Design and Analysis of Broadband High Isolation of Discrete Packaged PIN Diode SPDT Switch for Wireless Data Communication.

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Design and Analysis of Broadband High Isolation of

Discrete Packaged PIN Diode SPDT Switch for

Wireless Data Communication

N. A. Shairi, B. H. Ahmad, A. C. Z. Khang

Telecommunication Engineering Department, Faculty of Electronics and Computer Engineering,

Universiti Teknikal Malaysia Melaka (UTeM), Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, Malaysia.

[email protected],[email protected], [email protected]

Abstract— A design and analysis of broadband high isolation of discrete packaged PIN diode Single-Pole Double-Throw (SPDT) switch is presented. Many applications of wireless data communication are implementing Time Division Duplex (TDD) switching such asUMTS-TDD (in 3G technologies), LTE (in 4G technologies), HiperLAN, HiperMAN, WiFi, WiMAX and WiBRO. A broadband high isolation of SPDT switch is needed to provide these multimarket applications at different frequency bands. In this paper, three different circuit topologies (Type A, Type B and Type C) targeted for broadband high isolation are designed and analyzed. The SPDT switches are based on the discrete packaged PIN diode that available in the market for switching control. Analysis of the various configurations is done in order to select the most suitable topology for broadband high isolation SPDT switch. As results, Type C has been chosen for prototyping and measurement. In simulation and measurement, the isolation is higher than 25 dB from 0.5 to 3 GHz compared with Type A and Type B.

Keywords-broadband high isolation, SPDT switch, switch topology.

I. INTRODUCTION

SPDT switch is commonly used in RF transceiver system to perform Time Division Duplex (TDD) switching for transmit and receive operation (see Figure 1). Many applications are implementing TDD such as UMTS-TDD (in 3G technologies), LTE (in 4G technologies), HiperLAN, HiperMAN, WiFi, WiMAX and WiBRO. Therefore, a broadband high isolation for SPDT switch is needed to provide these multimarket applications at different frequency bands. This design will reduce the cost of design and manufacturing.

By using discrete packaged PIN diode that available in the market for switching control, this paper therefore focused on circuit topology in order to get broadband high isolation. The PIN diodes of HSMP-389Y from Avago Technologies are used for switching control and three different topologies (Type A, Type B and Type C) are designed and analyzed for the performance. Generally, there is almost difficult to get broadband high isolation if using single discrete packaged PIN diode. This is due to inductance of bondwires and lead frames, ground inductance and coupling between the RF line and the

surrounding package lead frame [1] affecting especially at high frequencies; but with circuit design techniques, a good topology for broadband performance is possible to achieve.

Figure 1. SPDT switch in RF transceiver system

II. SELECTION OF CIRCUIT TOPOLOGY

The selections of SPDT switch are depend on the requirement of low insertion loss, high isolation, small circuit size and low operating current [1-2], but with another requirement of broadband high isolation, designers have to select a suitable circuit topology to meet these requirements. Thus there are different types of broadband high isolation SPDT switch configuration such as shunt connection and combination of series shunt connection as discussed below.

A. Single Shunt Switch (Type A)

Paper in [3] proposed single shunt PIN diode connection with quarter wavelengths (λ/4) of transmission line as shown in Figure 2. The insertion loss of such connection is depending on its junction capacitance (Cj), while the isolation is depending on its series resistance (Rs) in the PIN diode. Such connection will get better isolation compared to series PIN diode connection [4-5].

In order to get wider bandwidth of isolation, a quarter wavelength transmission line at f0 is placed between the

common junction and antenna port with some value of impedance, Z1, below 50 Ω. The specific value of impedance

that is chosen will determine the SWR and bandwidth of the switch [3].

This work was supported by Centre For Research And Innovation Management (CRIM), Universiti Teknikal Malaysia Melaka (UTeM). Melaka, Malaysia under Short Term Research Grant.

2011 IEEE International RF and Microwave Conference (RFM 2011), 12th - 14th December 2011, Seremban, Malaysia

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Figure 2. Single shunt switch with quarter wavelenght transmission lines [3].

B. Series-Shunt Switch (Type B)

Higher isolation can be achieved by using a series-shunt connection where isolations greater than the sum of that obtained with a single series and a single parallel diode may be obtained [4]. The series PIN diode can act as high impedance in reverse bias, thus increasing the isolation.

However, there is an increase in bias circuit complexity because all diodes are not simultaneously biased in one state or the other [5].

The series-shunt PIN diode connection is added with three quarter wavelengths as shown in Figure 3. This circuit topology can have a broadband high isolation with the combination of series-shunt PIN diode and the impedance of quarter wavelength, Z1 where the functional and operational of

Z1 is the same with Type A.

Figure 3. Series-shunt switch with quarter wavelenght transmission lines.

C. Series-Shunt-Shunt Switch (Type C)

In [6-8], the series-shunt-shunt topology has been implemented for SPDT switch of wireless data communication. As shown in Figure 4, the circuit topology consists of single series PIN diode and two shunt PIN diode of each transmit and receive part. The two cascaded shunt PIN diode will increase the isolation of 6 dB of each additional shunt PIN diode [4].

However, the isolation of series PIN diode will decrease when the frequency is increased. This is due to the rise of junction capacitance (Cj) or parasitic capacitance as a function of frequency. To overcome this isolation problem at high frequencies, the compensation concept in [9] is used where a series resistor and inductor are connected in parallel with series PIN diode (Figure 4). When PIN diode is in off-state, the parasitic capacitance of the PIN diode resonates with the

inductance. This resonance reduces the capacitive effect near the resonance frequency and consequently increases the isolation performance at that particular frequency. The resistance from resistor is used to reduce the quality factor of the resonance thus to improve the positive effect introduced by the inductance for a wideband of frequencies [6].

Figure 4. Circuit topology of series-shunt-shunt connection

III. SIMULATION RESULT AND ANALYSIS

The SPDT switch of Type A, Type B and Type C have been simulated in Advanced Design System (ADS) software. The topologies are simulated with the ideal component except the PIN diode (based on HSMP-389Y from Avago Technologies). Since all the topologies are symmetry, the isolation and insertion loss results in transmit and receive mode are the same. Thus only the transmit mode is analyzed in this paper. The simulation results of insertion loss (S21) and isolation (S13) in transmit mode are shown in Figure 5 where these topologies are plotted in the same graph.

In Figure 5(a), Type A and B give better insertion loss compared with Type C which is in average at 0.5 dB of losses. A main issue in Type A and B is the limitation of quarter wavelength of Z2 and Z3 during the on-state of shunt PIN

diode. The Z2 and Z3 operate at very narrow bandwidth. This

issue has been stated in [3]. This cause a very limited bandwidth of insertion loss of Type A and Type B compared with Type C. Thus any quarter wavelength transmission line should be avoided in transmit and receive arm in broadband SPDT switch design.

The insertion loss of Type C is about 1 dB which is slightly higher than Type A and Type B. This is due to series resistor connected in parallel with series PIN diode in both transmit and receive arm. This resistor causes a high impedance of the transmission line and consequently attenuating the RF signal. However, the insertion loss of Type C is almost constant across the frequency band.

As shown in Figure 5(b), Type C gives higher isolation which is higher than 20 dB across the frequencies compared with Type A and Type B. Two elements have contributed to the higher isolation in Type C, which are the parallel inductor with the series PIN diode and the additional shunt PIN diode in the both transmit and receive mode. The series resistor parallel with series PIN diode is purposely used to get broadband isolation at high frequencies. That is why in Type B, with the series PIN diode, it has shown better improvement of isolation compared with Type A but without a series resistor and 2011 IEEE International RF and Microwave Conference (RFM 2011), 12th - 14th December 2011, Seremban, Malaysia

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inductor parallel with series PIN diode, the isolation has dropped significantly at 3 GHz and above.

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Figure 5. S-parameter simulation of Type A, B and C in transmit mode (a) Insertion Loss (b) Isolation.

In summary, Type A gives advantages in term of simplicity of circuit and has lower insertion loss, but lacks in term of isolation performance. Type B has high isolation and low insertion loss but in very limited bandwidth only at low frequency. In Type C, the broadband high isolation has been achieved but lacks in term of insertion loss performance. Thus, this is a trade-off of between isolation and insertion loss performance in this topology. Since this paper is targeted for broadband high isolation, Type C has been selected for fabrication and prototyping.

IV. SWITCH PROTOTYPING AND MEASUREMENT RESULT

As shown in Figure 6, the circuit topology Type C has been fabricated on FR4 substrate having thickness of 1.6 mm, a dielectric constant r of 4.7, microstrip line thickness of 0.035

mm and a loss tangent, tan of 0.019. The microstrip line

width of 3 mm is used for the characteristic impedance of 50

Ω. The Type C prototype has been measured using network analyzer for the actual insertion loss and isolation performance.

Figure 6. SPDT switch prototype (Type C)

Figure 7 shows the comparison between simulation and isolation of insertion loss and isolation. Since the ideal component of inductor, capacitor and transmission line have been used in the simulation, the measured insertion loss is slightly higher than the simulated result. This includes the losses of SMA connectors.

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Figure 7. S-parameter simulation versus measurement (a) Insertion Loss (b) Isolation.

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In the isolation graph, the measured isolation is higher than 25 dB. Take note that the isolation of SPDT switch is mainly influenced by PIN diode. The differences between simulation and measurement are depending on the PIN diode modeling in the simulation and the actual PIN diode in the measurement. The actual packaged PIN diodes have tolerance values thus resulting in different measurement result compared with simulation.

V. CONCLUSION

Three different SPDT switch topologies targeted for broadband high isolation SPDT switch have been selected and simulated in ADS software. There are single shunt topology (Type A), series-shunt topology (Type B) and series-shunt-shunt topology (Type C). The PIN diodes which are HSMP-389Y from Avago Technologies have been used for switching control. From the simulation results, Type C has shown broadband high isolation from 0.5 GHz to 4 GHz compared with Type A and Type B. Then, the Type C topology has been fabricated on FR4 board and measured with network analyzer for verification. The measured isolation has agreed with the simulation which is higher than 25 dB across the frequencies.

ACKNOWLEDGMENT

The work described in this paper was fully supported by Centre For Research And Innovation Management (CRIM), Universiti Teknikal Malaysia Melaka (UTeM). Melaka, Malaysia under Short Tearm Research Grant.

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