Microwave noncontact identification transponder using subharmonic interrogation and method of using the same

A credit-card sized microwave transponder for "wireless key" and surveillance applications uses a subharmonically-pumped quasi-optical mixer. The transponder is activated by a C-band interrogation beam to upconvert and radiate a digitally modulated identification tone at X-band frequencies nonharmonically related to the interrogation signal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to the field of small microwave transponders for 
"wireless key" and surveillance applications, and in particular to 
transponders that are activated by an interrogation beam to upconvert and 
radiate a digitally modulated identification tone. 
2. Description of the Prior Art 
The need for automatic identification of objects and personnel has grown 
rapidly in recent years with the increased use of computerized systems for 
security and control tasks. Noncontact identification schemes using radio 
frequencies (RF/ID systems) have several advantages over comparable 
optical systems, such as better penetration of obstructing materials 
(e.g., clothing, soot) and easier electronic manipulation of the 
identifying signals. Microwave frequencies in particular are attractive 
due to relatively low radio noise and interference levels, wide available 
bandwidth for high-speed data transfer, and physically small high-gain 
antennas. 
In a typical system, transponders (ranging from electronic ID badges to 
antitheft tags) are read, or interrogated, by a microwave beam which 
causes them to emit a coded response. Various types of response signals 
are in use, including simple back scatter with modulation in amplitude, 
("An Automatic Vehicle ID System for Toll Collecting," Lawrence Livermore 
National Laboratory, Report No. UCRL-TB-113409, April 1993), phase (P. de 
Bruyne and P. Leuthold, "Radar Surveillance of Autobahn Toll), or both 
(i.e., SSB) (T. Ohta, H. Nakano, and M. Tokuda, "Compact microwave remote 
recognition system with newly-developed SSB modulation," IEEE MTT-S 
Digest, pp.957-960, 1990), or generation of a continuous or modulated 
harmonic of the interrogation signal (R. Page, "A Low Power RF ID 
Transponder," RF Design, pp. 31-35, July 1993). 
In the prior art systems where an active response is generated, the 
responding signal was usually close to the frequency of the interrogating 
signal or was harmonically related thereto and therefor lay within the 
interference band of the interrogating signal. Also the transponder card 
needs a microwave source. As a result, false detection resulting from an 
interrogator receiving reflections of its own transmitted harmonics could 
arise. 
What is needed is a transponder in the form of an ID card which is 
interrogated at frequency fi and generates a response frequency modulated 
with an identification code at microwave frequencies apart from the 
interrogation signal, away front interference in the interrogation band, 
and without the need for a microwave source on the transponder card. The 
frequencies should not be harmonically related to the interrogation signal 
in order to avoid the problem of false detection resulting from an 
interrogator receiving reflections of its own transmitted harmonics. 
BRIEF SUMMARY OF THE INVENTION 
The invention is an improvement in a contactless transponder having a 
modulation oscillator, a quasi-optical mixer having a first input coupled 
to the modulation oscillator, and an antenna having an input coupled to an 
output of the mixer. The improvement comprises an identification code 
generator coupled to a second input of the mixer. The mixer modulates an 
interrogation signal received by the antenna where the interrogation 
signal has a frequency, f.sub.i, with a data carrier signal having a 
frequency, f.sub.d, which is generated by the modulation oscillator. The 
result is that supressed-carrier double sideband response signals are 
transmitted through the antenna. Each sideband response signal is 
modulated by an identification code generated by the identification code 
generator. The data frequency, f.sub.d, is at a nonharmonic value of the 
interrogation frequency, f.sub.i, so that false detection arising from 
reflections of transmitted harmonics of the interrogation signal is 
avoided. 
The antenna is a bow-tie antenna and the suppressed carrier frequency is at 
a second harmonic, 2f.sub.i, of the interrogation frequency, f.sub.i, 
while the double sideband response signals have frequencies of 2f.sub.i 
.+-.f.sub.d. 
In the illustrated embodiment the response signal generated by the 
transponder is characterized by two or more upper and lower modulated 
sideband response signals each. 
In another embodiment the modulation oscillator generates a plurality of 
data carrier signals, f.sub.di, so that the transponder generates a 
corresponding plurality of upper and lower modulated sideband response 
signals. 
The improvement further comprises a source of power coupled to the 
transponder, a slot antenna, a diode coupled to the slot antenna, and a 
detector circuit coupled to the diode for detecting when an interrogation 
signal is received by the slot antenna and diode. The detector circuit 
generates a detection signal indicative of interrogation of the 
transponder. The interrogation signal is coupled to the transponder to 
change the state of operation of the transponder from a power-save mode 
with minimum power consumption to a more power consuming operating mode. 
The slot antenna is tuned for the interrogation frequency, f.sub.i. 
The invention can also be defined as a method for providing an 
identification response to an interrogation signal comprising the steps of 
generating a data carrier signal having a frequency, f.sub.d, and 
modulating the data carrier signal with an identification code. An 
interrogation signal having a frequency, f.sub.i, is received. The 
modulated data signal at frequency, f.sub.d, and interrogation signal at 
frequency, f.sub.i, are mixed to obtain a suppressed-carrier double 
sideband modulated response signal. The data carrier signal is a radio 
frequency signal and the interrogation signal is a microwave signal. As a 
result, a reliably detectable response signal is generated in the 
transponder. 
The step of generating the data carrier signal is performed at a frequency 
which is a nonharmonic of the interrogation frequency, f.sub.i, so that 
false detections resulting from reflected interrogating signals of 
transmitted harmonics is avoided. 
The method further comprises the steps of receiving the interrogation 
signal at frequency, f.sub.i, detecting the received interrogation signal, 
and activating the transponder from a power-save mode to an operable mode 
where power is consumed when the interrogation signal is received. The 
step of receiving the interrogation signal is received with a tuned slot 
antenna. The step of detecting the interrogation signal is detected in a 
Schottky diode coupled to the tuned slot antenna. 
In one embodiment the step of generating the data carrier signal at the 
data frequency, f.sub.d, comprises generating a plurality of distinct data 
carrier signals, each at a different frequency, f.sub.di, to permit 
subsequent simultaneous interrogation of multiple transponders. 
The invention may now be better visualized by turning to the following 
drawings wherein like elements are referenced by like numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A credit-card sized microwave transponder for "wireless key" and 
surveillance applications uses a subharmonically-pumped quasi-optical 
mixer. The transponder is activated by a C-band interrogation beam to 
upconvert and radiate a digitally modulated identification tone at X-band 
frequencies nonharmonically related to the interrogation signal. 
In the illustrated embodiment a transponder, generally denoted in the 
schematic of FIG. 1 by reference numeral 10, is packaged in the form of an 
ID card (note shown) which is interrogated at frequency f.sub.i (e.g., 6 
GHz), from which it upconverts a locally generated data signal at f.sub.d 
(e.g., 10 MHz) to the response frequencies 2f.sub.i .+-.f.sub.d (11.990 
GHz, 12.010 GHz) which are radiated. The data carrier signal f.sub.i, and 
hence the response signal, is amplitude shift keyed (ASK) modulated with 
an 8-bit identification code, which can be easily expandable to over 32 
bits. The advantages of this scheme include the creation of new microwave 
frequencies an octave apart from the interrogation signal, away from 
interference in the interrogation band, without the need for a microwave 
source on transponder card 10. Unlike in harmonic-generation transponder 
systems, these frequencies are not harmonically related to the 
interrogation signal. Therefore, the problem of false detection resulting 
from an interrogator receiving reflections of its own transmitted 
harmonics is avoided. The carrier tone f.sub.d and the data sequence can 
be used independently or jointly to uniquely identify the card. As the 
response frequency is a direct function of the interrogation frequency, 
this scheme is inherently compatible with a frequency-hopped spread 
spectrum interrogation approach. In addition, the subharmonic 
interrogation method makes this approach attractive for millimeter-wave 
applications, allowing use of lower frequency sources. 
Transponder card 10 is based upon a quasi-optical mixer structure first 
proposed by Stephan and Itoh for imaging arrays and telemetry 
applications. See, K. Stephan and T. Itoh, "A Planar Quasi-Optical 
Subharmonically Pumped Mixer Characterized by Isotropic Converion Loss," 
IEEE Trans. Microwave Theory Tech., vol. MTT-32, pp. 97-102, January 
1984). See also K. Stephan and T. Itoh, "Inexpensive Short Range Microwave 
Telemetry Transponder," Electronic Letters, Vol. 20, No. 21, pp 877-78 
(October 1984). 
This structure is comprised of an antiparallel Schottky diode pair 13 
mounted at the terminals of a planar bow-tie (triangular dipole) antenna 
12 as shown in FIG. 1a, which is an enlarged depiction of portion 1a--1a 
of FIG. 1. The broadband nature of this structure allows it to receive the 
interrogation signal, to mix the interrogation signal with an AC 
modulation signal as controlled by a DC biasing signal, and to transmit 
the response an octave apart on a modulated frequency near the doubled 
interrogation signal which is suppressed. 
The mixer as shown in FIG. 1a is comprised of bow-tie antenna 12 and a pair 
of low-barrier Schottky diodes 13 connected head to tail in parallel 
across the antenna terminals 15. Low frequency modulation as discussed 
below is fed to diodes 13 via supporting wires 17 through ferrite beads or 
spiral inductors 19, which help to confine the RF currents to the antenna 
structure. The bow-tie antenna's terminal impedance is relatively stable 
over an octave bandwidth. Antenna 12 is therefore reasonably efficient 
both as a receiver of the fundamental frequency interrogation signal at 
f.sub.i and as a transmitter of the response signals around the suppressed 
second harmonic at 2.sub.fi. 
Suppose transponder 10 is now irradiated by an interrogation system's 
transmitter with a wave at frequency f.sub.i. Let the modulator, discussed 
below, apply an AC wave of frequency f.sub.d whose peak voltage is chosen 
so as to cause optimum signal frequency mixing in each diode 13. When one 
diode 13 is forward biased, the other diode 13 is cut off and can be 
ignored. As the modulating signal changes sign, diodes 13 alternately 
generate current at 2f.sub.i to create a double-sideband suppressed 
carrier signal with sideband of frequencies 2f.sub.i .+-.f.sub.d. Phase 
reversal between positive and negative modulation peaks is provided by the 
antiparallel connection between diodes 13 and antenna 12. 
Once the modulated second harmonic signal is generated, bow-tie antenna 12 
operates at its upper end frequency range to radiate the signal back to 
the interrogations system's receiver (not shown), where either or both 
sidebands may be demodulated. 
As the antiparallel diode pair 13 has only odd terms in its nonlinear i-v 
curve, it generates only odd order mixing products in response to a 
multitone input. Apart from the linear scattering of the interrogation 
signal, the dominant mixing product is the desired third-order term at 
2f.sub.i .+-.f.sub.d. All even order currents, including the second 
harmonic and DC, circulate only in the loop circuit between by diodes 13. 
In order to conserve the battery power required for the data generation 
circuitry, an additional diode receiver 20 is used to generate a DC 
control signal in response to an interrogation. A microstrip-fed one 
wavelength slot 14 is cut in the ground plane of the card and is 
impedance-matched to a Schottky diode 16 to form a C-band receiver, 
generally denoted by reference numeral 20, with a wide beamwidth 
comparable to that of bow-tie antenna 12. The DC signal from this receiver 
is detected by conventional detection circuitry 22 which generates a 
signal to activate transponder 10 from a power-conserving standby state. 
Low-power CMOS (74HC) was used in conjunction with discrete transistors on 
the card of transponder 10 for generation of the 10 MHz modulated data 
signal from oscillator 18. Operating and standby currents were 1 mA and 25 
.mu.A, respectively, from a +1.5 V silver oxide wristwatch battery 24. 
Reducing the standby current to 1 .mu.A could increase the battery life 
from 1-2 years to 10 years, when interrogated one minute daily. 
The locally generated 10 MHz modulating signal from oscillator 18 was then 
amplitude shift keyed (ASK) by modulator 21. Modulator 21 in turn is 
controlled by an identification code generator 23 which outputs a fixed, 
programmable 8 bit code as timed by bit rate clock 24 operating at 3 
kbaud. Additional or an extended code may be output if desired by means of 
a supplementary code generator 27 also driven and appropriately timed by 
clock 25 and having its output coupled to generator 23 or coupled in 
parallel thereto. 
The frequency spacing between 2f.sub.i and f.sub.d can be varied among a 
plurality of transponders 10 to allow the number of ID code bits to be 
expanded and also to allow the number of distinguishable transponders 10 
to be increased in any given interrogation view. For example, although the 
range of interrogation in the illustrated embodiment is 10 feet, the 
presence of multiple transponders is contemplated. The modulated carrier 
frequencies, f.sub.d, for the transponders can then be spaced at 1 MHz 
intervals to allow for a series of distinguishable transponders carrying 
the same set of codes. A set of 256 transponders can be supported by an 
eight bit ID code. However, if two transponders with different IDs were 
within interrogation view both would respond at the same modulated 
sideband frequencies with different IDs and hence provide an ambiguous 
signal. By offsetting the modulated sideband frequencies from each other, 
multiple sets of 256 transponders can be supported and simultaneously 
interrogated without ambiguity. 
Consider now the operation of transponder 10. Transponder 10 is 
interrogated from an interrogator system (not shown) at a distance of 5 
feet. The transmitted power was 125 mW into a +15.5 dBi pyramidal horn 
(+36 dBm EIRP), and a +22 dBi horn feeds the HP8562A receiver, which with 
preamplifier has a sensitivity of -105 dBm. The scattered response signal 
is shown in FIGS. 2a and b. FIG. 2a shows the carrier suppressed double 
sideband signal retransmitted from transponder 10 in response to an 
interrogation signal, f.sub.i, at 6.003875 GHz. A suppressed center 
frequency 26 at 12.00775 GHz is shown with upper and lower sidebands 28 at 
12.03775 and 11.9775 GHz, which are in turn carrier signals modulated by 
the ASK ID code. An additional second harmonic 29 of upper and lower 
sidebands 28 is also shown and can be used as well. FIG. 2b shows the 
demodulated sideband 28 in which the ID code, 10110101, has been 
transmitted. Therefore, by spacing the modulated data signal, f.sub.d, 
and, if desired, using different ones of the four sidebands, multiple 
simultaneous detection of different sets of transponders can be performed. 
For an eight bit code, each modulated data frequency can then accommodate 
1024 transponders, and the number of transponders which can be 
simultaneously detected expanded to 262,144 transponders with a 16 bit 
code. 
The equivalent circuit was subjected to a harmonic balance analysis to 
determine the effect of the embedding impedances presented to the mixer 
comprised of diodes 13 and bow-tie antenna 12. The devices employed were a 
Hewlett-Packard HSCH-5530 low-barrier Schottky diode pair in a beam-lead T 
package. The measured device parameters in conjunction with published 
impedance data for the 90-degree flared bow-tie were used, and an optimum 
antenna electrical length of 60-100 degrees at 6 GHz was obtained. 
For RF/ID system calculations, the transponder can be modeled as a 
frequency converter with conversion loss Lc, with a receiving antenna 
(gain Gr) and a transmitting antenna (gain Gt) for the interrogation and 
response waves, respectively. Whereas Lc can be obtained via nonlinear 
circuit analysis, only the ratio Gr*Gt/Lc can be measured experimentally. 
It is therefore helpful to define a conversion radar cross section (CRCS), 
for the transponder, equivalent to the conventional radar cross section, 
but with nonequal incident and scattered frequencies corresponding to the 
interrogation and response signals. The following relation then applies: 
EQU CRCS=(wavelength.sup.2)*Gr*Gt/(4*pi*Lc) (1) 
or equivalently in decibels, where CRCS=3 dB cm.sup.2 -Lc for 6 GHz 
interrogation and an isotropic transponder. This quantity can be used 
directly in link calculations, and was measured for both E and H plane 
cuts of the transponder's scattering pattern as shown in FIGS. 3a and b 
respectively. Since the signals applied to the mixer are relatively small 
(&lt;-10 dBm), a convenient expression for Lc can be obtained in terms of 
Volterra kernels for the nonlinear equivalent circuit. This expression 
indicates that the transponder's conversion loss for the weakly nonlinear 
case (away from the interrogator) is inversely proportional to received 
interrogation power. Consequently, the back-scattered response power 
received by the interrogator will vary as 1/r.sup.6, where r is the 
distance to the transponder, rather than the 1/r.sup.4 characteristic of a 
range-independent radar cross section. The resulting maximum range is 
approximately 10 feet as illustrated in FIG. 4 where received response 
power is graphed on line 32 against distance between the interrogation 
system and transponder 10 for the test interrogation system described 
earlier. Line 30 is the -105 dBm receiver sensitivity limit in this 
example. The complex region 34 is the zone where in the test situation of 
this example the interrogation system received multipath response signals 
from transponder 10. Increasing the EIRP, or using an optimum-bandwidth 
receiver could increase this range to over 15 feet. For fixed-frequency 
operation, additional tuning elements may be added to the quasi-optical 
mixer to further reduce the conversion loss. 
The practicality of a self-contained identification transponder using 
subharmonic interrogation is demonstrated by the illustrated embodiment. 
Integrating the ancillary circuitry using applications specific integrated 
circuit (ASIC) technology could reduce size and power requirements to the 
point where the battery could be hermetically sealed inside the card or 
eliminated altogether through beam powering, resulting in a commercially 
viable identification device. 
Many alterations and modifications may be made by those having ordinary 
skill in the art without departing from the spirit and scope of the 
invention. Therefore, it must be understood that the illustrated 
embodiment has been set forth only for the purposes of example and that it 
should not be taken as limiting the invention as defined by the following 
claims. 
The words used in this specification to describe the invention and its 
various embodiments are to be understood not only in the sense of their 
commonly defined meanings, but to include by special definition in this 
specification structure, material or acts beyond the scope of the commonly 
defined meanings. Thus if an element can be understood in the context of 
this specification as including more than one meaning, then its use in a 
claim must be understood as being generic to all possible meanings 
supported by the specification and by the word itself. 
The definitions of the words or elements of the following claims are, 
therefore, defined in this specification to include not only the 
combination of elements which are literally set forth, but all equivalent 
structure, material or acts for performing substantially the same function 
in substantially the same way to obtain substantially the same result. 
In addition to the equivalents of the claimed elements, obvious 
substitutions now or later known to one with ordinary skill in the art are 
defined to be within the scope of the defined elements. 
The claims are thus to be understood to include what is specifically 
illustrated and described above, what is conceptionally equivalent, what 
can be obviously substituted and also what essentially incorporates the 
essential idea of the invention.