When
you review specifications for
reed-relay switches you'll probably notice a bandwidth referenced to a 3-dB
attenuation of a signal at frequency x. But according to David Owen, business
development manager at Pickering Interfaces, "There's nothing magic about 3dB
as a bandwidth statement. By convention, people use 3dB as a bandwidth for
small devices, but often other factors limit a product's usable bandwidth. In
the case of switch that usually means the voltage standing-wave ratio (VSWR) or
crosstalk."
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According to Owen, "For a reed switch that passes very high frequency
signals, the architecture of the switching equipment rather than the -3dB point
limits bandwidth. For a common SPST reed relay, manufacturers can offer a large
bandwidth and even good VSWR numbers, but users rarely want a simple SPST
switch. They combine SPST switches to implement a complex switch matrix or
multiplexer, for example. Then an input connection ends up with several
transmission-line stubs connected to it, and those stubs increase VSWR and decrease
bandwidth. You need a switching arrangement that does more than break a signal
path and create an open-circuit discontinuity in the path, which leads to a
high VSWR."
The use of changeover, or
SPDT, switches can help avoid problems because they can divert a signal to an
alternate path, perhaps a resistive termination or dummy load. But again, the
isolation is not as good in a reed relay as in a larger electromechanical relay
due to close contact spacing. The poor isolation means that changes on the
disconnected path still affect signals on a selected path at high radio
frequencies. "Changeover switches are also much harder to make in a reed-relay
form and in our experience they are not as robust as SPST switches," adds Owen.
"Reed relays make very good
large switching systems for low- and medium-power signals," he says. "But when
you consider reed relays as switches for radio-frequency signals, you find they
have characteristics that limit their use above about 1 GHz to a few special
applications. Crosstalk and isolation also become problems when open relay
contacts cannot offer more than a 20dB attenuation of a signal on one contact
from another signal, an input, or an output on the other. Reed relays are
problematic in this area due to the small reed-contact separation."
Larger electromechanical relays can provide larger contact spacings and other
good characteristics for signals as high as 3 GHz. "You could never implement a
good switching system with reed relays for signals at that frequency," says
Owen. "If you attempt to use them for general switching at high frequencies,
their advantages - longer mechanical life and often more consistent
low-resistance contacts - tend to diminish when compared to higher VSWR and
lower signal isolation. So in the high-frequency realms they face competition from more capable electromagnetic
relays and
solid-state switches."
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