A low cost 2nd harmonic trap for integrated radios

A novel method is presented to reduce the 2^nd harmonic level in low cost radios with balanced output configuration. The common mode- which is usually dominant at the 2^nd harmonic frequency in differential configurations- is suppresed by a resonant trap which has no influence on the fundamental operation. The trap is applicable for differential antennas, baluns and matching networks and partly formed by them to minimize the cost. A novel matching balun circuit is also proposed for low cost ISM radio chips. With the 2^nd harmonic trap the experiments showed ~12 dB 2^nd harmonic suppression improvement at 434 MHz using standard E21 0603 SMD elements. The new method was also demonstrated to be successful with printed loop antennas.