A number of different strategies can be employed to avoid a situation were the coupling factor would be too high:

• The system can be designed whereby the base station antenna is much larger than the tag coil. This is generally the case for long-range systems.

• For a short-range system, where the sizes may be comparable, the communication range must be sacrificed to system reliability. This can be done by using a much lower Q for the tags. One can also deliberately mistune the tags.

• The most critical situation is for medium-range systems, where antenna sizes are not sufficiently different to prevent coupling factor frequency spreading at short range. In this case, one is advised to maintain, by means of a physical obstacle, a minimum distance between the tag and the antenna in order to maintain the system in a functional state. Also, the output power stage driving the base station antenna must be resistant to mismatch. Even medium-power stages can die quickly if they do not see the right load.

However, for short- and medium-range systems, the use of diodes will mitigate the coupling factor's actions. All tags have a maximal power supply voltage. To protect the chip from an overvoltage condition, chip manufacturers usually place a pair of zener diodes that start to conduct before the limit is reached in parallel with the coil inputs. When the diodes are placed in strong conduction, they completely destroy the tag Q factor. This is important because, in most cases, the coupling factor's deleterious effects are sufficiently alleviated to maintain the system in a functional state.

Denis Ruffieux is an application engineer at Melexis RFID business unit. He is responsible for hardware development and customer support. Ruffieux holds an engineering degree in electronics from Ecole d'Ingenieur du Canton de Vaud in Yverdon les Baines, Switzerland.

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