Does Switch Bandwidth Matter?

Whenyou review specifications forreed-relay switches you'll probably notice a bandwidth referenced to a 3-dBattenuation of a signal at frequency x. But according to David Owen, businessdevelopment manager at Pickering Interfaces, "There's nothing magic about 3dBas a bandwidth statement. By convention, people use 3dB as a bandwidth forsmall devices, but often other factors limit a product's usable bandwidth. Inthe case of switch that usually means the voltage standing-wave ratio (VSWR) orcrosstalk."

According to Owen, "For a reed switch that passes very high frequencysignals, the architecture of the switching equipment rather than the -3dB pointlimits bandwidth. For a common SPST reed relay, manufacturers can offer a largebandwidth and even good VSWR numbers, but users rarely want a simple SPSTswitch. They combine SPST switches to implement a complex switch matrix ormultiplexer, for example. Then an input connection ends up with severaltransmission-line stubs connected to it, and those stubs increase VSWR and decreasebandwidth. You need a switching arrangement that does more than break a signalpath and create an open-circuit discontinuity in the path, which leads to ahigh VSWR."

The use of changeover, orSPDT, switches can help avoid problems because they can divert a signal to analternate path, perhaps a resistive termination or dummy load. But again, theisolation is not as good in a reed relay as in a larger electromechanical relaydue to close contact spacing. The poor isolation means that changes on thedisconnected path still affect signals on a selected path at high radiofrequencies. "Changeover switches are also much harder to make in a reed-relayform and in our experience they are not as robust as SPST switches," adds Owen.

"Reed relays make very goodlarge switching systems for low- and medium-power signals," he says. "But whenyou consider reed relays as switches for radio-frequency signals, you find theyhave characteristics that limit their use above about 1 GHz to a few specialapplications. Crosstalk and isolation also become problems when open relaycontacts cannot offer more than a 20dB attenuation of a signal on one contactfrom another signal, an input, or an output on the other. Reed relays areproblematic in this area due to the small reed-contact separation."

Larger electromechanical relays can provide larger contact spacings and othergood characteristics for signals as high as 3 GHz. "You could never implement agood switching system with reed relays for signals at that frequency," saysOwen. "If you attempt to use them for general switching at high frequencies,their advantages - longer mechanical life and often more consistentlow-resistance contacts - tend to diminish when compared to higher VSWR andlower signal isolation. So in the high-frequency realms they face competition from more capable electromagneticrelays and
solid-state switches."

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