Complex RF Switching Architectures -- Part II

In our technological world, wireless appliances are becoming extremely common, and the need for radio frequency test systems is raising at a very high pace.

The GHz domain must be treated differently than the low frequency domain, with the introduction of new parameters and new considerations. The switching sector is not immune to this upsurge in radio frequency testing needs. In this article, we talk about the different solutions for these new needs.

General-Purpose Switching

Form C relay
With a form C relay, the signal present on the common terminal (COM) needs to be routed to one of the two poles (indicated commonly with Normally Closed -NC- and Normally Open -NO-)
A wealth of products are present on the market to address general-purpose switching in the GHz domain. Form C relays are available with a bandwidth generally of 1.3 GHz, 4 GHz, 18 GHZ, 26 GHz, and even in the optical domain.


Multiplexer
The multiplexer architecture has one common terminal (COM) and N switched terminals.

In this way, you can close the correct switch and connect the common terminal to any output point. Engineers in the automated test equipment business typically use this architecture, where you normally have one scope and many channels to acquire or one arbitrary waveform generator and many terminals to distribute the signal.



Generally, for RF devices, the available multiplexers are in the 4x1 or 6x1 dimensions. Rarely can you find a much higher number of switched terminals in an off-the-shelf RF multiplexer. Next, we analyze how to create more complex multiplexers starting from 4x1 or 6x1 building blocks.

More Complex Architectures
Let's assume a case where you need a 13x1 multiplexer, but only have 4x1 building blocks to use. In this case, the natural solution is to use a cascaded architecture so you can use the specific number of inputs. The following figure, taken from the SCXI-1190/SCXI-1191 user manuals, illustrates the point.



Next you might ask, "What happens to the three parameters discussed in Part I of this article -- insertion loss, VSWR, and isolation?" There is no easy rule or mathematical formula for all the three numbers, because the new combined system has to take into account the cables you use to build the multiplexer and the fact that some parameters do not have a linear relation with the number of stages in the new, more complex multiplexer.

The total insertion loss is generally close to the mathematical addition of the insertion losses of the single stages. VSWR is generally lower than the VSWR of a single stage, while the overall isolation parameter could remain close to the value of the single module.

In this case, the best suggestion is to consider every system as a specific and individual case. Talking with the switch manufacturer should help you determine the characteristics of the overall system.

Matrices

A matrix is the most versatile switch architecture because it gives you a grid with which you can connect any signal on one column to any signal on one row.

What is Generally Available?
The offering of RF matrices on the market is limited because of the complexity of the manufacturing process and the wide number of different configuration you could need. Generally, it's possible to build the matrix using multiplexers or general-purpose switches as building blocks.

Special Cases
A very interesting case of "matrix" is the following sparse matrix.

The figure represents one very easy way to create a special kind of 4x4 matrix with the SCXI-1190/SCXI-1191, that is to use two of the four multiplexers inside each module. The ComA terminal for instance is connected to the ComB terminal of another module. In this way, you can route any of the four inputs of module A to any of the four outputs on module B. The trick is that this is not a "full fledged" matrix. Only one signal (not multiple signals) can be routed at any given moment because the signal travels only on the coaxial cable present. Even with this limitation, you can use this architecture to potentially solve a good percentage of your needs.

How to Make a 3x2 Matrix with 4x1 Muxes
Sometimes the sparse matrix architecture is not enough to solve your problems. In this case, you need to use a complete matrix architecture. Generally, you can find RF multiplexers (6 to 1 or 4 to 1 are the most common configurations) or simple C-type switches on the market. The following figure is useful when trying to implement the correct connections to create a matrix. In the diagram, we show the case of a 2x3 matrix built starting from multiplexer modules (4 to 1 in this case).


In this schematic, you also can terminate the columns and rows via the R_T. In this architecture, you have to take into account the different value that parameters such as crosstalk, isolation, and VSWR assume in the complete system, as we discussed earlier in the article.

Conclusion

In this article, we talked about switching RF signals, treating waveforms in the GHz domain. We have introduced the basic concepts and parameters pertinent to this field and analyzed the different possibilities you have to implement different switching architectures using off-the-shelf components. We encourage the reader interested in more specific information on switching solutions to contact National Instruments for a custom analysis of the problem.

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