Electronic transponders are used in a wide variety of applications to store and transmit information. A transponder functions by receiving a transmission request and, in turn, transmitting a response. Typically, this response is an identification signal, often comprising a serial number.

In World War II, transponders were used to identify aircraft. The transponder assured the requesting aircraft that the associated aircraft was a friendly aircraft by transmitting an identification code. Early versions of electronic transponders supplied power by way of a battery or by a solenoid.

However, batteries and solenoids are relatively large, and therefore severely restrict the ability to reduce the size of electronic transponders. An antenna external to the transponder broadcasted identification information. This external antenna was necessary to generate an RF signal strong enough to be received and demodulated by a receiver. An external antenna, however, further increases the size of the transponder.

Today, transponders are used for a variety of purposes ranging from identification of wildlife to electronic article surveillance (EAS). Typically, transponders utilize a radio frequency identification (RFID) system. These systems operate without visual contact. For example, EAS systems typically employ a closed loop of a conductive substance that responds to a generated radio frequency (RF) field. These transponders, also called tags due to their ability to"tag"a consumer item to prevent shoplifting, are deactivated when a product is purchased. EAS tags are typically passive elements that respond only when placed in the appropriate RF or magnetic field. More advanced EAS systems transmit a description of the item to which the tag is affixed.

Transponders are also beneficial for applications where it is highly

desirable to reduce the size of the transponder to very small dimensions. For example, electronic transponders aid in the detection of biomolecules in samples when performing solid-phase assays. United States Patents 5,641,634,5,736,332,5,981,166, and 6,001,571 disclose the use of transponders for detecting biomolecules, determining the sequence of nucleic acids, screening chemical compounds, and performing multiplex assays for nucleic acids, and are herein specifically incorporated by reference. For these applications, the transponder must be significantly reduced in size.

When used in chemically hostile environments, as those often used in solid-phase assays, external antennas and power sources utilized in earlier prior art transponders needed to be protected. Therefore, the entire transponder, including the power source and antenna, would be enclosed in a protective material, such as a glass bead. This enclosure further added to the size of the transponder.

As disclosed in United States Patent No. 5,641,634, miniature transponders, also referred to as microtransponders, using photovoltaic cells to provide power have been developed. Photo-activated transponders enable smaller dimensions. Further, by providing a monolithic assembly that includes an antenna, the transponder disclosed in 5,641,634 further enables a reduction in size.

These transponders are typically formed on a silicon wafer and protected by a thin layer of silicon dioxide (SiO2). Si02 has the same chemical properties as glass with respect to chemically hostile environments.

Therefore, the transponder does not need to be enclosed in a glass encasement. Alternatively, the transponder may be coated with a variety of transparent or semitransparent materials, such as plastic or latex.

In many applications, it is desirable to have a small transponder that will output identification data. It is further desirable to create a purely passive device that does not depend on the operation of self-contained batteries.

Photo-activated transponders provided an advantage over the prior art due to their inactivity without light illumination. A narrowly focused laser light source may enable a single transponder at a time, even when a large number of

transponders are present in the assay. Only the illuminated transponder transmits data and other transponders are inactive. The reduction in the number of transmitting transponders significantly reduces noise level. If the light source is more broadly applied, an increased number of transponders may respond. Thus, the light source can be adjusted to control which transponders will respond during an assay.

SUMMARY OF THE PRESENTLY PREFERRED EMBODIMENT Notwithstanding the improvements provided by the monolithic photo- activated transponders previously disclosed, there remains a desire to further reduce the size of transponders and increase the accuracy of the transponder's transmission.

Prior art light-powered transponders utilize self-contained oscillators.

One function of these local oscillators is to provide a clock signal that drives the on-board circuitry and/or the transponder's transmission frequency.

Unfortunately, these oscillators are very sensitive to environmental conditions.

Temperature changes alone may dramatically increase frequency instability.

This broader range of transmitted frequencies, in turn, diminishes the signal quality of a system utilizing transponders.

If the transponders generate a wider range of frequencies, the receiver detecting signals from the transponders must operate with a wider bandwidth.

This wider bandwidth results in a lower signal-to-noise ratio. Consequently, the instability of local oscillators creates overall system degradation.

Additionally, a local oscillator in the transponder increases power consumption.

It would be beneficial to provide a manner in which input signals may be received by the transponder. In particular, it would be beneficial to allow for the transmission of a signal from which a clock signal can be generated; thereby eliminating the need for a local oscillator.

The present invention utilizes a modulated light signal. The modulated light signal may be separated into two components: a carrier signal and a modulating signal. In the presently preferred embodiment, the modulating

component of the light signal is used to generate a stable clock signal. A square wave or other periodic signal component is converted to an electrical clock signal that drives the on-board circuitry of the transponder. The frequency at which the transponder operates is controlled by the light source.

Amplitude modulated laser light may be applied to the transponder. A photosensitive element collects the laser light and generates an electrical signal. One photodiode is used for generating a clock signal and other photodiodes are used to supply power. Alternatively, one or more photodiodes may be used for both functions.

After the photosensitive element collects the laser light, a demodulation circuit isolates the modulating component of the light signal.

Clock circuitry constructs an appropriate clock signal from the isolated modulating component. This clock signal drives the identification data circuitry, which produces a unique series of bits. This series of bits is then encoded and modulated. Finally, the antenna on the transponder transmits the modulated signal using radio frequency (RF) signals.

The light source transmits an amplitude modulated signal that provides the basis for the clock signal used in the transponder. The on-board clock signal generator filters out noise and ensures that a proper waveform is generated. Inconsistencies present in a local oscillator based system are eliminated because the transponder constructs the clock signal from the modulated component of the light signal. Thus, the clock signals are uniform throughout a plurality of transponders. Further, these clock signals drive the on-board circuitry that generates the output transmitted through the antenna.

Consequently, variation in transmission frequency is also significantly reduced.

Additionally, the transponder may utilize the modulated light signal for a variety of purposes. The light signal may enable, affect, control, or modify the output transmission. One embodiment may utilize the detection of a sine wave, for example, to change the format of the signal transmitted through the transponder's antenna. In another embodiment, the detection of a triangular wave, for example, may yield an adjustment of the number of bits which are

read from memory. In yet another embodiment, the detection of a frequency or frequency range may affect the output signal generated by the transponder.

A plurality of modulated light signals may be used. The transponder may receive several modulated light signals, each of which may be used to operate or adjust the operation of the transponder.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A is a graphical representation of an incoming light signal in one embodiment.

FIG. 1 B is a graphical representation of a generated clock signal in one embodiment.

FIG. 1 C is a graphical representation of an RF signal transmission in one embodiment.

FIG. 2. is a top view of one embodiment of a transponder.

FIG. 3 is a block diagram of an embodiment of a monolithic, photo- activated transponder.

FIG. 4 is a block diagram of one embodiment of a system utilizing a photo-activated transponder.

FIG. 5 is a schematic diagram of one embodiment of a transponder.

FIG. 6 is a schematic diagram of the clock recovery circuit in the embodiment of FIG 5.

FIG. 7 is a schematic diagram of a logic circuit in the embodiment of FIG. 5.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS An amplitude modulated light signal is used as a source for a clock signal. The generated clock signal drives an internal memory. The contents of the memory are then accessed, and an output signal is transmitted through a radio frequency (RF) transmission. Figures 1 A through 1C depict representations of waveforms present in the operation of one embodiment.

As shown in Figure 1A, a modulated light signal 10 is formed by multiplying a

carrier signal with a modulating signal 20. In this case, the carrier signal is a sine wave. In the alternative, other types of carrier signals may be used at a variety of frequencies. The modulated light signal 10 may provide power and is the basis for an internal clock signal.

The modulated light signal 10 is converted into an electrical signal. A component representative of the modulating signal 20 is isolated. In one embodiment, the modulating signal 20 is similar to a clock signal waveform.

The similarity of the two signals reduces circuit complexity. The isolated signal is converted into an electrical clock signal operative to drive internal logic.

Referring now to Figure 1 B, a clock signal is depicted. This clock signal is composed of alternative high level 30 and low level 40 signals of equal periods. This clock signal drives the internal logic. The electrical clock signal is generated by adjusting the levels of the isolated signal and filtering to remove noise. In other embodiments, level adjusting and/or filtering may not be provided. In addition, other embodiments may require additional circuitry to generate an operative clock signal. In one embodiment, circuitry may isolate the peaks and troughs of a simple sine wave and then generate an acceptable square wave. In another embodiment, a circuit may be designed to identify various points of another periodic waveform, which may be used to generate a square wave. Since the clock signal is derived from an external source, a plurality of transponders may operate at the same frequency.

An identification number is recalled from memory after the activation of the transponder by the incoming light and generation of a clock signal. A memory stores an identification number represented by a series of bits. For example, a memory may store the hexidecimal number 5D. In the memory, this would be identified as 01011101. Next, this number is transmitted to a receiver that identifies the transponder's identification number. In the one embodiment, the transponder outputs a radio frequency (RF) wave generated from the clock signal. Alternatively, the output RF may be different than the clock signal, such as a frequency that is a multiple or division of the clock signal.

Figure 1 C depicts an RF transmission with the bits 01011101 stored in memory. In the one embodiment, phase shift key modulation is used. The transmitted signal is either sent in phase or 180 degrees out of phase with the clock signal. If the bit transmitted is a one, an out of phase waveform 50 is produced. If the bit transmitted is a zero, an in phase waveform 60 is produced. In the presently preferred embodiment, a preamble code is sent before the transmission of the identification number. This preamble identifies to the receiver when this data string begins. Alternative data formats may be used.

Numerous other signal methods of transmitting data from the transponder to a receiver may be used. For example, in one embodiment, an amplitude modulation technique similar to that represented in Figure 1A is used. In another embodiment, the transponder generates an output signal to represent a one bit, while failing to generate an output signal to represent a zero bit or vice versa. In yet another embodiment, differing frequencies may be utilized wherein each frequency represents a number or data string. In yet another embodiment, two significantly different waveforms may be used to distinguish between data elements, such as a one and a zero bit. Other methods may also be used for the transmission of the contents of a memory.

The transponder communicates externally using two modes of communication. The transponder receives a light signal with carrier and modulating components. In response, the transponder transmits a modulated radio frequency signal. By utilizing both optical and RF waves, separate modes of communication are used. In another embodiment, the transponder may generate any form of electromagnetic radiation. For instance, the transponder may generate a microwave signal.

Figure 2 shows a top view of one embodiment of a transponder 100.

This embodiment comprises a photovoltaic cell 110, a sync logic circuit 120, identification data ROM 130, a read logic circuit 140, a modulator 150, and an antenna 160. The photovoltaic cell 110 is a photosensitive element that receives the light signal and converts it to an electric signal. For example, the photovoltaic cell 110 may comprise a photodiode. The sync logic circuit 120

receives the modulating component of the light signal and constructs a clock signal. The identification data ROM 130 stores an identification number for the transponder. Driven by the clock signal, the read logic circuit 140 accesses the identification data ROM 130 to retrieve the unique identification number. Next, the modulator 150 modulates the encoded signal. Finally, the antenna 160 transmits the modulated RF signal.

Figure 3 shows a block diagram depicting the interconnections of various elements in the transponder of Figure 1. Many of these elements are also shown in the schematics incorporated into Figures 5 through 7. The transponder includes a photocell 200, a threshold circuit 210, a demodulator 220, a clock signal generator 230, an identification data circuit 240, an encoder 250, a modulator 260, and an antenna 270.

The photocell 200 is a photosensitive element that converts light into electrical signal by absorbing the energy of photons and, in turn, creating electron-hole pairs. In one embodiment, the laser light is in the visible spectrum. Alternative embodiments and associated photocells 200 may utilize ultraviolet or infrared light sources.

In addition, a plurality of photocells made be used. Each individual photocell may be assigned a different role. For example, one photocell receives the modulating component of the light signal and other photocells supply power to the transponder. In another embodiment, a plurality of photocells may be used to receive a plurality of modulated signals. In yet another embodiment, one photocell may be used to supply power and isolate one or more modulating signals.

When the photocell 200 is utilized to provide power to the transponder, the rate of electron-hole pair generation is not a concern, so long as sufficient power is supplied. When detecting amplitude modulated light, however, saturation in the photocell 200 is considered. If the photocell 200 generates electron-hole pairs too fast, the electron-hole pairs accumulate, and the photocell 200 may reach a saturation point. Saturation may prevent the generation of the proper waveform of the modulating signal 20. For example,

if saturation occurs, a clipped signal may be produced.

As depicted in Figure 3, the photocell 200 is connected with a threshold circuit 210 to avoid saturation. As used herein,"connected with"is defined to encompass both direct coupling between two components and indirect connection through one or more additional components. This threshold circuit 210 ensures that the electron-hole pairs generated by the photocell 200 are removed at an appropriate rate to avoid saturation. If electron-hole pairs are removed too fast, the waveform of the modulated component of the laser light may be distorted. In this case, the removal of electron-hole pairs may preclude any detectable variation in quantity.

The threshold circuit 210 may be implemented by connecting a resistor in parallel to the photodiode. If the photocell 200 and threshold circuit 210 operate correctly, the instantaneous value of the output signal of the threshold circuit 210 is proportional to the instantaneous intensity of the light.

In alternative embodiments, the threshold circuit is not used.

The output of the threshold circuit 210 is coupled to a demodulator 220. The demodulator 220 isolates the electrical component that corresponds to the modulating signal 20 in Figure 1A. The demodulator 220 is comprised of a comparator and a low pass filter. The demodulator 220 operates to extract the square wave component of the electrical signal. The demodulator 220 is implemented by coupling one input to the output of the threshold circuit 210 and coupling the other input to a low pass filter, which is coupled to the output of the photocell 200 or the threshold circuit 210. The low pass filter blocks the higher frequency components created, yielding a base DC component to one input of the comparator. The comparator amplifies the difference of the two signals to isolate and increase the amplitude of the modulated component.

The clock signal generator 230 is coupled to the output of the demodulator 220. The clock signal generator 230 filters out high frequency noise, constructs a square waveform, and shifts the level of the signal to be within acceptable logic circuit levels. In this embodiment, the modulating

component of the optical signal is a square wave signal similar to the clock signal outputted by the clock signal generator 230. In another embodiment, the modulating component comprises a sine, cosine, or other periodic waveform. Both the demodulator 220 and the clock signal generator 230 may be contained in a single circuit, as shown in Figure 6 and discussed below.

The identification data circuit 240 is driven by the clock signal generated by the clock signal generator 230. The identification data circuit 240 is comprised of two binary counters and a ROM. These components are described below with reference to Figure 7. Alternatively, the identification data circuit 240 may comprise any memory capable of storing and outputting a data string. The identification data circuit 240 outputs a data signal.

The encoder 250 prepares the data signal for modulation and transmission. The encoder 250 isolates the beginning of the data stream outputted by the identification data circuit 240 and inserts a start of frame pattern to the bit stream. The data outputted by the identification data circuit 240 is otherwise unaltered. Alternatively, the bit stream includes the header.

In another alternative, the encoder 250 compresses, transforms, or otherwise modifies the data signal to optimize the output of the transponder.

The encoder 250 and the clock circuitry 230 are coupled with a modulator 260. The design of the modulator 260 is dependent on the type of modulation utilized for transmission in the transponder. For phase shift key modulation, the modulator 260 comprises an exclusive-or gate followed by a modulation switch.

The modulation switch utilized in the modulator 260 is coupled with an antenna 270. When the modulation switch is on, current flows through the antenna 270. When the modulation switch is off, the flow of current stops.

Thus, an alternating current in the antenna is produced and associated electromagnetic field fluctuations are transmitted as an RF signal. As shown below in Figure 7, the identification data circuit 240, the encoder 250, and the modulator 260 may be contained in one circuit.

In another embodiment, additional information may be transmitted in

addition to the clock signal. For example, an additional amplitude modulated signal is transmitted at a different frequency and isolated through band-pass filters or other techniques familiar to one skilled in the art. This additional signal may be utilized to alter the data stream sent to the encoder 250 and/or the encoding format utilized. In the alternative, this additional signal may be utilized to modify the frequency of the RF transmission.

Figure 4 shows a block diagram of a system utilizing the transponder.

The oscillator 300 generates a frequency for the light source 310. The oscillator 300 is coupled with the modulation input of the light source 310 and the receiver 340. The oscillator 300 may be implemented with a set waveform output or by using a programmable signal generator. In the present embodiment, the oscillator 300 comprises a Hewlett-Packards 8116A Pulse/Function Generator set to produce a square wave output.

The light source 310 in the present embodiment is a laser light. The present embodiment uses a ThorLabs, Inc. #0221-202-00 laser light module powered by a Hewlett-Packards 6215A regulated DC power supply. The light source 310 transmits a modulated light signal to the transponder 320. The light source transmits an amplitude modulated light signal at 670 nanometers, but other wavelengths may be used.

The light signal is received by the transponder 320. One embodiment of the transponder 320 uses the light signal as a power and a clock signal source in order to ensure a precise operating frequency. The transponder 320 responsively generates a radio frequency transmission. The transponder transmission comprises a preprogrammed identification number.

The transmission is received by the receiver antenna 330. The receiver antenna 330 is coupled with the receiver 340. The receiver 340 can be implemented in a variety of ways. In one embodiment, a spectrum analyzer is used. In another embodiment, an oscilloscope is used. In yet another embodiment, the receiver 340 comprises a decoding circuit that analyzes the transponder transmission in order to store the transmission data in memory. This decoding circuit may further be connected with a computer

database to evaluate assay results.

Figures 5 through 7 show schematics of one embodiment of a transponder. Figure 5 is an overall schematic. The photodiode 400 is used for isolating the modulating signal 20 shown in Figure 1. The photodiode 400 is coupled with a clock recovery circuit 410, which in turn connects with a logic block circuit 420. The clock recovery circuit 410 comprises both the demodulator 220 and the clock signal generator 230 of Figure 3. The logic block circuit 420 comprises the identification data circuit 240, the encoder 250, and the modulator 260 of Figure 3. The logic block circuit 420 receives input from the clock recovery circuit 410 and from the fusible links 430. In this embodiment, the memory comprises fusible links 430 and is a read-only memory (ROM). The logic block circuit 420 outputs the encoded and modulated signal to the antenna 440 for RF transmission.

Referring now to Figures 3,5, and 6, Figure 6 shows a schematic of a clock recovery circuit 410. The clock recovery circuit input 500 is connected with the output of the photodiode 400. The clock recovery circuit 410 contains CMOS transistors and amplifiers for implementing both the demodulator 220 and the clock signal generator 230. The demodulator 200 in the clock recovery circuit 410 uses a comparator and a low pass filter to isolate the modulating signal 20. The clock signal generator 230 in the clock recovery circuit 410 shifts the levels of the isolated signal and uses a low pass filter to reduce unwanted noise. The clock signal generated is outputted through clock recovery circuit output 510.

Referring now to Figures 3,5,6, and 7, Figure 7 shows a schematic of the logic block circuit 420. The logic block circuit input 600 connects with the clock recovery circuit output 510. The logic block circuit 420 is driven by the clock signal generated by the clock signal generator 230. The logic block circuit implements the identification data circuit 240, the encoder 250, and the modulator 260. The identification data circuit 240 is comprised of two binary counters and a memory. In the presently preferred embodiment, the memory is comprised of the fusible links 430. The first binary counter 610 is three bits

long and is comprised of three D-type flip flops. This first binary counter 610 generates a bit rate frequency of one eighth of the modulation frequency.

The second binary counter 620 is six bits long and implemented through the use of six D-type flip flops. This second binary counter 620 counts at the bit rate frequency. The output of the second binary counter is coupled with the decoder 630 which is in turn coupled to the fusible links 430. The fusible links 430, comprising the ROM, are organized as sixty-four addresses, each address holding one bit of data. As the second binary counter circulates through the sixty-four possible outputs, the decoder 630 outputs a corresponding data output. Each transponder ROM is individually programmed to produce a unique identification number for each transponder.

The content of the ROM is repeatedly read. In the alternative, a transponder ROM may be identical to another transponder ROM.

The output of the decoder 630 is sent to the encoding logic 640, which inserts start of frame data. Next, the encoded signal is sent to the modulating logic 650, which in the presently preferred embodiment comprises an exclusive-or gate with one input connected with the clock signal 600 and the other input connected with the data stream generated by the previously discussed elements of Figure 7. As discussed earlier, the modulation technique used in the presently preferred embodiment generates an output signal that is either in-phase or out-of-phase of the recovered clock signal.

The modulated signal is sent to the antenna 440 via logic block circuit output 660.

Through the use of a modulated light signal, the transponder generates a stable, consistent clock signal. Consequently, the frequency of the signal transmitted by the transponder remains within a narrow range and allows a narrow bandwidth to be utilized in receiving the transmitted signal. In other embodiments, other modes of modulation may be utilized for either the light signal received or the RF signal transmitted by the transponder.

It is to be understood that a wide range of changes and modifications to the embodiments described above will be apparent to those skilled in the

art and are contemplated. It is, therefore, intended that the foregoing detailed description be regarded as illustrative rather than limiting, and that it be understood that it is the following claims, including all equivalents, that are intended to define the spirit and scope of the invention.