RF design awardsA Low Power RF ID TransponderBy Raymond Page batteries require periodic field into a data-modulated signalWenzel Associates replacement, and the solar cell which can be transmitted back to option would be both expensive the reader contributes to the low This is the Grand Prize winner and vulnerable to the environment. manufacturing cost of thisin the design category of the 1993 A passive design eliminates the transponder design. The circuitRF Design Awards Contest. This need for batteries by rectifying uses only one inexpensiveentry exhibited both innovative energy from the interrogating RF microwave semiconductor (ause of RF technology and an field to power the circuitry. The diode) and allows all parts to beelegant implementation of that harsh environment presented to an mounted on an FR-4 printed circuittechnology. The author was RF device mounted on the side of board with the patch antennasawarded a NOISE COM model a rail car is a challenging problem. (Figure 1). By contrast, otherUFX-BER noise generator for bit Minimum clearance requirements, approaches use expensiveerror rate testing. dirt, weather, vibration and an microwave parts, including SAW extremely large chunk of ferro- devices, oscillators, mixers, filtersF or some time railroad magnetic material near the and amplifiers. Designs involving companies have been antenna have to be considered. more RF circuitry tend to be powerwrestling with the problem of Additionally, the unit should be hungry, requiring increased RFtracking rail cars. This has encapsulated.. Microstrip patch interrogation fields.traditionally required manual log antennas have come to the rescue. Figure 2 shows the blockentry of identification numbers They afford a low profile and can diagram of the low powerdisplayed on the cars as they pass be made with an ordinary double- transponder. A 915 MHz receivethrough the switching yard. Some sided printed circuit board. The antenna powers theyears ago, an effort was patch antenna is on the top and a rectifier/frequency doubler/AMundertaken to use an optically ground plane is on the bottom, modulator. It provides a rectifiedscanned ID system. Dirt and thereby eliminating the effects of DC source to the MCU whichoptical registration problems led to the steel mounting surface. returns data to be AM modulatedits demise, forcing railroad onto the doubled frequency. Ancompanies to revert to the manual A Low Cost Transponder 1830 MHz antenna transmits thesystem. RF engineers have come An unusually simple method of modulated carrier.up with a solution, using converting the interrogating RF A reader, incorporating antransponders mounted on the sideof the cars which are read byinterrogating transceiverspositioned along the track.Design Considerations A practical transponder designmust include minimal maintenance,a rugged low profile and low cost.The most elusive of these hasbeen low cost. Presented here is adesign which meets theserequirements along with a briefdiscussion on the current state-of-the-art in passive RF identificationtransponders. An important designconstraint is that the transponderrequire little or no maintenance.Since no power is available fromthe rail car, the only conventionaloptions are batteries or solar cellsthat maintain rechargeable Figure 1. The complete transponder, with the 74AC00 testbatteries. The non-rechargeable oscillator.
dB) and received with an antenna915 MHz 1830 MHz unmodulated 915 MHz gain of 2 (3 dB) allows the interrogation transmitter with low transponder to function from as far (< -60 dBc) second harmonic as 20 feet away. This suggests that Rectifier distortion and an 1830 MHz AM just over 1 mW is adequate to Frequency Doubler receiver, is placed a relatively short energize the transponder. AM Modulator distance away from where the The transponders surprisingly Power Data transponder will pass (Figure 3). low power requirement is due to its The amount of transmitted RF efficient means of rectification, interrogation power needed to frequency doubling and MCU make the system function properly modulation. All of these functions at a given distance can be are accomplished by a single estimated by equation 1: microwave diode. A hybridFigure 2. Block diagram of the schematic in Figure 4 details thepassive transponder. Pr = PtGtGrλ2/(4πR)2 (1) circuit. The 915 MHz patch 10 antenna has two connections, a Where Pr is the received power, DC return path connected at the and Pt is the transmitted power zero impedance point and a 915 MHz radiated by an antenna of gain Gt. transmission line matched to the Gr is the gain of the receive 120 ohm impedance at the edge of antenna, h is the free space the antenna. The transmission line Reader 1830 MHz transceiver wavelength and R is the distance routes the signal to CR1 for between transmitters. Gt and Gr rectification. A DC tap on the 1830 are the gains over an isotropic MHz antenna provides the power radiator. A sufficient second connection for the MCU. (See side harmonic return path signal will bar on microstrip patch antennas.) occur for any combination of power Careful placement of CR1 along gain and distance capable of the transmission line is crucial inFigure 3. RF ID reader and energizing the MCU. creating the proper ACtransponder with rail car. One watt of power transmitted impedances for efficient frequency with an antenna gain of 31.6 (16 doubling. The 1830 MHz antenna becomes a 90 degree open stub at 915 MHz at the cathode of CR1, effectively giving the 915 MHz Trace length 2L signal a low impedance trap to 90 open stub (including diode) work against (Figure 5). Since the at 1830 MHz Zo = 120 ohms transmission line does not provide a similar low impedance on the anode side of CR1, a 90 degree open stub at 1830 MHz must be 1830 MHz CR1 added. 915 MHz Less than 100 uA are required to power the MCU (Figure 6). Consequently, little second harmonic is produced by CR1, C1 leaving plenty of modulation CR2 headroom. Increased frequency R1 multiplication occurs when the output port of the MCU goes low +VCC MCU providing a path to ground for Output Test Oscillator C2 rectified current via the 1 kohm Port resistor, R1. Varying the value of VCC R1 controls the modulation depth. CR1, HP2811 CR2 and C2 work together to CR2, HP5711 maintain sufficient voltage to the C1, 100pF MCU while the voltage at C1 is C2, 0.1 uF being pulled down by the R1, 1K modulation action. FB1 & FB2, SMT Ferrite Bead MCU, MC68HC04Figure 4. Hybrid schematic of transponder circuit.
Performance Improvements oscillators, phase locked frequency As previously noted, the system Inherent compatibility with sources, multipliers and dividers. Incan operate up to 20 feet away. spread spectrum is provided by addition to having fun withHowever, performance is this design since the returned electronics, he enjoys outdoormeasured at the 10-foot separation signal frequency is derived directly sports and music. He can berequired during normal operation. from the interrogation signal. reached at Wenzel Associates. byFor test purposes, a spectrum Frequency spreading is limited only equation 1 at a distance of 10 feet.analyzer functions as the receiver. by the bandwidth of the patchA 74AC00 gate oscillator in Figure antennas. With the simple addition Appendix A:4 is substituted for the MCU to of a micro-power line receiver and Rectangular Microstrip Patchsimulate load and logic level the associated communications Antennaconditions. The oscillator simplifies software, the transponder can beconfirmation of the concept. Three upgraded for two-way information The rectangular patch antenna iskHz modulation is used for easy applications. Size reduction can be essentially a resonant microstripdetection by the analyzer. accomplished by increasing with an electrical length of 1/2 the The transponder transmits data operating frequency at the expense wavelength of the frequency to beat 94 percent AM modulation. of costlier substrate material. transmitted or received. MicrostripMeasurements of the rectified Borrowing technology from missile patch antennas work well forvoltage (2.7 VDC) and current and aircraft radar technology the applications requiring a low profile,(1.45 mA DC) give 3.9 mW total transponder could be made a part offering a height equal to thepower which correlates nicely with of the "skin" of its host. thickness of the printed circuitthe received power (5.3 mW) board from which they are made.predicted by equation 1 at a Summary PTFE substrates are normally useddistance of 10 feet. This paper has described the to minimize dielectric losses which design, operation and application affect the efficiency of patch of a low-power RF identification antennas. However, FR-4 is a cost transponder. The simple design is effective alternative for low power spectrum friendly, requiring applications at frequencies below 2 minimal interrogation power and GHz. Microstrip antennas come in allows easy conversion to spread all sizes and shapes. A rectangular spectrum without modification to patch is chosen for its simple the transponder. Designed with geometry and linear polarization one inexpensive microwave part on when fed from the center of an a single piece of FR-4 substrate, edge. The input impedance varies component and manufacturing as a function of feed location. The costs are kept down, potentially edge of a 112 wavelength antenna opening up markets served has an input impedance ofFigure 5. Equivalent AC circuit exclusively by bar coding approximately 120 ohms whichof transponder showing RF technology. Other uses include drops to zero ohms as the feedtraps. automatic tolling, inventory tracking point is moved inboard to the and military vehicle security. center of the antenna. This allows easy impedance matching and References provides a convenient means of 1. Howard W. Sams & Co., DC tapping the antenna as seen in Reference Data for Radio the transponder design. For Engineers, Chapter 27, Sixth simplicity, the dimensions of the Edition, 1977. microstrip patch antennas in Figure 2. Robert E. Munson, “Conformal 9 are in terms of L, which is equal Microstrip Antennas,” Microwave to 1/2 the electrical wavelength of Journal, March 1988, pp. 91-109. the receive antenna (915 MHz). L 3. Alan Tam, “Principles of can be determined by equation 2: Microstrip Design,” RF Design, June 1988, pp. 29-34. L = 0.49 ( λ / εR ) (2) Our Design Contest Winner where λ is the free-spaceFigure 6. Current vs. clock fre- Raymond Page is a design wavelength and εR is the relativequency for a typical 68HC04 engineer for Wenzel Associates, a permittivity of the printed circuitMCU. manufacturer of high performance board. Bandwidth is determined by crystal oscillators and frequency the substrate thickness and can be standards. Ray specializes in low approximated for an SWR of less noise designs for devices including than 2 by equation 3:
BW = I28 f2 t (3)BW is in MHz, f is the operatingfrequency in GHz, and t is thesubstrate thickness in inches.Applying equations 1 and 2 to thetransponder design using 0.125inch FR-4 substrate material withan effective permittivity of 4.7results in a value of 2.92 inches forL and a bandwidth of 13.4 MHz at915 MHz.