AVI TRANSPONDER

A newly developed AVI transponder is to be used in Automati, Vehicle Identification (AVI) system, which provides identification of moving vehicles passing by a certain location as to its registration number, and classification etc. This AVI system is characterized by the fact that it utilizes cerami resonators in the composition of a transponder identification code, and a single microwave carrier to convey an interrogation signal to, and a response signal from, that transponder. The transponder is a completely passive device free of any power supply, and operates as a "Reflector". Overall external dimensions of transponder are 7 10 cm with a weight of 150 grams, the unit is compact, light weight and can be mounted easily on the vehicle, and is easy to carry. All of the circuitry required for the transponder is provided in the form of microstrip lines utilizing MIC (Microwave Integrated Circuit) technology on the PTFE-glass fibre laminate substrate. The transponder circuitry, which consists of printed antenna, microwave modem circuit and medium frequency circuit, is patterned on both surfaces of the single substrate. In this paper, operation principle, circuitry and construction of the transponder will be described.


INTRODUCTION
In recent years, in step with the significant progress of highway and railroad transportation systems, high speed and high volume of traffic has become the general trend. This trend has brought about an urgent need for a means to automatically identify vehicles while in motion, as to their type or vehicle number.
Heretofore, the authors have developed ACI (Automatic Car identification) system for railroads utilizing electromagnetic induction. 1,2 This system has been proven as practical and its usefulness is being assessed. However, there is an inherent drawback to this system in that the transponder cannot be attached directly to a metal surface. Another drawback lies in the short distance of several decimeters, permissible between the road-side loop antenna and the transponder's loop. Such drawbacks have practically precluded application of electromagnetic systems, in the identification of automotive vehicles and containers in motion. Also in the past, several systems utilizing light rays have been developed 3-5 and a few have been put into practical usage; however this system has the drawback of being affected by climatic and environmental conditions such as snow, and dust etc.
With the above described background, the advent of AVI (Automatic Vehicle Identification) systems utilizing microwaves with their inherent advantages, 137 and the development of such systems have come to be conducted on a world-wide basis. [6][7][8] These microwave systems can be grouped into those utilizing microwave cavity resonators, and those utilizing active elements. The former group usually utilizes a plurality of cavity resonators and thereby possesses, the disadvantage of becoming bulky, and of having difficulty in changing their encoding. On the other hand, the latter group has the problem of supplying power to the active elements of the transponder, either by transmitting large amounts of microwave power in order to supply the necessary DC power, or by equipping the transponders with batteries.
The newly developed microwave AVI system is characterized by the transponder. This new transponder has the advantages that it is a completely passive device, small in size, of flat construction, light in weight, and has a considerably long operation range from the interrogator antenna. This has enabled the transponder to provide an ideal system for the identification of automotive vehicles, rail cars and containers.

GENERAL DESCRIPTION OF THE SYSTEM
This system utilizes a combination of "ceramic resonators" to buildup the "encoded" identification code of a passive transponder, and the transmission of the interrogating and responding information is achieved through a microwave carrier. Use of ceramic resonators as identification elements has been put into practice in the past in systems using electromagnetic induction coupling. Figure 1 (a) shows the basic configuration of such a system. In this system the transponder consists of a plurality of ceramic resonators with different resonance frequencies, and the identification code consists of a particular combination of the resonance frequencies. First the interrogator emits an interrogation signal consisting of a sweep-frequency through its transmitting loop-coil, which is then received by the transponder and impressed on its resonators. At this instant the resonators are excited and driven to resonate at their own resonance frequencies, and this generated response signal is next received by the loop-coil. The resonance frequencies in the received signal are detected and the encoded code is identified. On the other hand, in the microwave carrier system, the signal sent and received through the inductively coupled loop-coils in the electro-magnetic system is done so by means of a microwave carrier, so that a large increase in the distance between the interrogator antenna and transponder is made possible. 1) Identification over a considerable distance and range is possible.
2) The transponder is of the completely passive type, is compact, will withstand severe vibrations and shocks and has high reliability under adverse environmental conditions.
3) The transponder can be attached directly to a metal surface. 4) Because of the microwave carrier, complete isolation from electrical engine-noise is possible.
5) The RF microwave power can be relatively small. Figure 2 shows an example of this AVI system. The system is applied for non-stop toll collection system, in which the transponder is mounted inside the windshield of a vehicle, and the interrogator antenna is installed on the canopy of a toll gate.

OPERATION OF THE SYSTEM
Operation of this system is shown in the block diagram of Figure 3. The system consists of the interrogating system; made-up of the interrogator and the antenna unit, and the transponder. System operation is described according to signal flow.

Transmission of the Interrogating Signal
A sweep-frequency signal fs, generated by a sweep-frequency signal generator, modulated the microwave carrier f0 by the modulator, and produces the microwave interrogation signal containing frequency components such as f0 and f0 --f. This microwave interrogation signal is amplified to a suitable level and transmitted through the transmitting antenna toward the transponder.

Response of the Transponder
The transmitted signal is received through the antenna of the transponder and is fed to the diode. As a result of nonlinearity of the diode, the sweep-frequency signal f is demodulated and is applied to the resonators having different resonance frequencies. At this instant, each resonator starts to resonate at its own resonance frequency and produces the "ringing" signal having its own resonance frequency fn (n 1, 2 n).
These ringing signals are again applied to the diode being excited by the microwave carrier f0, to cause components, f0 --fl, f0 ---f2 f0 --fn to be generated.
This forms the response signal which is transmitted through the transponder's antenna toward the receiving antenna of the interrogator.

Receiving the Response Signal
The response signal reflected by the transponder is received by the receiving antenna of the interrogator and is fed to the demodulator. The signal is demodulated at the demodulator which performs "homodyne detection" using the carrier frequency f0 as the reference, and ringing signals with components fl, f2 fn is obtained. In order to effectively detect the resonance points of the transponder, those ringing signals are processed by using a synchronous detector, Figure 4 shows the wave forms during this detection process. The ringing signals which have frequencies fa, f2 fn are synchronously detected by using sweep-frequency signal fs as the reference, and are converted to "ringing beat" signal having the frequencies fs fa, fs f2, f fn, respectively. The "k"th resonating point's ringing beat signal's waveform can be represented by the following equation.
where, V: constant ok: "k"th resonance angular frequency k: "k"th resonance starting time QL: Q of the transponder's resonance circuit h: sweep speed of the sweep-frequency The waveform takes on the same shape for all of the resonance points, thus, these waveforms are converted, one by one into information pulses by means of a "matched filter". These information pulses are located in a row corresponding to the resonance points on the sweep time axis. By processing these pulses,  signal is applied to the ceramic resonators through the transformer, and make the ceramic resonators start "ringing" one by one successively at their own resonance frequencies. Referring to one of the ceramic resonators, the instant when the frequency of the sweep-frequency signal coincides with its resonance frequency, the resonator starts "ringing". The ringing lasts for a period determined by the Q-value of the resonator even after the sweep-frequency signal has passed the resonance frequency point.
Thus the ceramic resonators produce so called "ringing signals" which have different frequencies from each other corresponding to their own resonance frequencies, and as long as the "ringing" continues, the ringing signal currents will flow through the diode. The ringing current produced by the "k"th ceramic resonator can be represented by: exp[-jnh(t r)2] exp(jnht2) (2) where,

I: constant
Since the diode is excited by the microwave carrier component exp(/'o0t) and is in a non-linear state, when ringing current flows through the diode, the microwave carrier will be amplitude-modulated by the ringing current and a modulated signal component irexp(joot) will be generated. This signal component is transmitted through the antenna as the response signal.

Requirements for the Diode
The diode in the transponder is used to modulate and demodulate the interrogating signal and the response signal by means of its non-linear characteristic. Conversion efficiency of the diode depends to a high degree on the carrier wave power that excites it. As a system, a diode that requires a small current for its excitation is desirable and this is one of the factors in its selection. The dependency of the conversion efficiency on the excitation power is determined by the V-I characteristic, and the lower the barrier height the smaller the excitation current. In general the V-I characteristic of a diode can be represented by: is: saturation current 0: constant of the diode If "g" is the conversion conductance, dI g-0ise (4) dV From the above the larger the value of and V, the better the conversion efficiency rises, when the V-I curve becomes steeper, and the excitation voltage becomes higher. However, under actual conditions, Eq.
(3) will not hold in the range of high diode current, and the non-linear characteristic disappears, when a certain voltage is exceeded, the conversion efficiency becomes constant and will not rise. Figure 6 shows typical V-I characteristics of various mixer diodes. Among these, those with the lowest barrier voltage values, or those that can be excited at low voltages are the "zero bias" Schottky diode. In this system, the zero bias Schottky diode was used permitting proper functioning of the diode at a low power level of about -15dBm.

Circuit Configuration
The equivalent circuit of the transponder is shown in Figure 7. In this circuit, two elements that significantly affect the performance of the transponder are, the matching circuit; at the input of the diode, and the transformer; for coupling the ceramic resonator group. the diode and the resonator group is desirable for maximum transfer of the demodulated sweep frequency signal energy, to obtain maximum resonating current. This resonating current has an amplitude that decays exponentially with exp(--tOk/2QL)t, SO that a value of QL is sufficiently high to produce ample ringing is desirable. Proper matching and the value of QL is determined by the design of the transformer. The "k"th resonating element is represented in the resonance circuit model shown in Figure 8 here, current build up starts at time r k and the resonance current I can then be represented as, Esn Ir Q. Q o/2. Therefore, if the value of Q is sufficiently large, then the optimum value for "n" will be x/-7. At this time, Zm will be a function of the carrier wave's power level just as Z and Z b and ZIF will increase when the power level decreases. Thus it becomes necessary to determine transformer ratio "n" in accordance with the value of ZF at low power level, the same as for the "matching circuit".

Conversion Loss of the Transponder
The conversion loss is defined as the ratio between the interrogating signal and the response signal at the input of the transponder circuit. Putting the power level of the interrogating signal s P(f0 --f), and the power level of the response signal or, the "ringing modulated signal" as P(f0 --fr). Then, the conversion loss L can be given by: P(fo +-fr) L 10 log (riB) (7) P(fo +-Actual transponder conversion loss data is presented in Figure 10. The data indicates that the transponder is capable of operating with a carrier wave power level of as low as 15dBm.

Encoding by Ceramic Resonators
This system uses a sweep-frequency signal in the normally, designated in decimal numbers, each decimal resonators, and in order to hold the number of ceramic resonators to a minimum, 5C2 encoding is employed. Table I is the 5C2 encoding system used in this system. Each digit is given 5 different resonance frequencies and by selecting 2 resonance frequencies out of the five frequencies, a decimal number is expressed. Therefore, in order to represent an N digit decimal number, 5N kinds of resonance frequencies will be necessary. Table  II is an example of the "frequency allocation" for the transponder of seven decimal digits.  transponder circuitry are patterned as an integral unit on the single substrate. No adjustments are required so that the unit is easy to produce in large quantities. Furthermore, no connections other than for the diode, transformer, and ceramic resonators mounted on the substrate require soldering so that a high degree of reliability can be expected. The circuit pattern developed for the transponder of this AVI system is shown in Figure 11, and the assembly is shown in Figure 12. The identifying information capacity for this unit, consists of seven digits of decimal number. An external view of the transponder is shown in Figure 13. The antenna substrate, and reflector plate are Fore (slotted antenna) Back (feeder and circuit pattern) FIGURE 11 Transponder circuit pattern. contained in a plastic case. Overall external dimensions are 7 10 1 cm with a weight of 150 grams, the unit is compact, light weight and can be mounted easily on the vehicle, and is easy to carry.

CONCLUSION
The AVI transponder described in this paper is characterized by no-battery operation and simple construction which based on its unique operation principle, and by small light weight, and compactness which are realized by use of MIC (Microwave Integrated Circuit) and HIC (Hybrid Integrated Circuit) technology. This has enabled the transponder to have a wide range of applications in the automatic vehicle identification systems.