Portable, batteryless, frequency divider consisting of inductor and diode

A batteryless, portable, frequency divider consists of a single resonant circuit consisting of an inductor and a diode. The resonant circuit detects electromagnetic radiation at a first predetermined frequency and responds to said detection by transmitting electromagnetic radiation at a second frequency that is one-half of the first frequency. The circuit is resonant at the second frequency when the voltage across the diode is zero. The frequency divider is utilized in a presence detection system that uses a tag containing the frequency divider. The system transmits electromagnetic radiation at the first frequency into a surveillance zone, and detects the second frequency to detect the presence of the tag in the surveillance zone.

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. Such 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; 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; and detecting the removal of 
sentitive and valuable materials, such as a computer tape containing a 
data base or computer program, from a secure area by detecting the 
presence of such materials having electronic tags attached thereto in a 
surveillance zone at the exit of the secured area. 
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 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. 
A frequency divider that may be operated without a battery or any other 
external power supply that is suited for use as an electronic tag in a 
presence detection system is described in U.S. Pat. No. 4,481,428. Such 
frequency divider 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 a 
sub-harmonic of the first frequency for transmitting electromagnetic 
radiation at the second frequency; and a semiconductor switching device 
having gain coupling the first and second circuits for causing the second 
circuit to transmit electromagnetic radiation at the second frequency 
solely in response to unrectified energy at the first frequency provided 
in the first circuit upon receipt of electromagnetic radiation at the 
first frequency. 
SUMMARY OF THE INVENTION 
The present invention provides an improved portable, batteryless, frequency 
divider that is useful in a presence detection system. The improved 
frequency divider of the present invention is less complex and less 
expensive than the frequency divider described in the aforementioned U.S. 
Pat. No. 4,481,428. 
The batteryless, portable frequency divider, of the present invention 
consists of a single resonant circuit consisting of an inductor and a 
diode connected in parallel with the inductor to define a resonant circuit 
that detects electromagnetic radiation at a first predetermined frequency 
and responds to said detection by transmitting electromagnetic radiation 
at a second frequency that is one-half of the first frequency. The circuit 
is resonant at the second frequency when the voltage across the diode is 
zero. 
The reason why the resonant circuit transmits electromagnetic radiation at 
the second frequency is believed to be because of the nonlinear 
capacitance characteristic that is inherent in a diode. 
The frequency divider of the present invention is utilized in a presence 
detection system that uses a tag containing the frequency divider. The 
system transmits electromagnetic radiation at the first frequency into a 
surveillance zone, and detects the second frequency to detect the presence 
of the tag in the surveillance zone. 
Additional features of the present invention are described with relation to 
the description of the preferred embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, one preferred embodiment of the frequency divider of 
the present invention consists of an inductor L1 connected in parallel 
with a diode D1 to define a parallel resonant circuit. The values of these 
components are chosen to define a parallel resonant circuit that detects 
electromagnetic radiation at a first predetermined frequency and responds 
to said detection by transmitting electromagnetic radiation at a second 
frequency that is one-half of the first frequency. 
The diode is a Model MV1405 diode manufactured by Motorola. Other diodes 
may be used provided that the diode which is chosen has a relatively high 
rate of change of capacitance with respect to voltage characteristic, 
dC/dV, at the zero-voltage axis crossing. 
The inductor is rated at 5.39 millihenries and has 330 turns of #32 AWG 
wire having a resistance of 59 ohms. 
The frequency divider of FIG. 1 is utilized in a preferred embodiment of a 
presence detection system according to the present invention, as shown in 
FIG. 5. Such system includes a transmitter 10, a tag 12, and a detection 
system 14. 
The transmitter 10 transmits an electromagnetic radiation signal 16 of a 
first predetermined frequency into a surveillance zone 18. 
The tag 12 is attached to an article (not shown) to be detected within the 
surveillance zone 18. The tag includes a batteryless, portable, frequency 
divider constructed as described above with reference to FIG. 1. 
The detection system 14 detects electromagnetic radiation 20 at the second 
frequency in the surveillance zone 18, and thereby detects the presence of 
the tag 12 in the surveillance zone 18. The second frequency is one-half 
of the first frequency. 
Measurements have been made of capacitance as a function of voltage for 
several diodes. This data was fitted to the following curves for reverse 
and forward capacitance to this data. 
EQU C=K.sub.1 e.sup.K 2.sup.V c 
EQU V.sub.c &gt;0.41 volts 
EQU C=C.sub.o +C.sub.1 (V.sub.c +10).sup.b 
EQU V.sub.c &gt;0.41 volts 
where 
C=diode capacitance 
V.sub.c =voltage across diode 
K.sub.1 =3.86.times.10 (typical value) 
K.sub.2 =40.098 (typical value) 
C.sub.o =21 pf (typical value) 
C.sub.1 =0.5268 pf (typical value) 
b=2.92 (typical value) 
The diode current vs. voltage relationship is given by: 
EQU i.sub.D =I.sub.s e .sbsp.K.sup.3 .sbsp.V.sup.D- I.sub.s 
where 
I.sub.s =8.7.times.10.sup.-9 (typical value) K.sub.3 =29.749 (typical 
value) 
i.sub.d =Current thru diode 
V.sub.d =voltage across diode 
The equivalent circuit of the frequency divider of FIG. 1 is shown in FIG. 
6. 
The circuit analysis for the equivalent circuit of FIG. 6 results in two 
simultaneous nonlinear differential equations. 
##EQU1## 
Those two simultaneous nonlinear differential equations may be solved using 
a numerical method called Runge-Kutta Method. The recursive relations are 
given by: