Analog/Mixed Signal

Challenges in Expanding an RF Matrix Switch

15 October 2018

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RF matrix switches have various uses, including radio antenna switching and automated test equipment. Matrix switches can be built with different construction methods and frequency ranges that impact both the size and cost. Different matrix switches have a variety of inputs and outputs that may correspond to the number of devices under test and test and measurement equipment.

As the need for more test protocols or communication expands, additional switching is needed. Expanding an existing matrix switch is possible, but the process has various pitfalls associated with it. These pitfalls need to be considered when deciding the best matrix switch for a current and future task.

Expansion Is Not Just Adding

Figure 1: RF matrix switch with rack-mount enclosure. Source: JFW IndustriesFigure 1: RF matrix switch with rack-mount enclosure. Source: JFW IndustriesThe number of connections in a matrix switch is the product of the inputs and outputs (assuming a standard, blocking configuration); therefore, a 4 x 4 matrix switch has 16 paths while an 8 x 8 matrix switch has 64 paths. This means that an 8 x 8 matrix switch is four times the size of the 4 x 4 matrix switch. Because of this, expanding a 4 x 4 matrix switch to an 8 x 8 matrix switch means a drastic increase in both cost and size. For this reason, the idea of expansion is a problematic issue for small matrix switches and an even bigger one for large matrix switches.

Physical Size Issues

Physical hardware size is another issue. A 4 x 4 blocking matrix is built with four one-pole four-throw (1P4T) switches on the input and four 1P4T switches on the output. To expand a 4 x 4 to an 8 x 8, there are two options. Option 1 is to eliminate all 1P4T switches and replace them with eight one-pole eight-throw (1P8T) switches on the inputs and eight 1P8T switches on the outputs. Option 2 is to use the existing 1P4T switches and add 1P4T switches and one-pole two-throw (1P2T) switches to build 1P8T switches. Both options require more room inside the chassis and the ability to control more switches than the original matrix switch contained.

Plan Ahead

Planning for maximum potential size means, in many cases, that the starting matrix size will cost more than a non-expandable unit. The chassis needs to be larger to accommodate the future parts, the power supplies need to be larger to power the future parts and more room will need to be allotted for the significant increase in the number of RF cables and switches.

Design with Care for Future Expansion

Non-blocking matrix switches use power dividers. If planning for expansion upfront and putting in eight-way power dividers for a 4 x 4 matrix switch, terminate all of the unused ports so that all of the eight-way dividers are properly loaded. This means, however, that a 4 x 4 matrix switch designed to be expandable to an 8 x 8 matrix switch will have the signal loss of an 8 x 8 because of the eight-way power dividers already in place. Designing for expansion can make expansion easier in the future but it is not without tradeoffs.

Figure 2: Block diagram of a 3 x 3 blocking matrix switch. Source: JFW IndustriesFigure 2: Block diagram of a 3 x 3 blocking matrix switch. Source: JFW Industries

Miniaturization of Components and Bandwidth

To build large RF matrix switches, one must rely heavily on miniaturization to avoid an unmanageably large size after expansion. In many cases, the RF specifications of small components are inferior to their larger counterparts. Small components typically have higher insertion loss and lower isolation. Many customers are looking for a switch with a bandwidth of DC to 18 GHz. To obtain that bandwidth, electro-mechanical DC to 18 GHz switches are necessary. Due to the physical size of these switches, miniaturization is impossible and expandability is largely unfeasible.

One Solution Does Not Fit All

Most requests for large matrix sizes are based on a desire to cover all possibilities with one solution. In several cases a single large matrix is not the best solution. For example, if a user has many different radios operating in different bands and several band-specific antennas, using a single large matrix does not make sense when many of the radio-to-antenna combinations are invalid. Breaking the one large matrix down into several smaller matrix switches makes more sense. This keeps the costs down by eliminating the invalid combinations, and using the smaller matrix switches can give better RF performance as they can be tuned over smaller bands.

Conclusion

Considering these pitfalls and options will help mitigate the work and cost required when it comes time to expand. For help selecting a switch, contact JFW Industries.



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