ib technology 1 data sheet antenna_1356.pdf 6 pages last revised 18/02/08 micro rwd mf (mifare) antenna specification the microrwd mf (mifare) module has been designed t o interface to a simple high q antenna coil of around 1uh inductance together with some capacitor and resistor components. the antenna coil and passive components form the tuned rlc (resistor- inductor-capacitor) circuit, which is designed to r esonate at the 13.56 mhz carrier frequency and have a q factor of 35-40. for the 125 khz family of microrwd modules, the low q antenna (700uh) has a relatively wide tolerance of inductance so the capacitor components are fixed and mounted on t he rwd module itself C pins an1 and an2 connect to an external antenna coil directl y. for the 13.56 mhz microrwd modules, the antenna q is higher and the inductance tolerance is narrow. for this reason the capacitor components are external to the module to allow fine-tuning and adjustment of the lc circuit as shown below. the external resi stors are used to limit the antenna current and correct the antenna q to 35-40 range. for maximum range and performance the following fac tors should be considered: 1) maximum range and coupling between transponder a nd rwd is based on the ratio of their antenna diameters. very approximately the rwd antenna loop diameter should be 2-3 times the diameter of the transponder coil. the basic method of communication is via magnetic flux linkage (like an air-cored transformer) so the more lines of flux that intersect the transponder c oil, the better the overall performance. for iso card transponders there is lit tle benefit in using an rwd antenna larger than 10cm diameter. circular antenna coils generally give a more uniform flux distribution microrwd mf an1 (pin 9) an2 (pin 12) approx 3cm max gnd (pins 7/13) centre tap not required for simple antenna 65mm diameter coil, 2 turns 0.45mm diameter enamelled cu wire (26 swg), approx 1uh 5-50pf trimming capacitor c2 = 22pf c1 = 22pf c3 = 220pf c4 = 220pf directly connected antenna arrangement all capacitors ceramic 50v minimum npo/cog t ypes +/- 5% or better rext rext typically < 0.5r
ib technology 2 2) the micro rwd is designed to generate a 13.56 mh z carrier frequency in short bursts of energy with a peak-to-peak voltage of up to 60 volts. this rf field is only turned-on whenever there is card communication and for the remainder of time the rf field is off. this dramatically reduces the aver age current consumption to around 20ma with short bursts of up to 200ma. 3) generally for 65mm diameter wound antenna or sim ilar sized pcb antenna the range is around 5 cm, larger antenna (10 C 15cm dia meter) can achieve up to 10cm range under ideal conditions. lower burst current c an be achieved by adjusting the series resistors in the antenna loop circuit (ensur ing q value is around 35-40). 4) sample antenna supplied are for evaluation only. the characteristics of an antenna for emc approval will vary according to shape of co il, type of wire, style of winding, bobbin material, spacing between windings or pcb track design and pcb material etc. 5) the antenna coil (pcb or wire wound) should be p ositioned as close to the capacitor components as possible and the coil-capacitor (lc) arrangement should be no more than 3cm from the microrwd module for optimum perfo rmance. for longer distances a screened twisted-pair antenna cable can be used with an impedance matching transformer, this technique is not describ ed here. 6) the position and environment of the antenna in t he final applications should be taken into account and a trimming capacitor can be used to adjust the capacitance to fine tune the lc arrangement back to 13.56 mhz reso nance. an oscilloscope should be connected across the antenna coil connections. with no transponder card in the field a pulsed 13.56 mhz sine wave will be seen with a peak-to-peak voltage of up to 60 volts. the trimming capacitor should be ad justed to achieve the highest amplitude of sine wave. the antenna resistance shou ld then be checked to ensure the quality factor is correct. the capacitor and coil i nductance values have been carefully chosen for optimum performance so any cus tom antenna should be designed to be as close to this arrangement as poss ible. 60v peak-to-peak measured across the antenna coil antenna waveform 13.56 mhz no card in field C rf duty cycle 7.5 ms 100ms (default polling delay) note that optimum performance (best signal to nois e ratio) is achieved when peak-to-peak antenna voltage is approximately 50 volts.
ib technology 3 7) for optimum performance the antenna q (quality f actor) should not exceed 40. practically the quality factor effects the shape of the waveform envelope and if too low or too high then read/write performance (range) will be poor or the system will not work at all. the formula for calculating q = 2*pi*fl / rant = 1 02 / rant where f = resonant frequency, 13.56 mhz, l = antenn a inductance, 1.2 uh rant = overall antenna resistance = rdriver + rext + (rcu + rrf) rdriver = resistance of fet driver (from ic s pec) rcu = resistance of copper (coil) rrf = rf resistive component (eddy current lo sses etc) for q = 35, rant = 102 / q = 102 / 35 = 3 r by measurement (and ic spec) rdriver + (rcu + rrf) = approx 2r therefore rext = 1r 8) the micro rwd has been designed to work at optim um performance with a quality factor of 35-40 using 1uh inductor and rant < 3 r. for practical purposes rext will be <1r and may be omitted and a thinner gauge of wire used for the coil. (note that rext would be shared between two series resistors of value rext/2) 9) when designing and winding antenna coils, the wire gauge and its dc resistance must be considered to keep close to the design opti mum. significant differences in rant will affect emc and overall performance. rwd to transponder communication C pulse pause modu lation, iso14443a timing specification and waveform envelope for ideal quali ty factor 0.7 C 1.4us
ib technology 4 ferrite shielding when the rwd antenna coil is positioned close to me tal objects such as the reader housing or even the pcb ground plane then the rf fi eld induces eddy currents in the metal. this absorbs the rf field energy and has the effect of detuning the antenna (reducing the inductance). both these factors can v ery significantly affect the performance of the rwd system. therefore for operat ion in metallic environments it may be necessary to shield the antenna with ferrite . this shielding has the effect of concentrating the magnetic field lines close to the ferrite material, which introduces a fixed field component that detunes the antenna syst em (which can be accounted for in antenna design). practically, there is a compromise between the shielding effect (reduction of eddy currents) and the concentration of the magnetic field (reduced range), so the ferrite plane should only slightly overlap t he antenna coil. the optimum size of the ferrite plane, the distance from the coil and t he degree of overlap are very hard to calculate and must be determined practically. tests have shown that best performance is achieved when the antenna coil and ferrite plane ov erlap by around 5mm. antenna design a useful formula is given below for calculating the approximate number of turns required to achieve a particular inductance. l n = (approx) or l = 2 . a . ln(a / d). n 2 . a . ln(a / d) l = required inductance (nh) i.e h x 10 a = antenna circumference (cm) ie. pi x diameter (cm) d = wire diameter (cm) e.g 0.0450 cm n = approx number of windings metal plane magnetic field around antenna coil eddy currents eddy currents absorb energy and cause detuning of antenna metal plane magnetic field around antenna coil optimum field distribution, fixed antenna detuning with minimum loss of energy ferrite plane 5mm overlap 1.9 -9 1.9
ib technology 5 example antennas a) wire wound coil 0.450mm diameter (26 swg) enamelle d cu wire. antenna 6.5 cm internal diameter, 2 turns. inductance approx 1.2uh (micro henry) maximum range: approx 5cm with iso card transponde rs c1/c2 = 22pf c3/c4 = 220pf, rext = 0r (zero ohm) b) pcb antenna dimensions: 51 x 42 mm track width: 1.27 mm, 35um cu thickness distance between 2 turns: 1.75 mm number of turns: 4 inductance approx 1.2uh rext = 0.5 r 10mm between coil and pcb assembly 65mm 30mm long connecting wires with stripped tinned ends (ribbon cable). tx1 tx2 gnd antenna coil 65mm diameter, 2 turns 0.45mm diameter en cu wire (26 swg). wire covered in contact adhesive (e vo-stik) or bound in ptfe tape to stiffen coil assembly. ibt evaluation kit antenna
ib technology 6 reducing emc emission to reduce the emc emission and transmission of stra y harmonic frequencies, the antenna should have partial electrical shielding. t his is easiest to implement using a pcb antenna and a 4-layer board. the shielding loop s are on the top and bottom layers and cover the antenna track area. these shielding loops must not be closed. the shielding must be connected to system ground at one point only. the coil is routed on the first inner layer. the optional centre tap of t he coil is connected to ground with a via as shown. the connection of the coil ends to the ma tching circuit should be routed close together to avoid additional inductance. no responsibility is taken for the method of integr ation or final use of micro rwd more information on the micro rwd and other product s can be found at the internet web site: http://www.ibtechnology.co.uk or alternatively contact ib technology by email at: sales@ibtechnology.co.uk
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