Coaxial Mechanical Switch Selection Guide

Requirements for Low Insertion Loss , High Levels of Isolation and High-Power Handling (one or all of the above) without the need for solid state switching speeds are generally the main drivers for the selection of a Coaxial Mechanical Switch as the signal routing choice for RF applications. Once the Coaxial Mechanical Switch solution is selected, there remain many other considerations in the switch selection process in order to fully define the product and the function. The purpose of this technical article is to point out those additional considerations, and to define, describe and contrast them accordingly.  

Actuation Related Attributes:

DC Voltage and Current

Regardless of Switch Configuration, all Electro-Mechanical Switches rely on the Electro-Motive Force (EMF)  generated by an energized coil (via the magnetic field created) to physically engage, or dis-engage RF conductor(s) from RF contact(s) within the switch. The level of EMF required can vary depending upon the configuration/design of the individual RF path mechanism, but once that level of EMF is established, it can be achieved by various combinations of coils and voltages, so long as reasonably same DC Power results. While it is clearly critical for the power supply being used to support the required current draw for proper actuation, it is also critical to understand that for any one coil selection, there is a range of DC voltages that when applied, have the desired outcome. The pull-in voltage is the point of actuation of the selected path as the DC voltage is increased. Once the energized coil provides enough EMF to overcome existing spring tension on the contact, the selected position can engage or connect. As the DC voltage changes from higher to lower, the drop-off voltage is the point where the force of the spring tension is higher than the force provided by the coil, and the position thus disengages/ disconnects. Of course,  there is a maximum voltage as well, where excess heating and or damage to the device can occur if maximums are exceeded.

Additionally, when energized coils discharge the collapsing magnetic field causes a reverse voltage spike or “back EMF” across the coil. Suppression Diodes (fast recovery rectifiers) are deployed in parallel with the coil to prevent that spike from doing any damage. Typically, suppression diodes are deployed with TTL control and Latching Configurations, but they are generally available as options whenever required as well. 

Path Selection

The basic method of path selection is just the application of voltage across the appropriate coil windings, creating the EMF needed to mechanically engage the selected position. In reality, it is always this voltage (typically a positive voltage between 12 and 28 volts DC) that does the work needed to change position, but the application of the voltage is often triggered by some other type of control signal, working in conjunction with this DC voltage source. Typical types of control signals range from straightforward TTL control, through BCD, RS-422, Ethernet USB and others. The main point here is that in addition to the application of the control signal, the aforementioned DC voltage must always be applied as well as it is the control signal that determines which coil “sees” the voltage. An obvious exception to this rule is the manually actuated Electro-Mechanical Switch where a front panel knob or toggle is provided to physically change switch positions. Another important path selection criterion is that generally, only one port should be selected at any one time. Attempts at actuation of more than one, or all positions in a switch simultaneously will most certainly result in drastic impedance changes, but may in fact result in physical damage. Actuation schemes must therefore avoid multiple path selections. In some cases, however, provisions are possible to allow for one or more paths to be engaged at one time, without the risk of damage. Unless otherwise discussed during the specification process, however, the rule remains, one port at a time.

Actuation Modes

With Failsafe Operation (normally associated with SP2T or 2P2T switches) the switch is in a pre-determined position, known as the normally closed (NC) position without the application of DC power or a control command. Once DC power and the control command (when applicable) are applied, and moreover, maintained, the switch moves to the other position, which is the normally open (NO) position. As soon as the DC power or Control Signal (as applicable) is removed, the switch goes back to the Normally Closed position accordingly. Normally Open actuation, typically associated with multi-position switches (SP3T and higher order) is as it seems, where unless DC Voltage and or the control command (again as applicable) is applied and maintained to a specific position, the switch will sit with all RF ports open. Normally Open Failsafe to Position X ( X meaning any one of the “N” available positions of the switch) is also a popular actuation mode, similar to the SP2T failsafe condition above but with a fixed choice of the failsafe or power off position, position 1 being the most common. Finally, we have the Pulse Latching actuation mode, where an initial short duration ( typically 50 ms. or so ) voltage/ control signal (as applicable) is applied to select a position and when that voltage/control signal is removed, the switch will latch (lock) in this selected position and remain there until another position is selected, even if all DC power is removed from the device.

Another popular variant of the Pulse latching actuation configuration is the incorporation of “self-cutoff” circuits, which allow longer or indefinite application of voltage/control signals as the DC current draw automatically folds back after a short period of time. Other than this foldback, sometimes called “Latching Cutthroat” or “Self-De-energizing” the latching feature remains the same with respect to holding a position until another position is selected.  Since it is easier to provide a continuous voltage or control signal rather than a pulsed one, this latching cutthroat feature is a good choice for a simpler actuation scheme, and the automatic reduction in current draw possible with any position selected is quite useful where DC power limits or excess heat generation concerns exist as well.

In closing out actuation-related specifications and features, consider that most coaxial Mechanical switches on the market today are designed to be “break before make” meaning the engaged position opens before the next position closes. Given this, there are concerns whenever appreciable RF power is being switched. For power levels of one watt and under,  “Hot switching“ (hot, meaning RF is on while the position is being changed) is generally not a concern, but once the RF power level exceeds one watt, there are generally going to be implications to the life of the switch,  the severity of which are directly proportional to the level of RF Power as one would expect.   For example, for 5 Watts, certain switches are able to manage up to 10,000 cycles or more without issues, but certainly, with power levels of 10 Watts and much higher, switch life is considerably affected. The RF contacts and the bridge between them that closes the switch (typically called the reed or leaf in most engineering circles) are gold-plated conductors that require a pristine environment in order to reach expected repeatability and life.  With hot switching and elevated RF power levels, arcing can and will occur across the narrow gaps that occur just prior to making a connection. This arcing fouls the pristine environment and reduces the useful life of the device accordingly. In extreme cases, for example, accidentally hot-switching with 2kW of CW RF applied, the results will be quick and catastrophic rather than gradual.

Additional Feature Considerations: 

Indicator Circuits can be very useful in a mechanical switch application,  as they can be used to provide some “external” to the switch feedback to “tell” which RF position is currently engaged. The indicator circuit provides a common node for each RF position via a form C relay that is internal to the switch. External to the switch are the pinouts where the indicator common pin is internally shorted to the indicator pin (representing the engaged position), and internally open to the remaining positions. Using external circuits this internal relay can route positional information (envision a power supply and current limiting resistor connected to the indicator common and an LED to ground connected to the other indicator pins as a simple means of “showing” the engaged position, in this case as a lit LED) and or routing DC power elsewhere within the system as a function of the switch position. Typical limits of these indicator circuits would be on the order of 28-30 Volts at 300-500 mA, and most indicator circuits have a total DC power limit to be mindful of as well in the range of 3 to 5 watts.

Power vs. Frequency and Connector Type is another very important consideration. In viewing typical industry-wide power curves as provided by the manufacturers, one can see that Coaxial Mechanical Switches manage very considerable CW power levels, but it’s also readily apparent that power handling diminishes as operating frequency increases. This diminishing power handling is a function of the skin effect, where higher frequency energy propagates more on the surface of the RF Conductor presenting additional heat dissipation challenges, and the fact that higher frequency coaxial structures are more gracile and handle much less RF Power than the more robust low-frequency structures to begin with.

As with other types of component solutions and certainly not limited to the switching function, the classic CW RF power handling vs. frequency dilemma is very much a reality in Coaxial Switch Design criteria. As for Peak Power Handling in pulsed signal applications,  coaxial mechanical switch designs conservatively handle a range of peak powers from 1-2 kW. to 10 kW or more as limited by the smallest gap that occurs between any RF conductor and the ground plane of the particular switch design. As typically with all coaxial RF components, design selection for high average and peak power handling must also factor in the Power derating due to Increasing altitude and load VSWR as applicable.  

In many switch applications, the unengaged positions of the switch are required to present a good 50-ohm load at all times when “looking into “ the output(s). When this is the case, the outputs are absorptive, rather than reflective. Said another way,  the device that the switch was feeding continues to see a 50-ohm load at its input,  even when that position is not selected. Absorptive switches are equipped with Internal 50-ohm Terminations that typically have 1-2 Watts of power handling, needed provisions (a threaded coaxial connector typically) for externally applied terminations with higher power handling are generally available as needed.

Summary:

In addition to the obvious considerations, such as RF Frequency Range, RF Performance Parameters, and desired connector type, there are many things to consider in order to completely specify a coaxial Mechanical Switch that will provide the performance and function required for the particular application being considered. As a major and trusted supplier of Coaxial and Surface Mount Mechanical switches for over six decades and the only QPL-certified provider of the same on the market, we welcome your inquiries in the future and do hope that this overview was found to be useful!