High Isolation of Single Pole Single Throw Switch using Defected Ground Structure

Abstract

Fig. 1. Equivalent circuit of discrete PIN diode on DGS during (a) ON state and, (b) OFF state During ON or OFF state, the general total impedance of the equivalent circuit is Therefore, from (1), the total impedance of the equivalent circuit during ON state is From (2), using ABCD matrix and the conversion between ABCD to S-parameter the S 21 was derived as The same steps were used on the equivalent circuit during OFF state where Then, from (4), the S 21 was derived as Meanwhile, by refering to (1), a resonant frequency of discrete PIN diode on DGS occurs when Therefore, from (6), the resonant frequency can be calculated as From (3) and (5), the resonant frequency during ON and OFF states would be a different resonant frequencies to each other due to the presence of C j during OFF state of PIN diode. Thus, the resonant frequency during OFF state is theoretically would be shifted to lower frequency due to larger values of C T compared to the resonant frequency during ON state. In the next step, a circuit simulation was performed in Advanced Design System (ADS) software based on the equivalent circuits in Fig. 1(a) and (b). In the simulation, the PIN diode model was based on commercial PIN diode from NXP Semiconductors (part number: BAP64-02). The voltage supply of +5 V and-5 V were used to turned ON and turned OFF the PIN diode respectively. The PIN diode model has the parameters of C j = 0.35 pF, L s = 0.6 nH, R r = 5 Ω and R f = 1 Ω. Meanwhile, for the equivalent circuit of DGS, it is well known that the effective inductance of square dumbbell DGS increases with larger square areas (a and b), while its effective capacitance increases with a narrower gap width in the middle (g) [18] (see Fig. 5(b)). Therefore, the DGS design was only obtained through parametric studies as reported in [19], [20] and can be found in the other designs as well like antennas [21], [22] and microwave absorber [23]. This is due to no specific synthesis to obtain the actual layout size of the DGS. The inductance and capacitance of the DGS were varied in the simulation to be resonating at 4.0 GHz where five values of inductance were chosen as follows; 10 nH, 8 nH, 6 nH, 4 nH and 2 nH. Then, the capacitance of the DGS was tuned at 4.0 GHz. Fig. 2 shows the simulated results for the resonant frequency of the DGS while Table I tabulates the calculated different values for the inductance and capacitance at 4 GHz. Fig. 2. Resonant frequency of DGS at 4.0 GHz with different values of inductance and capacitance according to Table I. TABLE I. Inductance and capacitance values of DGS at resonant frequency of 4.0 GHz. Inductance, L (nH) Capacitance, C (pF) 10.0 0.16 8.0 0.20 6.0 0.26 4.0 0.40 2.0 0.78 From Table I, the values of L and C of DGS were used to investigate the resonant effect of the PIN diode model (BAP64-02) during ON and OFF states on DGS. Thus, Fig. 3 shows the simulation results of the discrete PIN diode on DGS during ON and OFF states. It was found that different resonant frequencies were produced during ON and OFF states of the PIN diode. During ON state, the resonant frequency was shifted to higher frequencies while during OFF state it was