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What is a Blocking Matrix?

Hardware: Switches>>Multiplexer/Matrix

Problem:
What is a Blocking Matrix? Will my application benefit from the use of a block matrix?

Solution:
A typical matrix will have stubs (the name for the unused, unconnected portion of the matrix row or column). These stubs are un-terminated and allow reflections which degrade VSWR, discussed below.

When waves propagate between mediums of different density, reflections occur (wave theory). The greater the variation in mediums, the greater the reflection that will be seen. Voltage Standing Wave Ratio (VSWR) is a measure of this reflection, since a standing wave is formed by the superposition of two waves propagating in opposite directions.

In electrical systems, reflections can occur when signal propagates through components with varying characteristic impedances (connectors, relays, traces, etc.) or when a signal path is left un-terminated, finishing in air with an infinite impedance.

VSWR measures the power of this reflected signal. VSWR is frequency dependent. This means that, while VSWR is not an important factor for signals at low frequencies, managing stubs is important in High Frequency applications like those in RF systems.

Blocking Matrices are arranged in such a way so as to not have stubs. To do this, we use a series of form C multiplexers to make one continuous path, from source to load with no stubs.

A blocking matrix will require N Multiplexers of size (N-1)x1. In the example above, we use 4 multiplexers of at least size 3x1, or (4-1)x1. You may notice that we have used a third form C relay in the multiplexer. This results in a fourth terminal that remains unused, but ensures the three channels of the multiplexer pass through the same number of relays and have uniform insertion loss, VSWR, isolation, etc. 

With a blocking matrix, you can connect any terminal (A-D in the example above) to one other terminal.  For example,  A can connect to either B, C, or D, but not to more than one other terminal simultaneously (A cannot connect to B and C).  However, you can have up to N/2 simultaneous connections (A connected to B and C connected to D, etc).

Currently, NI does not offer a native blocking matrix. However, a blocking matrix can be built by using National Instruments multiplexers. For example, the blocking matrix pictured above can be built with four NI PXI-2554s, a 4x1 RF Multiplexer.

As previously stated, blocking matrices are used in applications where VSWR is of concern; therefore, consider using switches specifically designed with VSWR in mind. National Instruments' line of RF Switches will switch signals with very low insertion loss. In addition, the excellent VSWR and isolation parameters make these modules the perfect choice in any system geared toward high-frequency applications.



Related Links:
Products and Services: RF Switches
Developer Zone Tutorial: The Guide to Selecting an RF Switch
Developer Zone Tutorial: Chapter 1: Understanding Key RF Switch Specifications
Developer Zone Tutorial: Chapter 4: Effects of Impedance Matching and Switch Quality on RF Test System Performance

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Report Date: 10/05/2010
Last Updated: 11/10/2010
Document ID: 5E4F019G

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