TEM Cells

A Guide to Switch Considerations - Digital Switching

Published: 13th July 2021

High speed digital signals exhibit RF behavior in real-world devices, which creates a need for RF or microwave components when routing these signals in test systems. Digital (and optical) engineers must be aware of RF parameters in product development and production testing.

Digital Design Considerations

The first consideration when switching digital signals is providing adequate bandwidth to preserve the frequency harmonics of the digital pulse and their power spectral distribution. The switching solution should have as a minimum the bandwidth of the DUT, and preferably equal to the bandwidth of any higher frequency source and measurement equipment. Because the –3dB specifications of switch cards are based on sinewave
signals, an approximation is needed for square waves.

If the frequency of the square wave is known, the required bandwidth is determined by the highest harmonic that needs to pass without distortion, whether that is the third, fifth, seventh, or higher harmonic.

If the frequency of the square wave is unknown, the following equation approximates the minimum bandwidth for digital signals:

Digital switching has some other considerations in addition to those listed in the section for “RF Switch Design Considerations”:

Arcing: The RFI (radio frequency interference) generated by high voltage switching may disrupt high-speed logic circuits.

Hot switching: When digital signals are being switched, they may change state with any length of power interruption. Using hot switching prevents digital state changes.

Make-before-break: Use Form A isolated switches instead of Form C relays to allow for make-before-break operation.

MCS Test are the approved UK partner for Tektronix
Content produced by Keithley, A Tektronix Company: Test and Measurement Equipment | Tektronix

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