What are the differences between DC Versus RF Signals in RF Switch?

Before you can design an RF Electromechanical Switch, it is necessary to understand the difference between how signals propagate in a DC (direct current) condition compared with an RF condition.

In the RF switching system since the routed signals (on the transmission line) in DC condition is at low frequencies with constant voltage or current, signals at different points on a coaxial cable change minimally. By contrast, the routed signals in RF condition vary along the frequency as they go higher.

Another question you may ask: “Does DC have frequency?” The answer is NO. However from mathematical perspective, it does have a frequency which is 0Hz.

Since the RF signal wavelength is shorter in comparison to the length of the coaxial cable, thus multiple cycles of the signal simultaneously propagate through the coaxial cable. As a result, the amplitude of an RF signal varies (since it is a wave), while it is constant for a DC signal.

Compensation for signal degradation due to RF signal reflection and power losses must be taken into account when dealing with RF signals. In addition, as a switch matrix system cascades switches at least two levels, these RF characteristics increase in relation to each other and deteriorate the overall system performance, unless resulting tradeoffs are understood.

What is Electromechanical RF microwave switch?

An “Electromechanical RF microwave switch" is a passive device that allows you to open and close an electric circuit. This lets you turn something on or off, or select one of several choices.

Moving blade contact or stripline microwave coaxial switches have been available in many configurations since the middle of World War II.

They are miniature and lightweight, and available in configurations ranging from
SPST to SP14T, including the multi-position switch, matrix switch, transfer switch and bypass switch configurations.

Actuation is usually accomplished by small linear solenoids, one solenoid being provided for each output position.

Transition from the coaxial input and output transmission lines to a well matched stripline contact assembly with 50 ohm impedance, operating in a microwave cavity that has the characteristics of a waveguide beyond cutoff, occurs at the connectors.

With good design and high precision manufacturing, microwave coaxial switches with
stripline blade contact assemblies can provide acceptable performance up to 67 GHz.

Excellent values of isolation can be obtained within the practical limits of cavity cross section and length, with values often exceeding 100 dB. These values of isolation prove to be substantially in conformance with the theoretical values, and exhibit virtually complete absence of resonant spikes (moding) or any of the various forms of leakage which tend to degrade the performance of other switch designs.

Switching times of 15 to 20 milliseconds or less are practical.
VSWR ratings below 1.7:1 up to a frequency of 26 GHz are common, with values below 1.2:1 possible.

Insertion loss approaches the theoretical minimums at the lower frequencies, and almost never exceeds one-half dB, except at the millimeter wave frequency ranges.

Power handling ability is reasonable, normally in the range of watts to hundreds of watts. Designs that have been optimized for power handling capability are capable of handling 3 to 5 KW average power at the lower frequencies, and up to 200 to 300
watts average power at X-Band ( 7 to 12.5 GHz)