A compact sized LTCC diplexer with high-band selectivity and high isolation for GSM and CDMA multi-band applications

In this work, a compact diplexer to separate GSM (global system for mobile) from CDMA (code division multiple access) band has been presented using a LTCC (low temp. co-fired ceramic) multi-layer technology. In order to achieve high selectivity of the bands, a shunt capacitor and inductor are designed in the high-pass filter (HPF) and low-pass filter (LPF), respectively. A transmission zero is designed at the stopband of the LPF using a shunt inductor for improving isolation characteristics. The diplexer is realized in a 6-layer LTCC substrate with a relative dielectric constant of 7. The size of the fabricated diplexer including CPW pads is 3.45 × 4.0 × 0.7 mm³. An insertion loss (IL) and return loss (RL) of the GSM band are less than -0.54 dB and -10.50 dB, respectively. In the case of CDMA band, the IL of -1.13 dB and RL of below -6.16 dB are archived.