Low loss microwave switches

Cambridge, UK-based Plextek has designed a series of low loss microwave switches (12 to 19 GHz) with discrete PIN diodes to replace higher loss, bulkier and significantly more expensive connectorised components.

The switches include both Single Pole Double Throw (SPDT) and Single Pole 4-Throw (SP4T) variants. Unlike commercially available MMIC based products in this frequency range, the switches use packaged components and printed transmission lines to realise a microwave circuit function, therefore they avoid the use of bare die and chip and wire assembly. The measured insertion loss is 0.85 dB for the SPDT and 1.8 dB for the SP4T.

The switches have been designed for a radar application but they can also be used for VSAT equipment and microwave point-to-point links. Although FET and PHEMT switch MMICs are commercially available in the 12 to 19 GHz frequency range, they tend to be bare die components that require wire-bond assembly. The switches are realised on a low cost Rogers RO4003 substrate using packaged beam-lead PIN diodes, SMT resistors and capacitors and printed components.

The SPDT insertion loss of 0.85 dB is much lower than the typical 1.5 dB of commercially available microwave switch MMICs. Although the SP4T loss is higher at 1.8dB, SP4Ts are not readily available as MMICs and a loss of 1.8 dB is still lower than that of connectorised Ku-band SP4Ts. These tend to have losses greater than 2 dB and are also quite bulky. The SP4T switch has a simple realisation based around an input SPDT that feeds two output SPDTs. The latest version of the SP4T also incorporates a switch driver on the same PCB. This allows two-line control of the 4-way switch using standard CMOS logic levels.

A presentation on the design, implementation and measured results of the new switches will be given at the ARMMS RF & Microwave Conference on 7th and 8th November at the Harben House Hotel in Newport Pagnell, UK.