ANT-125K

ib technology Data Sheet ANTENNA 125.PDF 4 Pages Last Revised 21/03/06 Micro RWD 125kHz Antenna Specification The Micro RWD has been designed to interface to a simple low Q (10-20) antenna coil of around 700uH (micro Henry) inductance operating at a carrier frequency of 125 kHz. For maximum range and performance the following factors should be considered: 1) Maximum range and coupling between transponder and 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 coil, the better the overall performance. For ISO card transponders there is little benefit in using an RWD antenna larger than 10cm diameter. Circular antenna coils generally give a more uniform flux distribution. 2) The Micro RWD is designed to give up to 400ma pulse current with a peak voltage of up to 200v. This can give a 20cm read/write range with HT2 ISO card transponders. Lower burst current can be achieved by inserting an additional series resistor in the antenna loop circuit (22R in series would typically reduce pulse current/voltage by about half using example antennas). This is recommended in order to give the optimum signal-to-noise ratio and Q value. 3) Sample antenna supplied is for demonstration only. The characteristics of an antenna for EMC/WT approval will vary according to shape of coil, type of wire used, style of winding, bobbin material, compaction of windings etc. 4) If the antenna is to be positioned remote to the Micro RWD a screened twisted pair cable can be used to connect them. The screen should be connected to the Antenna GND pin on the Micro RWD. Cable lengths up to 1.5 metres have been tested and perform well with no significant system degradation. 5) The capacitance and inductive effect of antenna cable and physical positioning of the antenna should be taken into account when designing the antenna coil and if necessary the inductance of the coil should be adjusted to compensate. The 125KHz tuned circuit is basically an RLC network with the C element fixed on the Micro RWD board. Tuning the antenna inductance (L) for optimum performance is quite simple, the positioned antenna system should be connected to the Micro RWD and power applied. An oscilloscope should be connected between the AN 2 pin and the GND pin. Without a transponder in the field a pulsed 125kHz sine wave will be seen with a peak voltage of up to +/-180v, with small switching steps visible near the peaks and troughs of the sine wave. 1ib technology Optimum tuning and performance is achieved when these steps are seen exactly at the maximum and minimum points of the sine wave, the peak voltage will also be at its maximum value. The antenna series resistor (R) should be set to limit the peak voltage to 110v maximum in order to give the best Q value and optimum signal-to- noise ratio (For practical purposes use +/-100v peak voltage as limit). Detuned antenna. Micro RWD driver Low antenna L too low (<700uH) 110v switching steps voltage peak 125kHz correctly tuned antenna waveform with Detuned antenna. Low antenna Ra = 22 R to limit peak L too high (>700uH) voltage voltage AN1 MicroRWD 22R 700uH antenna Antenna connection pins AN2 6) For optimum performance the antenna Q should not exceed 20 and to achieve reliable tuning at 125kHz the antenna inductance should be around 700uH. Higher Q and inductance values will still function but with a reduced range and performance. The formula for calculating Q = 2*pi*fL / Rant = 549 / Rant where f = Resonant frequency, 125 kHz, L = Antenna inductance, 700uH Rant = Overall antenna resistance = Rdriver + Ra + (Rcu + Rrf) pi = 3.14159 etc Rdriver = 3.5 R (from IC spec) and Ra = 22 R (series resistor in antenna loop) Rcu = Resistance of Copper (coil and cable) and Rrf = RF resistive component (eddy current losses etc) By measurement at 125kHz, (Rcu + Rrf) = approx 6R Therefore Rant = 3.5 + 22 + 6 = 31.5 Ohms, Q = 549 / 31.5 = 17 Max peak antenna current (with 22R series resistor), Iant max = 4Vdd / pi*Rant = 20 / pi*31.5 = 200ma Max peak antenna voltage, Uant max = Iant max . (2*pi*fL) = 110v 7) The Micro RWD has been designed to work at optimum performance with Rant = 31.5 (Ra = 22R), Uant max = 110v and Iant max = 200ma. When designing and winding antenna coils, the wire gauge and its DC resistance must be considered to keep close to the design optimum. Significant differences in Ra will affect resilience to EMC and overall performance. 2