This page describes how to manage the screen connection that appear on switch modules designed to support screens that lower the noise in test systems. The topic of screen use is one that is debated by many engineers - the use of a screen rather a coaxial arrangement that has a ground at both ends can often (in the opinion of EMC engineers) indicate there are design issues with the system. Systems engineers may experience problems with ground loops if two or more grounds are connected together in a large system. This page offers no opinion of the subject.
Configurations which include screened relays have separate screen connections brought to the user connector. For matrices these screens are connected together, but are not connected to ground. For multiplexers with multiple poles or banks each bank or pole has its own screen and the screen is brought out by each user connection. For general purpose switches the screens are kept separate and are available on the user connector.
The user should connect the screen in the way that is most appropriate for the application.
Where the user needs to maintain the best possible BW we recommend the screen connections are returned to front panel ground. This ensures that no ungrounded screen is present that might have an impact on the transmission line impedance of the wiring and the switch module.
In some applications, particularly where ground loops may be present, users may wish to connect the screen to an external ground connected to the instrumentation or the UUT in the system.
Where there is a need to preserve the BW and break any low frequency ground loops the screen can be capacitor coupled to the module front panel ground. The inductance of the capacitor can have a crucial impact on the effectiveness of the capacitor in providing a high frequency ground, and may lead to resonances at certain frequencies. However, it does provide a ground loop "break" for low frequencies (e.g. 50, 60 or 400 Hz related) which are the most common sources of problems
One issue that users sometimes find confusing is distinguishing between a transmission line and a screen.
In the transmission line case the "screen" provides a defined transmission line impedance and is connected to RF ground at least at both ends.
A screen does not provide a defined transmission line impedance and may only be connected at one place, and for a switching module the screen is not usually grounded inside the module. A screen can therefore result in a resonant structure, and the longer the length of the ungrounded screen (including its extension inside the switching module) the lower the frequency of the resonance.
We do not recommend leaving screens floating since it can degrade isolation and crosstalk and can create RF structures that influence the frequency response of the system.