first reference frequency N and for
transmitting that first carrier respectively to the receiving means of the
module receivers;
each transmitting means of the base transmitter and the module receiver
also including means for generating when transmitting a second carrier of
second frequency N/n derived from the first reference carrier, where the
divisor n is greater than 1, means for modulating the respective second
carrier with a respective digital modulating signal in accordance with
digital information to be transmitted respectively from the base
transmitter and the module receivers, and means for transmitting the
digitally modulated second carrier therefrom;
dividing means at each receiving means receiving the first carrier from the
respective transmitting means and dividing it by the divisor n to produce
a corresponding demodulating signal of frequency N/n; and
a demodulator at each receiving means receiving the respective second
digitally modulated carrier and the demodulating signal and demodulating
the second digitally modulated carrier with the demodulating signal to
generate a resulting digital information signal.
15. A system as claimed in claim 14, wherein the said first frequency N is
in the range 10 kHz-500 kHz.
16. A system as claimed in claim 14, wherein the divisor is an even whole
number integer.
17. A system as claimed in claim 14, wherein said second carrier is phase
modulated by a digital modulating signal.
18. A system as claimed in claim 14, wherein said second carrier is phase
modulated by a coded binary digital modulating signal.
19. A signal communication system as claimed in claim 14, and comprising
with each receiving means a visible display unit receiving the digital
information signal and displaying information thereof.
20. A signal communication system comprising a base transmitter including
transmitting means and also including receiving means, and comprising a
plurality of module receivers, each of which includes a respective
receiving means and also a respective transmitting means;
the transmitting means of the base transmitter including means for
generating a first carrier of a first reference frequency N and for
transmitting that first carrier respectively to the receiving means of the
module receivers, wherein the said first carrier is generated at the base
transmission means and transmitted therefrom in the form of sequential
discrete envelopes;
each transmitting means of the base transmitter and the module receiver
also including means for generating when transmitting a second carrier of
second frequency N/n derived from the first reference carrier, where the
divisor n is greater than 1, for generating a data word to be transmitted
thereby, means for modulating the second carrier with the said data word,
and means for transmitting the resultant modulated second carrier
therefrom;
dividing means at each receiving means receiving the first carrier from the
respective transmitting means and dividing it by the divisor n to produce
a corresponding demodulating signal of frequency N/n; and
a demodulator at each receiving means receiving the respective second
digitally modulated carrier and the demodulating signal and demodulating
the second digitally modulated carrier with the demodulating signal to
generate a resulting data word;
the receiving means at each module receiver detecting each received data
word and the module receiver in response t its termination generating a
timing period of length such that the sum of the length of the received
data word, the length of the timing period and the length of a module data
word to be transmitted by the module transmitting means is equal to the
length of a respective envelope of said first carrier frequency;
the transmitting means at the module receiver modulating the respective
second carrier with its data word to be transmitted and transmitting the
resultant, modulated second carrier at the termination of the said timing
period.
21. A system as claimed in claim 20, wherein the broadcast transmitter in
receiver mode detects the leading edge of the module data word at the end
of said timing period following the termination of the broadcast
transmitter data word by reference to the transmission of the base data
word and the timing period.
22. A system as claimed in claim 20, wherein the said first frequency N is
in the range 10 kHz-500 kHz.
23. A system as claimed in claim 20, wherein the divisor is an even whole
number integer.
24. A system as claimed in claim 20, wherein he said second carrier is
phase modulated by a digital modulating signal.
25. A system as claimed in claim 20, wherein the said second carrier is
phase modulated by a coded binary signal modulating signal.
26. A signal communication system as claimed in claim 20, and comprising
with each receiving means a visible display unit receiving the digital
information signal and displaying information thereof. |
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Claims |
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Description |
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FIELD OF THE INVENTION
The present invention is concerned with improvements in or relating to
signal broadcast communication systems, and in particular to a new low
power system providing broadcast communication between a number of
individual display modules and a central base station transmitting
information to the modules and also receiving information therefrom.
REVIEW OF THE PRIOR ART
There have been a number of prior proposals to automate in some way the
provision of item price information in a retail grocery store. Such a
system is attractive to store operators because of the economic benefits
that result, for example,, from reduction or elimination of the labour
costs associated with maintaining the shelf labels and signs up-to-date;
reducing or eliminating the need to provide price tags on the individual
items; reducing or eliminating loss on stock due to price change lags and
the difficulty of quickly repricing a large number of individually priced
items; and to permit optimization of price distribution in the store with
the possibility of rapid and economical provision of time limited specials
To this end there have been a number of proposals for such systems.
Several important technical problems have prevented the cost effective
development of such systems For example, the shelves that are now used in
most retail industries are constantly being rearranged. Any direct wiring
therefore becomes an expensive impracticality. Moreover, cost
considerations make it important that individual display modules be priced
low, and expensive anti-fretting gold connectors used to connect the
modules to the wiring would overprice the units. Nevertheless, much effort
has been focused on the creation of clever connectors, and wiring schemes
as the solution. Wireless systems including infrared, acoustic and radio
broadcast have been proposed, but most have assumed that such a system
would simply be too unreliable for transmitting important pricing and
merchandising information.
U.S. Pat. No. 4,002,886, issued to Sundelin, discloses an "electronic
shelf" consisting of modules that are attached to the front edge of the
shelf and supplied by wire connections with the data required for display
It teaches that as an alternative to wiring each of 10,000 or more modules
directly to the master computer, a simple address decoding system could be
used where a unique address is first transmitted followed by the data.
Each module in turn has its own unique address and, if the transmitted
address corresponds to the module address, then the data is accepted by
the module
U.S. Pat. No. 4,028,537, issued in 1977 to N.C.R., proposes that a serial
addressing scheme be used. Each module is serially connected to the next
module similar to a Christmas tree string of lights, and they propose that
address decoding be accomplished by subtracting 1 from the current number
before sending it on to the next module. The module that receives a zero
accepts the data as being its own.
U.S. Pat. No. 4,500,880 issued in February 1985 to Motorola and proposes
that the UPC code used as the address, in place of an arbitrary number.
DEFINITION OF THE INVENTION
In accordance with the present invention there is provided a signal
broadcast system comprising a transmitter and at least one receiver, the
system comprising:
means for generating at the transmitter a first carrier of a first
reference frequency N and for broadcasting that first carrier;
means for generating at the transmitter a second carrier of second
frequency N/n derived from the first reference carrier where the divisor n
is greater than 1, for modulating the second carrier with a digital
modulating signal in accordance with digital information to be transmitted
thereby, and for broadcasting the digitally modulated second carrier;
means at the receiver for receiving the first carrier and for dividing it
by the divisor n to produce a corresponding demodulating signal of
frequency N/n; and
a demodulator at the receiver receiving the second digitally modulated
carrier and the demodulating signal and demodulating the second digitally
modulated carrier with the said demodulating signal to generate a
resulting digital information signal.
Preferably the transmitter is a base transmitter and also comprises
receiving means for receiving said first and second carriers, and the
receiver is a module receiver and also comprises means for generating
first and second carriers respectively of reference frequencies N and N/n
and for transmitting said carriers, wherein the said first carrier is
generated at the base transmitter and transmitted therefrom in the form of
sequential discrete envelopes;
the base transmitter when transmitting generates a base data word to be
transmitted modulates the second carrier therewith and transmits the
resultant modulated second carrier within the respective envelope;
the receiving means at the module receiver detects the base data word and
in response to its termination generates a timing period interposed
between the received base data word and an associated module data word to
be transmitted by the module receiver;
the transmitting means at the module receiver modulates the second carrier
with the module data word and transmits the resultant modulated second
carrier at the termination of the timing period.
Also in accordance with the invention there is provided a signal
communication system comprising a base transmitter including transmitting
means and also including receiving means, and comprising a plurality of
module receivers, each of which includes a respective receiving means and
also a respective transmitting means;
the transmitting means of the base transmitter including means for
generating a first carrier of a first reference frequency N and for
transmitting that first carrier respectively to the receiving means of the
module receivers;
each transmitting means of the base transmitter and the module receiver
also including means for generating when transmitting a second carrier of
second frequency N/n derived from the first reference carrier, where the
divisor n is greater than 1, means for modulating the respective second
carrier with a respective digital modulating signal in accordance with
digital information to be transmitted respectively from the base
transmitter and the module receivers, and means for transmitting the
digitally modulated second carrier therefrom;
dividing means at each receiving means receiving the first carrier from the
respective transmitting means and dividing it by the divisor n to produce
a corresponding demodulating signal of frequency N/n; and
a demodulator at each receiving means receiving the respective second
digitally modulated carrier and the demodulating signal and demodulating
the second digitally modulated carrier with the demodulating signal to
generate a resulting digital information signal.
Further in accordance with the invention there is provided a signal
communication system comprising a base transmitter including transmitting
means and also including receiving means, and comprising a plurality of
module receivers, each of which includes a respective receiving means and
also a respective transmitting means;
the transmitting means of the base transmitter including means for
generating a first carrier of a first reference frequency N and for
transmitting that first carrier respectively to the receiving means of the
module receivers, wherein the said first carrier is generated at the base
transmission means and transmitted therefrom in the form of sequential
discrete envelopes;
each transmitting means of the base transmitter and the module receiver
also including means for generating when transmitting a second carrier of
second frequency N/n derived from the first reference carrier, where the
divisor n is greater than 1, for generating a data word to be transmitted
thereby, means for modulating the second carrier with the said data word,
and means for transmitting the resultant modulated second carrier
therefrom;
dividing means at each receiving means receiving the first carrier from the
respective transmitting means and dividing it by the divisor n to produce
a corresponding demodulating signal of frequency N/n; and
a demodulator at each receiving means receiving the respective second
digitally modulated carrier and the demodulating signal and demodulating
the second digitally modulated carrier with the demodulating signal to
generate a resulting data word;
the receiving means at each module receiver detecting each received data
word and the module receiver in response to its termination generating a
timing period of length such that the sum of the length of the received
data word, the length of the timing period and the length of a module data
word to be transmitted by the module transmitting means equal to the
length of a respective envelope of said first carrier frequency;
the transmitting means at the module receiver modulating the respective
second carrier with its data word to be transmitted and transmitting the
resultant, modulated second carrier at the termination of the said timing
period.
Thus, a wireless display module for an "electronic shelf" has four major
requirements:
1. Two Way Communication;
2. Long Battery Life (3-5 years +);
3. Minimal Error Rates; and
4. Low Cost.
To simultaneously achieve all four requires several compromises. First to
achieve low error rates and two way communication a phase modulation
system is used. This previously has required a very complex circuit both
to encode and decode the analog signal consisting of a phase locked loop
or square law device, several amplifiers and encoding and decoding
circuitry. A second major area of concern is that while with some
difficulty it is possible to create a one way link of base station to
module, the return signal from module to base station represents a major
challenge. Power consumption in any CMOS device is due largely to
capacitive discharge; thus, as the driving frequency for reception
increases so does the power consumption. However, as the transmission
frequency decreases, the efficiency for fixed transmission becomes very
poor.
These problems are reduced with this invention by a unique phase encoding
system employing a special reference carrier. This reference carrier is,
in a preferred embodiment, nominally 132 kHz and initially is activated to
frame the transmission from the base station in an envelope of
predetermined length. The module takes the 132 kHz carrier and divides it
by 2 using a conventional flip-flop to create a 66 kHz internal reference.
The base station can then transmit digital data by phase shifting a second
66 kHz carrier also derived from the reference. The module makes a direct
comparison with the 132 kHz divided by 2 signal to obtain a modulated
digital output. When the module transmits back it again uses the 132 kHz
signal as a reference and creates a 66 kHz carrier. This 66 kHz carrier is
phase modulated to encode the digital data. The module transmitted signal
is transmitted within the reference envelope a predetermined period after
the data is received from the base station. The base station has the
advantage that it therefore knows with a great deal of precision the
frequency and timing of the return signal. This makes it possible to
extract acceptable digital data with low signal-to-noise ratios with a
high degree of reliability.
Description of the Drawings
Particular preferred embodiments of the invention will now be described, by
way of example, with reference to the accompanying diagrammatic drawings,
wherein:
FIG. 1 is a perspective view illustrating a typical layout of part of a
store in which the apparatus of the invention is employed;
FIG. 2 is a transverse cross section through a shelf unit of FIG. 1 to
illustrate the enhanced broadcast field that is obtained;
FIG. 3 is a front elevation of a shelf module of the invention, some of the
internally mounted components thereof being shown in broken lines;
FIG. 4 is a schematic diagram of the operating circuit of one of the
modules;
FIGS. 5a through 5e illustrate the broadcast signals received by the
modules and the digital signals produced therefrom for operation of the
module;
FIG. 6 is a schematic illustration of the format of the operating binary
word that is transmitted to the module;
FIGS. 7a through 7e illustrate transmission of base station data to a
module and vice versa within a reference signal framing envelope;
FIG. 8 illustrates apparatus for investigating the best phase relationship
for transmitting and receiving for each module;
FIG. 9 is a plot of a typical table of the different transmit/receive phase
relationships in the modulator and detector at the base station;
FIG. 10 is a more detailed schematic circuit diagram of the pipper circuit
of FIG. 4;
FIG. 11 is a more detailed schematic circuit diagram of the decoder circuit
of FIG. 4;
FIG. 12 is a more detailed schematic circuit diagram of the encoder circuit
of FIG. 4;
FIG. 13 is a more detailed schematic circuit diagram of the sync logic
circuit of FIG. 4;
FIG. 14 is a more detailed schematic circuit diagram of the phase
detector/modulator circuit of FIG. 4; and
FIG. 15 is a circuit diagram of a chargeable circuit for replacement of the
battery of the circuit of FIG. 4.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention will be specifically described in its application to a
self-service retail food store, particularly one of the supermarket type,
in which typically there may be from about 5,000 to about 10,000 different
items to be sold, each requiring its price to be clearly and positively
identified, and each requiring that price notice to be readily changeable,
often at very short notice, to take account of seasonal changes, etc. in
wholesale prices, and to implement the marketing strategy of the store. It
will be evident, however, that the invention is also applicable to other
types of stores, such as clothing and general department stores, and to
completely different types of installation, such as industrial plants,
warehouses and distribution centres, exhibition and convention centres,
and the tool and supply cribs of manufacturing establishments.
FIG. 1 illustrates part of a typical retail store consisting of a plurality
of spaced parallel multiple shelf units 10, each having a plurality of
shelves 12, on the upwardly inclined front edge of each of which is
mounted a plurality of longitudinally spaced shelf unit modules 14, one
for each item whose price is to be displayed. The store also includes a
plurality of check-out stations 16, each of which includes a point-of-sale
terminal having a scanner able to read the bar code that is now almost
universally an integral part of the item labels, and to display and record
the corresponding price in the cash register. The stations 16 typically
are controlled from an in-store main computer 18 to which information may
be supplied as required via a telephone link 20 from a central office, or
by direct keyboard, EPROM, tape, or floppy disc input, as will be apparent
to those skilled in the art. This information is also supplied as required
from the main computer 18 to a system computer 22 (which may also have its
own similar input 23), which in turn is connected to a base station
transmitter/receiver 24. The computers and the base station between them
store the information required by the store in connection with the items
sold, such as:
(a) the identifying bar code;
(b) the item price that day;
(c) information as to previous price history;
(d) details of a temporary sale price to be offered that day at
predetermined times;
(e) the corresponding unit prices;
(f) the aisle, shelf and shelf position location;
(g) the number of facings on the shelf;
(h) the size of a standard unit for re-ordering;
(i) the list of words that each module can reproduce upon command; and
(j) the program that will result in announcements to be displayed on the
module, such as "ON SALE", "15% OFF", etc., and the times at which it is
to be displayed.
In this embodiment the base station 24 is a phase modulated radio frequency
transmitter, the output of which is fed via switches 26 controlled from
the station 24 via a separate control line 27 to the parallel segments 28
of the in-store broadcast antenna, which is disposed so that the parallel
loop planes of the segments are horizontal. Each immediately adjacent pair
of switches controls the antenna segment between them. Each of these
segments has the two horizontal power carrying leads of the respective
horizontal loop lying along the respective top surfaces of the two
associated row of metal shelf units 10 so that each is electromagnetically
coupled to its respective unit. With such an arrangement and at the
frequencies employed the transmission is principally near field inductive
and the practical range of each antenna segment does not extend much more
than its own dimension beyond the shelf unit. The switches 26 permit the
selection of the antenna segment or segments that are required to be
energized at any time, so as to avoid energization of modules 14 that are
not to be addressed, avoiding unnecessary operation thereof and power
consumption, as will become evident from the description below. In this
embodiment the connections to the antenna segments are taken through the
utilities space above the store suspended ceiling requiring downwardly
extending portions 30, but they could also be led through the floor and up
the ends of the shelf units.
The shelf units 10 of such a store are almost universally of thin sheet
steel because of load bearing requirements and it is found unexpectedly
that, at the frequencies at which it is preferred to operate the system,
which will be described in more detail below, placing part of the antenna
segment 28 in sufficiently close contact with the metal structure so as to
be electromagnetically coupled thereto results in greatly increased local
radiation fields at the outer longitudinal edges of the shelves on which
the modules 14 are located, as is indicated by the broken-line outlines 32
in FIG. 2. Thus, in a test installation voltages measured at the module
locations were expected to be in the range of 0.5-3 volts, but instead
were found to be in the range 1-9 volts, and moreover the voltages at the
lower shelves further from the antenna were higher than at the higher
shelves.
Referring now to FIG. 3, a shelf mounting module of the invention comprises
a plastic molded case 34 that is generally rectangular as seen in plan and
elevation, the front face of which has a rectangular aperture 36 behind
which is mounted an LCD display 38 that is capable of displaying the
required information upon suitable energization of the component segments
thereof. A label is applied to the front face, the upper part of which
contains item identification, while the lower part carries the
corresponding bar code and instructions for operation of a visible unit
price pushbutton 40. The manner of operation of the unit price button 40
is more specifically described in our U.S. Pat. No. 4,603,495, our
application Ser. No. 732,114, the disclosure of which is incorporated
herein by this reference.
The module also has mounted therein behind the label two "concealed"
pushbuttons 42 and 44 disposed respectively above and below the visible
button 40, which during normal shopping hours are usually disabled to
prevent their accidental operation by, for example, a child touching the
module. The functions and operation of these two concealed buttons when
they are enabled will be described below. The case 34 also mounts a low
impedance, low Q, air-cored receiving/transmitting loop antenna coil 46
disposed with the plane of the loop parallel to the casing front face and
with its longer sides parallel. The case further mounts a higher Q, higher
impedance ferrite-cored receiving loop antenna coil 48 disposed with its
loop plane at a right angle to the casing front face and thus at a right
angle to that of the coil 46; in this embodiment its longer loop side is
also parallel to the case longer edge. The loop 48 is positioned as
centrally as possible and, with the relative orthogonal placement,
minimizes the coupling between them. It will be noted from FIG. 2 that the
modules are mounted on the shelf edges inclined at an angle to the
vertical, so that the loop planes of the two antennae 46 and 48 are not
orthogonal to that of the loop antennae segments 28, but are inclined at
that angle, which is necessary for other than minimal coupling between
them. The above mentioned electromagnetic coupling is also found
unexpectedly to effectively increase this angle, as though the field is
being bent, so that the transmission efficiency from both of the coils to
the store antennae is increased with minimum coupling between the coils
themselves. Each module also contains a circuit board which is not
illustrated in FIG. 3 but is shown schematically in FIG. 4.
The power for each module is provided by a power source 50, which in this
embodiment is a lithium battery of 0.2 amp hour capacity having a
potential life for operation with the circuit of the invention of about
3-5 years. In view of the fact that a typical retail store will contain at
least 5,000 modules this is the extent of the life that is preferred by
the industry, since battery replacement of so many modules is a
time-consuming and costly operation. The manner in which the circuit of
the invention is able to obtain such an extended shelf life with such a
battery will be described below.
The base station transmitter transmits a first reference carrier signal of
frequency N, which in this embodiment is 132 kHz, the frequency being
determined by division down from a crystal controlled oscillator to,
obtain the desired stability. Provided the module is powered to receive a
signal, as will be described below, this is received by the smaller
ferrite-cored antenna 48, amplified by amplifier 52 and divided by an
integer n, which in this embodiment is 2, by divider 54 to produce a
demodulating or heterodyning signal of 66 kHz frequency (N/n) that is fed
to a circuit 56, to be described in more detail below, which is operative
alternatively as a bi-phase detector or a modulator. The divider output is
also used as a clock signal and for that purpose is fed to a pipper 58, a
divider 60 and a decoder 62. The transmitter also transmits an information
containing signal, to be described in more detail below, consisting of a
second carrier at 66 kHz, also derived from the same crystal standard,
phase modulated by a coded digital signal, this second modulated carrier
being received in the module by the larger air-cored antenna 46 and fed to
the circuit 56 configured as a phase detector. The output of the bi-phase
detector is an information-containing encoded, digital pulse signal with
pulses that are positive-going or negative-going with respect to ground
resulting from demodulation of the second modulated carrier signal from
antenna 46 employing as a demodulating reference the divided signal from
divider 54. This digital output signal is fed to a narrow band filter and
amplifier circuit 66, in which it is shaped as required and unwanted
frequency components (such as the 132 kHz harmonic) are removed. In this
embodiment a pass band filter of 3 kHz is employed.
A high Q, ferrite-core coil 48 is preferred for the reference frequency
antenna since it is relatively immune from the effects of ambient noise,
which is relatively high in the particular environment of a food store
with extensive lighting, refrigeration and air conditioning installations,
particularly to the effects of "spikes" which might otherwise cause
unwanted frequency and phase changes. On the other hand, a low Q air cored
coil is preferred for the receive/transmit antenna 46, particularly when
it is required to transmit, since more power can be radiated as compared
to a ferrite-cored antenna, and the receiver bandwidth can be greater to
permit higher BAUD rates to be used.
FIGS. 5a-5e show the sequence of signals beginning with that received by
the antennae and subsequently that obtained from the phase detector 56.
Thus, FIG. 5a shows a typical 132 kHz first carrier signal received by
antenna 48, and FIG. 5b shows the resultant divided demodulating reference
signal from divider 54. FIG. 5c shows a typical phase modulated signal
obtained from antenna 46 having two phase transitions at X and Y. The
signal at 5d is the output of the phase detector resulting from detection
using the reference frequency signal 5b, and that at 5e is the resultant
signal after smoothing and filtering, consisting of either positive- or
negative-going pulses about the zero volt line 0V. Since all of the
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