Batteryless, portable, frequency divider useful as a transponder of electromagnetic radiation

A batteryless, portable, frequency divider including a first LC circuit that is resonant at a first frequency for receiving electromagnetic radiation at the first frequency; a second LC circuit that is resonant at a second frequency that is one-half the first frequency; and a transistor coupling the first and second LC circuits for causing the second LC circuit to transmit electromagnetic radiation at the second frequency in response to the first LC circuit detecting electromagnetic radiation at the first frequency. The first and second LC circuits respectively include inductance coils that are positioned orthogonally to one another so as not to be mutually coupled. The frequency divider is operable solely from unrectified energy at the first frequency provided in the first circuit upon receipt of the electromagnetic radiation at the first frequency detected by the first LC circuit. The frequency divider is useful as an electronic tag for attachment to articles for enabling detection thereof when moved through a surveillance zone containing electromagnetic radiation at the first frequency and thereby is useful in shoplifting detection systems.

BACKGROUND OF THE INVENTION 
The present invention generally pertains to frequency dividers and is 
particularly directed to an improved frequency divider for use as an 
electronic tag in a presence detection system. 
A presence detection system utilizing a frequency divider as an electronic 
tag is described in United Kingdom Patent Application No. 2,017,454. Such 
system includes a transmitter for transmitting a scanning signal at a 
first frequency in a surveillance zone; an electronic tag including an 
active frequency divider for detecting electromagnetic radiation at the 
first frequency and for transmitting a presence signal in response thereto 
at a second frequency that is a submultiple of the first frequency; and a 
receiver for detecting electromagnetic radiation at the second frequency 
to thereby detect the presence of the electronic tag in the surveillance 
zone. The electronic tags are attached to articles of which detection is 
desired for enabling detection of the presence of such articles in the 
surveillance zone. Such presence detection systems are useful for 
detecting shoplifting, as well for other applications. 
A few examples of such other applications include detecting the presence of 
a person or vehicle carrying an electronic tag in a surveillance zone; 
detecting the presence of articles bearing electronic tags within a 
surveillance zone along an assembly line; and detecting the presence of 
keys attached to electronic tags in a surveillance zone at the exit of an 
area from which such keys are not to be removed. 
The electronic tag is encased in a small card-shaped container that can be 
attached to an article in such a manner that it cannot be removed from the 
article without a special tool. When used in a shoplifting detection 
system, a sales clerk uses such a special tool to remove the electronic 
tag from the merchandise that is paid for; and the surveillance zone is 
located near the doorway for enabling detection of articles from which the 
electronic tags have not been removed. 
The electronic tag described in the aforementioned patent application 
includes a complex frequency divider that must be powered by an expensive 
long-life miniature battery. Other prior art frequency dividers also 
utilize either a battery or an external power supply. 
SUMMARY OF THE INVENTION 
The present invention is a frequency divider that may be operated without a 
battery or any external power supply. Accordingly, the frequency divider 
of the present invention is portable, and inexpensive and is ideally 
suited for use as an electronic tag in a presence detection system. 
The frequency divider of the present invention includes a first circuit 
that is resonant at a first frequency for receiving electromagnetic 
radiation at the first frequency; a second circuit that is resonant at a 
second frequency that is less than the first frequency for transmitting 
electromagnetic radiation at the second frequency; and a transistor 
coupling the first and second circuits for causing the second circuit to 
transmit electromagnetic radiation at the second frequency in response to 
unrectified energy at the first frequency provided in the first circuit 
upon receipt of electromagnetic radiation at the first frequency. 
Additional feature of the present invention are described in the 
description of the preferred embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a preferred embodiment of the frequency divider of the 
present invention includes a first LC circuit consisting of a first 
inductance coil L1 and a first capacitance C1 connected in parallel with 
the first coil L1; a second LC circuit consisting of a second inductance 
coil L2 and a second capacitance C2 connected in parallel with the second 
coil L2; and a transistor Q1. The first LC circuit is resonant at the 
first frequency; and the second LC circuit is resonsant at a second 
frequency that is one-half the first frequency. 
The second coil L2 has a center tap 10 that is connected to one side 12 of 
the first LC circuit. The center tap 10 need not be at the center of the 
second coil L2, but may be positioned anywhere within approximately the 
middle third of the second coil L2. 
The transistor Q1 is a bipolar pnp transistor. The emitter of the 
transistor Q1 is connected to the other side 14 of the first LC circuit. 
The collector of the transistor Q1 is connected to one side 16 of the 
second LC circuit; and the base of the transistor Q1 is connected to the 
other side 18 of the second LC circuit. 
The first coil L1 is positioned orthogonally in relation to the second coil 
L2 so as not to be mutually coupled thereto. 
The operation of the frequency divider shown in FIG. 1 is described with 
reference to the waveforms of the voltages at the transistor terminals as 
illustrated in FIGS. 2, 3 and 4. The zero voltage reference point in the 
frequency divider is the center tap 10 of the second coil L2. These 
waveform were taken from an oscilloscope and show only the free running 
conditions. They do not show the starting conditions. 
At the start, all portions of the frequency divider are at zero volts. The 
transistor Q1 becomes turned on to enable conduction between the emitter 
and the collector when the emitter-to-base voltage exceeds 0.6 volts. 
Accordingly, when the first LC circuit L1, C1 received electromagnetic 
radiation at the first frequency of such intensity as to provide a voltage 
across the first coil L1 in excess of 0.6 volts, the transistor Q1 is 
turned on. Once the transistor Q1 is turned on, current begins to flow to 
the second coil L2 from the first coil L1. The resultant current build-up 
in the second coil L2, augments the forward bias of the transistor Q1 and 
the free running operation of the frequency divider commences. 
Referring to the waveforms of FIGS 2, 3 and 4, during the free-running 
conditions, the transistor Q1 is turned on at point A in each cycle when 
the emitter voltage is at approximately 0.3 volts and the base voltage is 
at approximately -0.3 volts. The emitter voltage then flattens out as 
current flows from the first inductor L1 to the second inductor L2. 
The transistor Q1 remains on and conducting until the voltage across the 
first coil L1 (as represented by the emitter waveform of FIG. 2) decreases 
to the point that the forward bias of the transistor Q1 cannot be 
sustained. 
At point B in each cycle, the transistor Q1 is off and not conducting 
because its base-to-emitter junction and its collector-to-emitter junction 
both are reverse biased. 
At point C in each cycle, the transistor Q1 is still off and not conducting 
because the collapsing field across the second coil L2 creates a positive 
bias on the base which is sufficient to prevent the transistor from 
becoming turned-on even though the emitter voltage rises above its value 
at point A. 
When point A in each cycle is reached again, the transistor Q1 is turned on 
and current again flows from the first inductor L1 to the second inductor 
L2. 
The frequency divider of FIG. 1 is operable at relatively high power 
levels. Even though high level signals detected by the first resonant 
circuit L1, C1 increase the emitter voltage at point C in each cycle, the 
correspondingly greater amount of energy transferred to the second coil L2 
causes the positive bias on the base of the transistor Q1 to also increase 
sufficiently at point C in each cycle to keep the transistor Q1 off. 
Excessive current between the base of the transistor Q1 and the other side 
18 of the second coil L2 can be limited by a resistance, a capacitance or 
a parallel combination thereof. 
The resonant frequency of the second circuit L2, C2 may be other than 
one-half the resonant frequency of the first circuit L1, C1. However, the 
frequency divider is more efficient when the frequency is divided in half. 
Efficiency is a measure of the power of the signal transmitted by the 
second circuit L2, C2 divided by the power of the signal detected by the 
first circuit L1, C1. 
An npn bipolar transistor can be substituted for the pnp transistor Q1 
without any loss in efficiency. The frequency divider also is operable if 
other semiconductor switching devices having gain are used in place of the 
pnp bipolar transistor Q1, but at varying efficiencies. For example, other 
types of bipolar transistors or field effect transistors can be used. 
It is not necessary that the first coil L1 be positioned orthogonally to 
the second coil L2. The relative positioning of the first and second coils 
L1 and L2 should be such that they are not mutually coupled. Mutual 
coupling means coupling to such an extent as to decrease the efficiency of 
the frequency divider. 
There is a decrease in the efficiency of the frequency divider if the 
center tap 10 of the second coil L2 is not located in the middle one-third 
of the second coil L2. 
The alternative preferred embodiment of the frequency divider of the 
present invention shown in FIG. 5 includes a first LC circuit consisting 
of a first inductance coil L1 and a first capacitance C1 connected in 
parallel with the first coil L1; a second LC circuit consisting of a 
second inductance coil L2 and a second capacitance C2 connected in 
parallel with the second coil L2; a transistor Q2; and resistances R1 and 
R2. The first LC circuit is resonant at the first frequency; and the 
second LC circuit is resonant at a second frequency that is one-half the 
first frequency. 
The second coil L2 has a center tap 10 that is connected to one side 12 of 
the first LC circuit. The center tap 10 need not be at the center of the 
second coil L2, but may be positioned anywhere within approximately the 
middle third of the second coil L2. 
The transistor Q2 is a programmable unijunction transistor (PUT). The anode 
of the transistor Q2 is connected to the other side 14 of the first LC 
circuit. The cathode of the transistor Q2 is connected to one side 16 of 
the second LC circuit; and the gate of the transistor Q2 is connected to 
the other side 18 is of the second LC circuit. 
The first coil L1 is positioned orthogonally in relation to the second coil 
L2 so as not to be mutually coupled thereto. 
The resistances R1 and R2 determine the switching threshold of the 
transistor Q2. 
The alternative preferred embodiment of the frequency divider of the 
present invention shown in FIG. 6 includes a first LC circuit consisting 
of a first inductance coil L1 and a first capacitance C1 connected in 
parallel with the first coil L1; a second LC circuit consisting of a 
second inductance coil L2 and a second capacitance C2 connected in 
parallel with the second coil L2; a transistor Q3; and resistances R3 and 
R4. The first LC circuit is resonant at the first frequency that is 
one-half the first frequency. 
The second coil L2 has a center tap 10 that is connected to one side 12 of 
the first LC circuit. The center tap 10 need not be at the center of the 
second coil L2, but may be positioned anywhere within approximately the 
middle third of the second coil L2. 
The transistor Q3 is an SCR. The anode of the SCR Q3 is connected to the 
other side 14 of the first LC circuit. The cathode of the SCR Q3 is 
connected to one side 16 of the second LC circuit; and the gate of the SCR 
Q3 is connected to the other side 18 of the second LC circuit. 
The first coil L1 is positioned orthogonally in relation to the second coil 
L2 so as not to be mutually coupled thereto. 
The resistances R3 and R4 determine the switching threshold of the SCR Q3. 
The alternative preferred embodiment of the frequency divider of the 
present invention shown in FIG. 7 includes a first LC circuit consisting 
of a first inductance coil L1 and a first capacitance C1 connected in 
parallel with the first coil L1; a second LC circuit consisting of a 
second inductance coil L2 and a second capacitance C2 connected in 
parallel with the second coil L2; a transistor Q4; and a resistance R5. 
The first LC circuit is resonant at the first frequency; and the second LC 
circuit is resonant at a second frequency that is one-half the first 
frequency. 
The second coil L2 has a center tap 10 that is connected to one side 12 of 
the first LC circuit. The center tap 10 need not be at the center of the 
second coil L2, but may be positioned anywhere within approximately the 
middle third of the second coil L2. 
The transistor Q4 is a p-junction, enhancement mode field effect transistor 
(FET). The source of the transistor Q4 is connected to the other side 14 
of the first LC circuit. The drain of the transistor Q4 is connected to 
one side 16 of the second LC circuit; and the gate of the transistor Q4 is 
connected by the resistance R5 to the other side 18 of the second LC 
circuit. 
The first coil L1 is positioned orthogonally in relation to the second coil 
L2 so as not to be mutually coupled thereto. 
The free running operation of the frequency dividers shown in FIGS. 5, 6 
and 7 is generally equavalent to that of the frequency divider of FIG. 1, 
as discussed above with relation to FIGS. 2, 3 and 4. 
The frequency divider of the present invention is encased within a 
card-shaped container for use as an electronic tag in a presence detection 
system.