12 Jul 2010

Latest RF power sensor technology - interview with Rohde & Schwarz

Our editor, Ian Poole interviews Thomas Reichel, Head of Power Meter Development at Rohde and Schwarz about RF power sensor technology and looks at key elements in their design.

One of the key parameters for any radio frequency system is the level of the RF power that is generated. These days tighter controls are required for many RF-based equipment from cellular handsets to base stations as well as satellite systems, and all forms of RF systems where RF power is present.

With the recent announcement by Rohde and Schwarz of their 67GHz coaxial power sensor, we thought it would be interesting to discuss power sensor technology with Thomas Reichel, the head of the Power Meter Development Group to get a real insight behind what makes a good RF power sensor and what the design challenges are.

It is clear from the way that Thomas Reichel talks that he knows both the marketplace and the technology behind power sensors in real depth. He also has an infectious enthusiasm for the subject.

Basic RF power sensor requirements

I asked Thomas about RF power sensors and what should people look for in an RF power sensor when they choose one? He replied: "Customers should look at what they really want the power sensor for - is it for universal use where a more general power sensor is needed, or is there a specific requirement for which a more specialised sensor may be required?"

He also notes that care should be taken in choosing the manufacturer of the RF power meter because they are normally used for very accurate measurements - full calibration, traceability and also support are essential. It is important to find a company that can provide all of these.

Right sensor for the right job

There are a number of different types of RF power sensor - what are the main types and what is each one particularly suited to?

"There are two main types of RF power sensor - thermal and diode sensors. The thermal types have been around for many years - twenty or thirty years ago they were the main type. However since the beginning of the 1990s the industry needed capabilities the thermal sensors could not provide because of the new types of modulation being used. Now eighty to ninety percent of all power sensors are diode based."

Looking at the different types of sensor, there must be good reasons for using the different types - what are the limitations of each type?

A power sensor in use

"Well, there is a lot to say about the way in which each type works. Although thermal sensors are slow over a relatively large portion of their dynamic range and they only can measure the average power of a signal, they still deliver the most accurate measurement results. One reason is the effect of harmonics and non-harmonics can be predicted very well. Diode power sensors are, in general, much more critical when it comes to modulation or superimposed spurious, but modern types like wideband power sensors or multi-path power sensors out-perform their simple predecessors very well in these aspects. So, today, diode based RF power sensors offer sufficient accuracy for the majority of applications.

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