Pulsed interrogation signal in harmonic EAS system

An electronic article surveillance system includes generating circuitry for generating an interrogation signal. The generating circuitry includes an interrogation coil for radiating the interrogation signal in an interrogation zone. A marker is secured to an article appointed for passage through the interrogation zone, and includes an active element for generating a marker signal including harmonic signal components at harmonics of an operating frequency of the generating circuitry. Detecting circuitry detects the harmonic signal components of the marker signal generated by the active element. The generating circuitry generates the interrogation signal in the form of discrete pulses and the detecting circuitry is operated concurrently with the generating circuitry.

FIELD OF THE INVENTION 
This invention relates to electronic article surveillance (EAS) systems 
and, in particular, to such systems in which EAS markers are detected on 
the basis of harmonic perturbations of an interrogation signal. 
BACKGROUND OF THE INVENTION 
It is well known to provide electronic article surveillance systems to 
prevent or deter theft of merchandise from retail establishments. In a 
typical system, markers designed to interact with an electromagnetic field 
placed at the store exit are secured to articles of merchandise. If a 
marker is brought into the field or "interrogation zone", the presence of 
the marker is detected and an alarm is generated. Some markers of this 
type are intended to be removed at the checkout counter upon payment for 
the merchandise. Other types of markers are deactivated upon checkout by a 
deactivation device which changes an electromagnetic characteristic of the 
marker so that the marker will no longer be detectable at the 
interrogation zone. 
One type of magnetic EAS system is referred to as a "harmonic" system 
because it is based on the principle that a magnetic material passing 
through an electromagnetic field having a selected frequency disturbs the 
field and produces harmonic perturbations of the selected frequency. The 
detection portion of the system is tuned to recognize certain harmonic 
frequencies, and, if such frequencies are present, an alarm is actuated. 
Examples of harmonic EAS systems are disclosed in, e.g., U.S. Pat. Nos. 
5,387,900 and 4,859,991. The assignee of the present application currently 
markets EAS systems of the harmonic type under the trademark 
"AISLEKEEPER". 
Although harmonic EAS systems have been successfully deployed and operated, 
improvement in the performance of such systems remains desirable. In 
particular, in such systems there is an inevitable trade-off between 
reliability in detecting active markers in the interrogation zone and 
susceptibility to false alarms. Significant effort has been devoted to 
improving the ratio of reliability in detection to false-alarm 
susceptibility. 
Another factor that must be taken into consideration is how strong an 
interrogation signal field may permissibly be generated. The latter factor 
has become increasingly important as regulatory authorities have proposed 
reductions in the strength of the signals transmitted by EAS systems. Much 
of the research effort has been directed to new developments in filtering 
or other signal processing techniques to be applied to the signal received 
in the EAS system, so that reliability can be enhanced or maintained in 
the face of reduced interrogation signal levels, and without increasing 
false alarms. 
Examples of some difficulties encountered in reliable detection of harmonic 
EAS markers will now be discussed with reference to FIG. 1. 
A conventional interrogation signal used in harmonic EAS systems, in the 
form of a continuous low-frequency sinusoidal signal, is shown as trace 10 
in FIG. 1(a). A typical frequency for the interrogation signal is 73.125 
Hz. When a marker is present in the interrogation zone, and the level of 
the interrogation signal at the point in the field where the marker is 
located reaches a certain positive or negative amplitude level, an active 
element in the marker is caused to "switch", i.e., to change its magnetic 
polarity. These points in the interrogation signal cycle are indicated by 
the vertical dotted lines in FIG. 1(a). When the marker switches, it 
causes a relatively sharp perturbation or "spike" in the field formed by 
the interrogation signal. These spikes (indicated by reference numeral 12 
in FIG. 1(a)) are rich in harmonics of the interrogation signal frequency, 
and can be detected by suitably tuned receiving equipment. 
Some of the difficulties encountered in harmonic EAS systems result from 
variations in the effective interrogation signal level from location to 
location within the interrogation zone. For example, in a typical system 
installation in which interrogation signal transmitting antennas are 
provided on opposite sides of a store exit, the interrogation signal field 
is strongest in locations that are close to one of the transmitting 
antennas, and is weakest at a central location that is substantially 
equidistant from the antennas. 
Trace 14 in FIG. 1(b) is indicative of the effective interrogation signal 
level at a point in the interrogation zone where the signal is lower in 
amplitude than the signal shown in FIG. 1. As indicated by the vertical 
dotted lines in FIG. 1(b), a marker exposed to the signal represented by 
trace 14 will switch at a point in the signal cycle that is closer to the 
peak of the cycle than was the case for a marker exposed to the 
higher-amplitude signal of FIG. 1(a). In comparing the marker switching 
points in FIG. 1(b) to those of FIG. 1(a), it will be observed that in 
FIG. 1(b) the gradient of the interrogation signal is lower at the 
switching points than in FIG. 1(a). As a result, the marker switches more 
slowly, and produces a marker signal (indicated by spikes 16) that is 
lower in amplitude than the spikes 12 of FIG. 1(a). The relatively 
low-amplitude spikes 16 of FIG. 1(b) are more difficult to detect than the 
higher-amplitude and sharper spikes 12 of FIG. 1(a). 
Another difficulty which results from the variation in field strength 
within the interrogation zone (and as also illustrated in FIGS. 1(a) and 
1(b)) is variation in the timing of the marker signal from cycle to cycle 
of the interrogation signal, as the marker is carried between locations of 
varying field strength. Because of this variation or "jitter" in the 
timing of the marker signal relative to the interrogation signal, it can 
be difficult for the receiving equipment to distinguish between the marker 
signal and random noise impulses. Also, it becomes necessary to operate 
the detection equipment either continuously or throughout large portions 
of the interrogation signal cycle. This increases the likelihood that the 
detection equipment will generate false alarms in response to noise. 
It could be contemplated to increase the amplitude of the interrogation 
signal in order to move the marker switching point further away from the 
peak of the interrogation signal cycle, thereby increasing the amplitude 
of the marker signal even when the marker is at a relatively low-strength 
portion of the interrogation field. It will be understood that the 
increased field strength itself, and also the larger gradient of the 
interrogation signal at the switching point, would both contribute to 
increase the amplitude of the marker signal. However, the above-mentioned 
regulatory constraints place limits on the amplitude of the radiated 
interrogation signal. 
Another possible solution would be simply to reduce the width of the 
interrogation zone (i.e., by moving the transmit antennas closer 
together), so that the signal at the point of minimum strength would be of 
higher amplitude, but this cannot be done without reducing the width of 
the store exit, which would cause inconvenience for store patrons and 
would not be acceptable to retailers, who are the customers for EAS 
systems. 
It could also be contemplated to increase the frequency of the 
interrogation signal (without increasing the amplitude), which would 
provide a higher gradient of the interrogation signal at the marker 
switching point, thereby increasing the amplitude and sharpness of the 
marker signal. But applicable regulations again come into consideration, 
because at higher frequencies the maximum permissible field strength is 
lower. This would make it necessary to reduce the signal amplitude if the 
frequency were increased, so that the width of the interrogation zone 
would have to be reduced. As noted above, this would not be acceptable to 
customers for the system. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is accordingly an object of the invention to provide a harmonic EAS 
system in which markers can be detected more reliably. 
It is a further object of the invention to provide a harmonic EAS system 
with reduced susceptibility to false alarms. 
It is another object of the invention to produce a harmonic EAS system in 
which higher-amplitude marker signals are generated. 
It is still another object of the invention to provide a harmonic EAS 
system in which the timing at which marker signals are generated, relative 
to an interrogation field signal, can be predicted with greater precision 
than in existing systems. 
It is yet a further object of the invention to provide a harmonic EAS 
system that is less subject to disruption by ambient interference signals 
than existing systems. 
Still a further object is to maintain or improve system performance while 
reducing the strength of the interrogation field signal. 
According to a first aspect of the invention, there is provided an 
electronic article surveillance system including generating circuitry for 
generating an interrogation signal, the generating circuitry including an 
interrogation coil for radiating the interrogation signal in an 
interrogation zone, a marker secured to an article appointed for passage 
through the interrogation zone, the marker including an active element for 
generating a marker signal including harmonic signal components at 
harmonics of an operating frequency of the generating circuitry, and 
detecting circuitry for detecting the harmonic signal components of the 
marker signal generated by the active element, wherein the generating 
circuitry generates the interrogation signal in the form of discrete 
pulses. 
Further in accordance with this aspect of the invention, the detecting 
circuitry operates to detect the marker signal generated by the active 
element concurrently with times during which the discrete pulses are 
generated by the generating circuitry. Moreover, the detecting circuitry 
may be arranged so that it does not operate to detect the marker signal at 
times that do not correspond to the discrete pulses. The discrete pulses 
may be such that each one has a pulse length that defines the operating 
frequency of the generating means, with all the pulses being equal in 
pulse length. The pulse length may have a duration that is within a 
preferred range from at least about 2 milliseconds to no more than about 
20 milliseconds. Furthermore, the generating circuitry may operate to 
provide between each pair of successive pulses a time gap that has a 
duration at least as long as, and possibly five times as long as, the 
pulse length of the pulses. Each pulse may be formed so that it is one 
cycle of a sinusoidal signal or a triangular wave. Also, the discrete 
pulses of the interrogation signal may be generated according to a binary 
code pattern so that a cycle of the interrogation signal is generated in 
each time period corresponding to a "1" value of the binary code pattern, 
and a pause in the interrogation signal is formed in each time period 
corresponding to a "0" value of the binary code pattern. 
Still further, the EAS system provided in accordance with this aspect of 
the invention may include circuitry for determining a level of the 
detected marker signal, and the generating circuitry may selectively vary 
a level of the pulses of the interrogation signal according to the 
determined level of the detected marker signal. For example, the 
generating circuitry may be operated to reduce the level of the pulses of 
the interrogation signal when the level of the detected marker signal 
exceeds a predetermined threshold value. 
Still further in accordance with this aspect of the invention, the system 
may include interference detecting circuitry for detecting a periodically 
recurring noise signal present in the interrogation zone, and the 
generating circuitry may be operated to adjust a timing at which the 
pulses of the interrogation signal are generated so that the pulses do not 
coincide with the periodically recurring noise signal. The periodically 
recurring noise signal may have a timing that corresponds to the power 
line operating frequency. 
According to another aspect of the invention, there is provided a method of 
operating a harmonic EAS system, including the step of generating a 
harmonic EAS system interrogation signal in the form of discrete pulses. 
Further in accordance with this aspect of the invention, the method may 
include detecting EAS marker signals concurrently with the discrete pulses 
of the interrogation signal, and refraining from detecting marker signals 
at times that do not correspond to the discrete pulses. 
By operating the transmitting circuitry of a harmonic EAS system in a 
pulsed or intermittent manner, the effective frequency, and thus the 
gradient of the interrogation signal at the marker switching point, can be 
increased, without exceeding regulatory limits on the average radiated 
power of the transmitting circuitry. A marker signal that is higher in 
amplitude, and therefore more easily detected, can thereby be produced. 
Furthermore, the pulsed generation of the interrogation signal makes it 
possible to limit the time windows during which tag signal detection 
operations must be performed, thereby reducing the possibility that the 
system will generate a false alarm in response to impulsive noise. 
Moreover, "jitter" in the timing of the marker signal can be reduced, 
thereby making it easier to distinguish between the marker signal and 
ambient noise. 
In addition, with the pulsed interrogation signal it becomes possible to 
shift the timing of the interrogation signal pulses relative to 
predictable noise (such as may be generated in relation to the power line 
signal) so that the timing of the marker signal is moved to a relatively 
low-noise time interval. Further, the amplitude of the pulses can be 
reduced when a high-amplitude "marker-like" signal is generated, to aid in 
distinguishing between actual markers and other objects (such as shopping 
carts) that mimic EAS markers. 
Still further, use of a pulsed interrogation signal makes it possible to 
operate the system at an over-all lower average power level, which permits 
the cost of the system to be reduced by decreasing the size of heat-sink 
structures on which transmitter power circuitry is mounted. 
The foregoing and other objects, features and advantages of the invention 
will be further understood from the following detailed description of 
preferred embodiments and practices thereof and from the drawings, wherein 
like reference numerals identify like components and parts throughout.

DESCRIPTION OF PREFERRED EMBODIMENTS AND PRACTICES 
Embodiments of the invention will now be described, initially with 
reference to FIG. 2. 
In FIG. 2, reference numeral 20 generally indicates a harmonic EAS system 
provided in accordance with the invention. The system 20 includes a 
transmit control circuit 22, a transmit antenna 24, a power amplifier 26 
connected between the transmit control circuit 22 and the antenna 24, a 
marker 28 including an active element 30, a receive antenna 32, and a 
receiver circuit 34 connected to the receive antenna 32. Signal paths 36 
and 38 are provided between the transmit control circuit 22 and the 
receiver circuit 34. 
The transmit control circuit 22 generates an interrogation signal waveform 
that is amplified by power amp 26 to form an antenna drive signal. The 
antenna drive signal is applied to energize the transmit antenna 24, which 
radiates a corresponding interrogation signal, as indicated at 40, into an 
interrogation zone 42. 
The marker 28 is present in the interrogation zone 42 and is exposed to the 
interrogation signal 40. The active element 30 of the marker 28 responds 
to the interrogation signal 40, by changing or "switching" magnetic 
polarity, thereby causing perturbations in the magnetic field formed by 
the interrogation signal. The perturbations are picked up at receive 
antenna 32 and fed to the receiver circuit 34. The receiver circuit 34 
analyzes the signal received at the antenna 32, detects the perturbations 
caused by the active element 30, determines that the marker 28 is present 
in the interrogation zone, and actuates an alarm. 
FIG. 3 illustrates the interrogation signal generated in the system of FIG. 
2, as well as the resulting marker signal. As seen from FIG. 3, the 
interrogation signal is made up of isolated pulses 44, each of which is 
separated from its respective preceding and succeeding pulses by a pause 
or time gap 46. 
According to the embodiment of the invention illustrated in FIG. 3, each of 
the pulses is a single cycle of a sinusoidal signal, having a pulse length 
t.sub.P which defines a nominal frequency f (=1/t.sub.P) of the signal 
pulses. Each time gap has a duration of t.sub.P0 which defines a 
repetition rate r (=1/(t.sub.P +t.sub.P0)) at which the pulses are 
produced. 
When a marker 30 is present in the interrogation zone, marker signals 48 
are generated at switching points indicated by the vertical dotted lines 
in FIG. 3. The receiver circuit 34 need not be, and preferably is not, 
operated during the time gaps between pulses, but is operable during 
periods in which the pulses are being produced. The transmit control 
circuit 22 accordingly provides a synchronizing signal to the receiver 
circuit 34 via the signal path 36, so that the timing of operation of the 
receiver circuit 34 is synchronous with operation of the transmitter 
portion of the system. 
Referring again to FIG. 2, blocks 50, 52, 54 and 56 represent functions 
carried out in the transmit control circuit 22. The pulse length t.sub.P 
and the repetition rate r (corresponding to the inverse of (t.sub.P 
+t.sub.P0)) are respectively determined at blocks 54 and 52. The power 
line synchronizing block 50 is connected to the AC power supply line (not 
shown) and provides phase synchronization of the pulse 44 (FIG. 3) with 
the power line signal. The transmit signal generating block 56 produces 
the interrogation signal waveform shown in FIG. 3 on the basis of the 
outputs of the blocks 50-54. The resulting waveform is then provided to 
the power amplifier 26 for generation of the desired antenna drive signal. 
It is to be understood that most or all of the functions illustrated by 
blocks 50-56 may be carried out using conventional digital circuitry, such 
as a suitably programmed microcontroller or microprocessor, coupled to the 
power amplifier 26 through digital-to-analog conversion circuitry (not 
separately shown). 
The antennas 24 and 32 may be the same as those used in conventional 
harmonic EAS systems, although the transmitter circuitry provided in 
accordance with the invention to drive the antenna 24 is arranged so as 
not to form a resonant circuit with the antenna 24. It is contemplated to 
provide two or more transmit antennas and two or more receive antennas. 
Some or all of the antennas may be used both for transmitting and 
receiving. 
In FIG. 4, the interrogation signal and marker signal waveforms produced in 
accordance with the invention as shown in FIG. 3, and the prior art 
interrogation signal and marker waveforms of FIG. 1(b), are presented 
together in combination, for ease of comparison. 
In addition to features previously described in connection with FIGS. 1(b) 
and 3, FIG. 4 also shows a time interval t.sub.C1 which represents a time 
interval from the zero crossing to the signal peak of the prior art 
interrogation signal 14, during which period the marker signal 16 may be 
produced, and a corresponding time interval t.sub.P1 for the interrogation 
pulse 44, which is the time period within which a marker signal 48 may be 
produced in accordance with the invention. If desired, in the system 
provided in accordance with the invention, the receiver circuit 34 may be 
operated only during time windows of length t.sub.P1 corresponding to the 
"up slopes" of positive peaks and the "down slopes" of negative peaks of 
the pulses 44. By contrast, in prior art systems using the continuous 
interrogation signal waveform 14, the receiver circuit, if not operated 
continuously, must be operated at least during windows of length t.sub.C1 
corresponding to the positive- and negative-going segments of the 
interrogation signal 14. It will be observed that the time period during 
which the inventive receiver circuit must be operated is much shorter than 
is required according to the prior art. The shorter receiver operating 
window made possible by the present invention significantly reduces the 
system's susceptibility to noise. 
The time intervals indicated by the symbols t.sub.CJ and t.sub.PJ 
respectively correspond to periods during which the continuous and pulsed 
interrogation signals are at or above the amplitude required to "switch" 
the marker. 
It will be observed that in FIG. 4 the conventional continuous-wave 
interrogation signal 14 and the pulses 44 are indicated as having 
substantially the same amplitude. However, the pulses 44, with their high 
nominal frequency, provide a larger signal gradient at the marker 
switching point than the conventional interrogation signal, resulting in 
marker signals 48 having an amplitude V.sub.P that is substantially higher 
than the amplitude V.sub.C of the prior art marker signals 16. The marker 
signals 48 are therefore much more readily detectable than the prior art 
signals 16. 
Further, as will be appreciated from the previous comparison of the 
interval t.sub.CJ with the interval t.sub.PJ, the marker signals 48 are 
much less subject to jitter as compared to the marker signals 16, which 
improves the ability of the inventive EAS system to detect the marker 
signals. 
It is also indicated in FIG. 4 that the repetition rate of the pulsed 
interrogation signal provided in accordance with the invention corresponds 
to the frequency of the conventional continuous interrogation signal 
(i.e., the period t.sub.C of the continuous signal is equal to the sum of 
the pulse length t.sub.P and the duration of the time gap t.sub.P0). If it 
is assumed that the conventional continuous signal is at the typical 
frequency of 73.125 Hz, then the repetition rate r of the pulsed 
interrogation signal in the example shown would also be 73.125 Hz. The 
nominal frequency of each pulse may be, for example, in the range of 
400-500 Hz, producing a time gap duration t.sub.P0 that is on the order of 
five times as long as the pulse length t.sub.P. It will be recognized that 
a nominal signal frequency f of about 400-500 Hz corresponds to a pulse 
length of about 2.0-2.5 milliseconds. 
Other combinations of pulse-length and repetition rate are also 
contemplated. For example, a nominal frequency f as low as 50 Hz (i.e., 
pulse length as long as 20 milliseconds) and a repetition rate r as low as 
25 Hz are also contemplated. Furthermore, for the contemplated range 
50-500 Hz of the nominal frequency f, a ratio of time gap duration to 
pulse length (t.sub.P0 /t.sub.P) as low as 1:1 is contemplated. A 
repetition rate r of 250 Hz or even higher is also contemplated by the 
invention. A preferred range for the repetition rate r is about 50 to 100 
Hz. 
It is also contemplated to provide pulses that are shaped differently from 
the sinusoidal pulses shown in FIGS. 3 and 4. For example, the transmit 
control circuit 22 of FIG. 2 could be arranged (e.g., by suitable 
programming of a microcontroller, which is not separately shown) to 
produce triangular wave pulses, as shown in FIG. 5. This waveform has the 
advantage of providing a fixed gradient, up to the peak of the signal, so 
that the gradient at the marker switching point can be known in advance. 
Other pulse shapes may also be used, although it is desirable to avoid 
square waves or other pulse shapes (such as high-frequency sinusoids) that 
produce very high gradients. Although, in general, a steep gradient is 
desirable because the amplitude of the marker signal is enhanced, if the 
gradient is too steep then objects other than the marker 30 may, upon 
exposure to the interrogation signal, generate signals that cannot readily 
be distinguished from the marker signal. Such objects may include keys, 
key rings, coins or EAS markers intended for use with different systems. 
As has already been noted, the pulsed-signal harmonic EAS system disclosed 
herein provides the advantages of enhanced marker signal, reduced signal 
jitter, limited receiver operating window and relative ease of compliance 
with regulatory restraints related to interrogation signal strength. 
Another beneficial feature that may be provided in a pulsed-signal EAS 
system is adjustment of the position of the interrogation signal pulses so 
as to avoid recurrent ambient noise signals. This feature will now be 
discussed with reference to FIG. 6. 
Shown at the first horizontal axis in FIG. 6 are interrogation signal 
pulses 44 and repositioned pulses 44', the latter being shown in phantom. 
The waveforms shown at the second horizontal axis represent, respectively 
an AC power line signal (dotted line trace 60), and a noise signal 
(indicated by trace 62) with periodically recurring components 66 related 
to the power line signal. 
At the third horizontal axis in FIG. 6 there are shown marker signals 48, 
as well as shifted marker signals 48' corresponding to the shifted 
interrogation signal pulses 44'. 
At the last horizontal axis in FIG. 6, trace 64 is indicative of a signal, 
received at the receiver circuit 34 (FIG. 2), and corresponding to a sum 
of the noise signal 62 and the un-shifted marker signals 48. The shifted 
marker signals 48' are also shown in juxtaposition with the signal trace 
64. 
As will be well understood by those who are skilled in the art, the 
receiving circuitry of conventional harmonic EAS systems includes 
capabilities for storing, in the form of digital samples, several "frames" 
(i.e., transmit signal cycles) of the signal received at the receive 
antenna, as well as the capability of analyzing the stored digital 
signals. According to the aspect of the invention illustrated in FIG. 6, 
the receiver circuit 34 (FIG. 2) is programmed to analyze the stored 
signal frames in order to detect recurring noise patterns such as the 
relatively high amplitude and quasi-periodic noise bursts 66 shown as part 
of trace 62. It will be observed that the noise bursts 66 are correlated 
with the beginning of the positive-going phase of the power line signal 
60. The noise bursts 66 occupy about 25% of the power line signal cycle. 
If the marker signals 48 happen to coincide with the noise bursts 66, the 
resulting signal, as shown in trace 64, might not be recognized by the 
receiver circuit 34 as including a marker signal. However, if the transmit 
pulses are shifted, as shown at 44', so as not to coincide with the noisy 
part of the power line signal cycle, then the resulting shifted markers 
signals 48' can be readily detected in the "quiet" intervals between the 
recurrent noise bursts. 
According to a preferred embodiment of the invention, the receiver circuit 
34 is operated to detect periodically recurring noise, and upon detection 
of a recurrent noise signal, the receiver generates a feedback signal 
which is supplied to the transmit control circuit 22 via the signal path 
38 (FIG. 2). In response to the feedback signal, the transmit control 
circuit 22 shifts the timing of the interrogation signal pulses to avoid 
the predicted occurrence of the noisy part of the power line signal cycle. 
Of course, the receiver circuit's "listening window" (i.e., the interval 
during which the receiver operates to detect marker signals) is also 
shifted to correspond to the adjusted interrogation pulse timing. 
This may be done either in response to a signal provided by the transmit 
control circuit on signal path 36, or based on the anticipated response of 
the transmit control circuit to the feedback signal. 
It will be noted that, for purposes of illustration, the repetition rate of 
the interrogation signal is shown in FIG. 6 as matching the power line 
signal frequency. However, in a preferred embodiment, the repetition rate 
is selected to be different from the power line frequency, and is altered 
in phase when required to prevent the interrogation signal pulse from 
coinciding with predicted noisy parts of the power line signal cycle. It 
should be understood that the pulse-shifting technique shown in FIG. 6 can 
also be applied to avoid recurrent noise that is not correlated with the 
power line signal. 
Still another advantageous technique that is made possible by use of a 
pulsed interrogation signal is illustrated in FIG. 7. In the example shown 
in FIG. 7, the pulses of the interrogation signal are generated in 
accordance with a predetermined digital code, so that marker signals 
corresponding to the code are produced. Such coded marker signals can 
readily be distinguished from noise or other forms of interference, 
thereby improving the ratio of the marker detection rate ("pick" rate) to 
the false alarm rate. 
It will be noted from FIG. 7 that the pulses of the interrogation signal 
may consist of one or more than one signal cycle. Moreover, the intervals 
between pulses are subject to variation, although such intervals between 
cycles are constrained to be equal in duration with, or an integral 
multiple of, the pulse length. In the example shown in FIG. 7, the coding 
is performed by time interval, with each time interval being assigned a 
value of "1" or "0". In the intervals having the value "1", one cycle of 
the interrogation signal is generated; in the "0" value intervals, a pause 
occurs. Where two or more consecutive "1" intervals occur, the signal 
pulse has a length that is the corresponding multiple of the interrogation 
signal cycle. Similarly, the length of each pause between signal pulses is 
determined by the number of consecutive "0" value intervals. It will be 
noted that the marker signals are generated in a pattern that corresponds 
both to the coded bit value and the interrogation signal. In the example 
shown in FIG. 7, it is assumed that the coded bit pattern is formed by 
continuously repeating the pattern "1101001110100". 
Of course, the polarity of the coded interrogation signal could be 
reversed, so that signal pulses correspond to 0's and pauses correspond to 
1's. 
As an alternative to, or in addition to, producing the interrogation signal 
in accordance with a binary code, the amplitude of the interrogation 
signal pulses may be varied when the receiver circuitry detects a signal 
that is similar in shape to a marker signal, but has an amplitude in 
excess of a predetermined threshold level. Specifically, the amplitude of 
the interrogation signal pulses may be reduced in such case, making it 
possible to distinguish between signals that are in fact generated by a 
marker, and signals generated by objects such as shopping carts that may 
tend to generate signals that mimic marker signals in response to 
high-level interrogation signals. 
Although the invention has, up to this point, been described in terms of 
application to harmonic EAS systems, it is also contemplated to employ a 
pulsed interrogation signal in other types of EAS systems in which 
continuous interrogation signals have conventionally been employed. In 
such cases, operation of the system receiver circuitry is carried out 
concurrently with at least a portion of the interrogation signal pulses, 
and preferably is inhibited during times when no interrogation signal 
pulse is being transmitted. 
Various changes in the foregoing apparatus and modifications in the 
described practices may be introduced without departing from the 
invention. The particularly preferred methods and apparatus are thus 
intended in an illustrative and not limiting sense. The true spirit and 
scope of the invention is set forth in the following claims.