By Raymond PageWenzel Associates

This is the Grand Prize winnerin the design category of the 1993RF Design Awards Contest. Thisentry exhibited both innovativeuse of RF technology and anelegant implementation of thattechnology. The author wasawarded a NOISE COM modelUFX-BER noise generator for biterror rate testing.

or some time railroadcompanies have been

wrestling with the problem oftracking rail cars. This hastraditionally required manual logentry of identification numbersdisplayed on the cars as they passthrough the switching yard. Someyears ago, an effort wasundertaken to use an opticallyscanned ID system. Dirt andoptical registration problems led toits demise, forcing railroadcompanies to revert to the manualsystem. RF engineers have comeup with a solution, usingtransponders 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 rechargeablebatteries. The non-rechargeable

batteries require periodicreplacement, and the solar celloption would be both expensiveand vulnerable to the environment.A passive design eliminates theneed for batteries by rectifyingenergy from the interrogating RFfield to power the circuitry. Theharsh environment presented to anRF device mounted on the side ofa rail car is a challenging problem.Minimum clearance requirements,dirt, weather, vibration and anextremely large chunk of ferro-magnetic material near theantenna have to be considered.Additionally, the unit should beencapsulated.. Microstrip patchantennas have come to the rescue.They afford a low profile and canbe made with an ordinary double-sided printed circuit board. Thepatch antenna is on the top and aground plane is on the bottom,thereby eliminating the effects ofthe steel mounting surface.

A Low Cost Transponder An unusually simple method ofconverting the interrogating RF

field into a data-modulated signalwhich can be transmitted back tothe reader contributes to the lowmanufacturing cost of thistransponder design. The circuituses only one inexpensivemicrowave semiconductor (adiode) and allows all parts to bemounted on an FR-4 printed circuitboard with the patch antennas(Figure 1). By contrast, otherapproaches use expensivemicrowave parts, including SAWdevices, oscillators, mixers, filtersand amplifiers. Designs involvingmore RF circuitry tend to be powerhungry, requiring increased RFinterrogation fields. Figure 2 shows the blockdiagram of the low powertransponder. A 915 MHz receiveantenna powers therectifier/frequency doubler/AMmodulator. It provides a rectifiedDC source to the MCU whichreturns data to be AM modulatedonto the doubled frequency. An1830 MHz antenna transmits themodulated carrier. A reader, incorporating an

RF design awards

A Low Power RF ID Transponder

F

Figure 1. The complete transponder, with the 74AC00 testoscillator.

unmodulated 915 MHzinterrogation transmitter with low(< -60 dBc) second harmonicdistortion and an 1830 MHz AMreceiver, is placed a relatively shortdistance away from where thetransponder will pass (Figure 3).The amount of transmitted RFinterrogation power needed tomake the system function properlyat a given distance can beestimated by equation 1:

Pr = PtGtGrλ2/(4πR)2 (1)

Where Pr is the received power,and Pt is the transmitted powerradiated by an antenna of gain Gt.Gr is the gain of the receiveantenna, h is the free spacewavelength and R is the distancebetween transmitters. Gt and Gr

are the gains over an isotropicradiator. A sufficient secondharmonic return path signal willoccur for any combination of powergain and distance capable ofenergizing the MCU. One watt of power transmittedwith an antenna gain of 31.6 (16

dB) and received with an antennagain of 2 (3 dB) allows thetransponder to function from as faras 20 feet away. This suggests thatjust over 1 mW is adequate toenergize the transponder. The transponder's surprisinglylow power requirement is due to itsefficient means of rectification,frequency doubling andmodulation. All of these functionsare accomplished by a singlemicrowave diode. A hybridschematic in Figure 4 details thecircuit. The 915 MHz patchantenna has two connections, aDC return path connected at thezero impedance point and atransmission line matched to the120 ohm impedance at the edge ofthe antenna. The transmission lineroutes the signal to CR1 forrectification. A DC tap on the 1830MHz antenna provides the powerconnection for the MCU. (See sidebar on microstrip patch antennas.) Careful placement of CR1 alongthe transmission line is crucial increating the proper ACimpedances for efficient frequencydoubling. The 1830 MHz antennabecomes a 90 degree open stub at915 MHz at the cathode of CR1,effectively giving the 915 MHzsignal a low impedance trap towork against (Figure 5). Since thetransmission line does not providea similar low impedance on theanode side of CR1, a 90 degreeopen stub at 1830 MHz must beadded. Less than 100 uA are requiredto power the MCU (Figure 6).Consequently, little secondharmonic is produced by CR1,leaving plenty of modulationheadroom. Increased frequencymultiplication occurs when theoutput port of the MCU goes lowproviding a path to ground forrectified current via the 1 kohmresistor, R1. Varying the value ofR1 controls the modulation depth.CR2 and C2 work together tomaintain sufficient voltage to theMCU while the voltage at C1 isbeing pulled down by themodulation action.

915 MHz

1830 MHz

10 '

Readertransceiver

CR1915 MHz

1830 MHz

Trace length 2L(including diode)Zo = 120 ohms

90 open stubat 1830 MHz

MCUC2

C1

R1CR2

+VCCOutputPort

CR1, HP2811CR2, HP5711C1, 100pFC2, 0.1 uFR1, 1KFB1 & FB2, SMT Ferrite BeadMCU, MC68HC04

Test Oscillator

VCC

Figure 4. Hybrid schematic of transponder circuit.

Rectifier

Frequency Doubler

AM Modulator

915 MHz 1830 MHz

MCU

Power Data

Figure 2. Block diagram of thepassive transponder.

Figure 3. RF ID reader andtransponder with rail car.

Performance As previously noted, the systemcan operate up to 20 feet away.However, performance ismeasured at the 10-foot separationrequired during normal operation.For test purposes, a spectrumanalyzer functions as the receiver.A 74AC00 gate oscillator in Figure4 is substituted for the MCU tosimulate load and logic levelconditions. The oscillator simplifiesconfirmation of the concept. ThreekHz modulation is used for easydetection by the analyzer. The transponder transmits dataat 94 percent AM modulation.Measurements of the rectifiedvoltage (2.7 VDC) and current(1.45 mA DC) give 3.9 mW totalpower which correlates nicely withthe received power (5.3 mW)predicted by equation 1 at adistance of 10 feet.

Improvements Inherent compatibility withspread spectrum is provided bythis design since the returnedsignal frequency is derived directlyfrom the interrogation signal.Frequency spreading is limited onlyby the bandwidth of the patchantennas. With the simple additionof a micro-power line receiver andthe associated communicationssoftware, the transponder can beupgraded for two-way informationapplications. Size reduction can beaccomplished by increasingoperating frequency at the expenseof costlier substrate material.Borrowing technology from missileand aircraft radar technology thetransponder could be made a partof the "skin" of its host.

Summary This paper has described thedesign, 'operation and applicationof a low-power RF identificationtransponder. The simple design isspectrum friendly, requiringminimal interrogation power andallows easy conversion to spreadspectrum without modification tothe transponder. Designed withone inexpensive microwave part ona single piece of FR-4 substrate,component and manufacturingcosts are kept down, potentiallyopening up markets servedexclusively by bar codingtechnology. Other uses includeautomatic tolling, inventory trackingand military vehicle security.

References1. Howard W. Sams & Co.,Reference Data for RadioEngineers, Chapter 27, SixthEdition, 1977.2. Robert E. Munson, “ConformalMicrostrip Antennas,” MicrowaveJournal, March 1988, pp. 91-109.3. Alan Tam, “Principles ofMicrostrip Design,” RF Design,June 1988, pp. 29-34.

Our Design Contest Winner Raymond Page is a designengineer for Wenzel Associates, amanufacturer of high performancecrystal oscillators and frequencystandards. Ray specializes in lownoise designs for devices including

oscillators, phase locked frequencysources, multipliers and dividers. Inaddition to having fun withelectronics, he enjoys outdoorsports and music. He can bereached at Wenzel Associates. byequation 1 at a distance of 10 feet.

Appendix A:Rectangular Microstrip PatchAntenna

The rectangular patch antenna isessentially a resonant microstripwith an electrical length of 1/2 thewavelength of the frequency to betransmitted or received. Microstrippatch antennas work well forapplications requiring a low profile,offering a height equal to thethickness of the printed circuitboard from which they are made.PTFE substrates are normally usedto minimize dielectric losses whichaffect the efficiency of patchantennas. However, FR-4 is a costeffective alternative for low powerapplications at frequencies below 2GHz. Microstrip antennas come inall sizes and shapes. A rectangularpatch is chosen for its simplegeometry and linear polarizationwhen fed from the center of anedge. The input impedance variesas a function of feed location. Theedge of a 112 wavelength antennahas an input impedance ofapproximately 120 ohms whichdrops to zero ohms as the feedpoint is moved inboard to thecenter of the antenna. This allowseasy impedance matching andprovides a convenient means ofDC tapping the antenna as seen inthe transponder design. Forsimplicity, the dimensions of themicrostrip patch antennas in Figure9 are in terms of L, which is equalto 1/2 the electrical wavelength ofthe receive antenna (915 MHz). Lcan be determined by equation 2:

L = 0.49 ( λ / εR ) (2)

where λ is the free-spacewavelength and εR is the relativepermittivity of the printed circuitboard. Bandwidth is determined bythe substrate thickness and can beapproximated for an SWR of lessthan 2 by equation 3:

Figure 5. Equivalent AC circuitof transponder showing RFtraps.

Figure 6. Current vs. clock fre-quency for a typical 68HC04MCU.

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.