A 5 To 6 GHz Switch Using Low-Cost Plastic Packaged PIN Diodes

Introduction
Short-range wireless LANs are being developed for use in the ISM frequency bands between 5.15 and 5.875 GHz. Using low-cost, plastic packaged, surface-mount components, such as PIN diodes, is problematic in this frequency range because of package parasitics. In traditional designs, using low-cost packages like the SOT-23, coverage is limited to about 2.5 GHz. In this Application Note, a new design for an SPDT switch in the 5–6 GHz range will be described using PIN diodes in a low inductance SOT-23 package. The design has performance approaching 1 dB insertion loss and 20 dB isolation.

PIN Diode Switch Fundamentals
PIN diodes are widely used in switches at frequencies below 2.5 GHz. A typical design for a simple SPDT switch, covering a fixed bandwidth, consists of shunt connected PIN diodes each connected a quarter wavelength from the common input port. Using low-cost plastic packaged PIN diodes, the isolation of this circuit is limited by package inductance. The expression for attenuation based on the inductance, L, and resistance, R, of PIN diode is shown above. A PIN diode in a SC-79 package with typical inductance of 0.7 nH and PIN diode resistance of 1 O results in approximately 9 dB isolation, in this switch design at 5.8 GHz. This level of performance is not satisfactory.

This paper describes the design and performance of an SPDT switch covering 5–6 GHz using low-cost, commercially available PIN diodes in SOT-23 packages that use a low inductance internal lead configuration. The design is based on a Libra IV simulation.

Click Here To Download:
•Application Note: A 5 To 6 GHz Switch Using Low-Cost Plastic Packaged PIN Diodes