PEPS LF antenna Application Notes

       The low-frequency communication frequency commonly used in PEPS systems is 125KHz. Therefore, the corresponding external low-frequency antenna is also 125KHz. This article will briefly introduce the types of PKE antennas and their advantages and disadvantages.

      
       The ideal antenna equivalent circuit is shown in Figure 1.

                                       Figure 1 The equivalent circuit of the ideal antenna

       The equivalent circuit includes: antenna resonance capacitor (C _ANT ), antenna inductance (L _ANT ), antenna coil equivalent series resistance (ESR _LANT ) and damping resistor R _{S. 。}

       In practical applications, there are three types of antennas, namely LC antennas, LCR antennas, and L-only antennas, as shown in Figure 2.

                                                                   Figure 2 Antenna type applicable to PKE

       As can be seen from the figure, the L-only antenna does not contain a resonance capacitor and a damping resistor. It needs to reserve a position in the circuit. The advantage is that the debugging flexibility is large, and it is also convenient to replace antennas with different frequencies and inductance values; LC antennas need to reserve the position of the damping resistor; LCR antenna resonance capacitance and damping resistance are alreadyBuilt into the antenna, the advantage is that it is easy to measure the antenna parameters and _S the non-vehicle (BCM) power consumption of R S, while the disadvantage is that the antenna cannot be optimized.

       Taking the NXP PEPS scheme as an example, for a typical JOKER (NJJ29C2) application, the shared PKE/IMMO antenna of TX4 is L-type, while the PKE antenna in the NXP demonstration system is LC-type. Other types of antennas, such as L-only and LCR types, can also be used for PKE. Specific types of antennas may have advantages over other types for specific applications. Users need to consider all factors before making a final choice.

       In practical applications, the series resistor RS is divided into two half-value resistors (RS/2), and can be placed between each TX output pin and the antenna lead, which reduces the power consumption of each resistor compared with a single resistor. In addition, the split half-value resistance improves the symmetry of the antenna circuit and reduces spurs.

       In practical applications, deploying antennas at different points inside and around the vehicle inevitably requires short-distance and long-distance wires, which often modifies the overall LRC parameters of the connected antenna. Therefore, it is recommended that all antennas connected to JOKER (NJJ29C2) should be tuned according to the different lengths of the connection line. At the same time, it is recommended to reserve the position of the resonance capacitor and damping resistor when drawing the schematic diagram and PCB to facilitate the final antenna tuning.

       Reference document: "an460220-Application Note-AN-SCA1602-B-NJJ29C0A-Joker (2.0)"