RF Switch Matrix

RF Switch Matrix or Microwave Switch Matrix or Switch Matrix
An RF/Microwave Switch Matrix is used in test systems, in both design verification and manufacturing test, to route high frequency signals between the device under test (DUT) and the test and measurement equipment. Besides signal routing, the RF/Microwave Switch Matrix may also contain signal conditioning including passive signal conditioning devices, such as attenuators, filters, and directional couplers, as well as active signal conditioning, such as amplification and frequency convertors. Since the signal routing and signal conditioning needs of a test system differ from design to design, RF/Microwave Switch Matrices typically have to be custom designed by the test system engineer or a hired contractor for each new test system. The Switch Matrix is made up of switches and signal conditioners that are mounted together in a mechanical infrastructure or housing. Cables are employed to interconnect the switches and signal conditioners. The switch matrix then employs some type of driver circuit and power supply to power and drive the switches and signal conditioners. The switch matrix uses connectors or fixtures to route the signal paths of the sourcing and measurement equipment to the DUT. The switch matrix is typically located close to DUT in the test system to shorten the signal paths to the DUT thus reducing insertion loss and signal degradation.

Benefits of an RF/Microwave Switch Matrix

The purpose of a switch matrix is to move the signal routing and signal conditioning to one central location in the test system versus having it all distributed at various places in the test system. Moving the signal routing and signal conditioning to a single location in the test system has the following advantages:
*Calibration plane between the DUT and test equipment becomes smaller and centralized making it easier to characterize.
*Switches and signal conditioners have similar power, mounting, and driver requirements so moving them to a single location means you will only need a single power supply and driver circuit to power and control them.
*Short signal paths reduce insertion loss and increase signal integrity.
*Exact length signal paths are possible to control phase issues.
*Simplifies service and support.

Making It vs Buying It

Switch matrices present a unique problem to test system designers because the signal conditioning needs, the frequency range, the bandwidth, and power aspects change from application to application. So test and measurement companies cannot provide a one size fits all solution. This leaves test system designers with two choices for their switch matrix design: Create an in-house solution or contract it out.Advantages of creating your switch matrix in-house:
*Proprietary concerns can be a big issue especially in the Aerospace Defense industry. Creating a switch matrix in-house makes proprietary concerns a non-issue.
*Company has spare suman resources to commit to the design.
*Being the first to develop an emerging technology into a finished product can be very profitable for a company. When building a switch matrix in-house the timely process of shopping around for the right contractor is bypassed. A company is in control of the amount of daily man hours spent developing a switch matrix.
*Successive switch matrix designs can be highly leveragable from design to design. The switch driver hardware and software, the mechanical designs, the power supply, etc. can all be leveraged from design to design with little or no modification.Contracting out advantages:
*Company lacks spare human resources.
*System integrators (contractors) tend to have more experience and expertise. They can design within tight specs and can handle complicated designs.
*System integrators can provide guaranteed work as well as product support.

ignal Routing

There are two types of switches typically used in switch matrices: Coaxial Electromechanical Switches and Solid State Switches, also known as electronic switches. Coaxial electromechanical switches can be further divided into two categories based on their architecture, latching relay and non-latching relay. Solid state switches come in three types: PIN diode, FET, and hybrid. The advantages of solid state switches over EM switches include they have much faster switching speed (at least 10,000 times faster), they have an almost infinite life, and they are very stable and repeatable. On the other hand, since solid state switches have non-linear portions over their frequency range their bandwidth is limited. Also, EM switches provide better insertion loss, VSWR, power handling, and isolation specifications. For these reasons EM switches are used much more often in switch matrix designs.

Example Applications

Custom Switch Matrices are used extensively throughout test systems in the wireless and aerospace defense sectors for design verification and manufacturing test. They can range from the simple to the complex. An example of a simple design switch matrix application would be a 1:16 MUX configuration that routes 12 satellite TV feeds to a single spectrum analyzer input that is used to perform signal integrity checks on the satellite feeds. Such a design would require 5 SP4T coaxial EM switches as well as interconnecting coax cable for the signal routing along with a mechanical infrastructure, power supply, and switch driver circuit to mount, power, and operate the switches.
An example of a more complex switch matrix is an application that is measuring jitter on multiple high speed serial data buses. The switch matrix inputs the data bus signals then provides the proper switching and signal conditioning for the signals before feeding the signals to test and measurement instruments. This custom switch matrix employed 14 EM switches and a number of different signal conditioners including: power splitters, amplifiers, mixers, filters, and attenuators.

Design Challenges

There are six main challenges when designing a custom RF/Microwave Switch Matrix from beginning to end:
#"Mechanical Design:" design of a electrically shielded enclosure or box, internal component mounting brackets, as well as component and cabling layout.
#"RF/Microwave Design:" RF/Microwave signal routing and signal conditioning design and testing. A calibration plan for the switch matrix would need to be developed to properly characterize the signal paths.
#"Power and Control Hardware": The power supply and switch driver circuitry will need to be designed and developed.
#"Software Control:" A software driver will need to be developed to provide an interface between the control hardware and test system program.
#"Documentation:" The whole switch matrix design will have to be documented to support maintenance and possible future design leveraging.
#"Servicing Plan:" A servicing plan will need to be developed to ensure the life of the switch matrix lasts as long as the life of the test system.

Test equipment manufacturers, such as Agilent Technologies, offer instruments that provide a power supply, driver circuitry, and software drivers that essentially saves a test system designer time and cost by eliminating two of the six switch matrix design challenges: power and control hardware design as well as software driver development. In early 2008 Agilent Technologies introduced a new product concept that aids in custom switch matrix design. The new product offers test system designers a power supply, driver circuitry, and software drivers all wrapped together in a mainframe. The mainframe provides flexible mounting for switches and other components as well as blank front and rear panel that can be easily modified to fit a design need. This new product eliminates 3 of the 6 design challenges: mechanical design, power and control hardware design, and software driver development

External Links

* [http://www.agilent.com/find/L4491A Agilent L4491A RF/Microwave Switch Platform - an instrument that assists in custom switch matrix development]
* [http://cp.literature.agilent.com/litweb/pdf/5966-2916E.pdf Information on Agilent Technologies custom RF/Microwave Switch Matrix design and build services]
* [http://www.senariotek.com/ Senariotek homepage - a company that designs and builds custom RF/Microwave Switch Matrices]
* [http://archive.evaluationengineering.com/archive/articles/0404/0404microwave.asp Information on switching in a microwave test system for VXI Technologies]

References

#

[http://www.microwaves101.com/encyclopedia/index.cfm Microwaves101: Microwave Encyclopedia]
# [http://cp.literature.agilent.com/litweb/pdf/5989-3322EN.pdf Application Note: Enhancing Measurement Integrity in RF/Microwave Test Systems]
# [http://cp.literature.agilent.com/litweb/pdf/5989-3323EN.pdf Application Note: Calibrating Signal Paths in RF/Microwave Test Systems]