Instrumentation and Test Expert

Calibration of an RF Test Platform (page 4)

We have developed a complete calibration procedure for our common RF test platform and continue to work to further improve it. The math behind it and the derivation of the procedure is beyond the scope of this paper. For fixed frequency measurements, the internal thru connection internal to the RFIU is used to verify calibration repeatability. By measuring the thru connection at the time of full calibration, the thru connection can be measured periodically and compared against the stored measurement value. Deviation limits between the initial measurement and subsequent measurement ensure the measurement system has not drifted outside the repeatability specification.

Measurement uncertainties generally fall into two categories: random uncertainties and systematic uncertainties. Random uncertainties, by definition, may not be calibrated out of the measurement. A first measure of random uncertainty is the repeatability of the measurement. The primary sources of random uncertainties in a general-purpose RF test system are the RF switches, thus, manufacturers of RF switches typically specify their repeatability. Systematic uncertainties may, to some extent, be calibrated out of the measurement. Manufacturers of measurement instruments assist the automatic test equipment designer in this regard by providing guidance in application notes, data sheets, guides and uncertainty calculators. The following discussion shows how the designer of a general-purpose RF test system can calculate the measurement uncertainty of their system using the data and tools provided by the instrumentation and switch manufacturers.

A. Frequency Measurement Accuracy 
The gold standard for frequency reference is the Rubidium oscillator. It is typically used as the 10 MHz reference for all of the test equipment. The source and measurement frequency accuracy using the Rubidium reference is 1E-10 or +/- 3.2 Hz at 32 GHz (+/- 0.1 Hz / GHz).

B. RF Output Power Uncertainty
The calculations below apply to measurement of the UUT output power. Data for the following calculations are derived from HP Application Note 64-1B, Fundamentals of Microwave Power Measurements, Agilent data sheets “Agilent N1913A and N1914A EPM Series Power Meters” and “Agilent E4412A and E4413A Power Sensor Operating and Service Guide”, and the Agilent tool “Average Power Sensor Uncertainty Calculator rev6 version1.xls”. The measurement uncertainty for the power meter is shown in the following paragraphs. Power measurement is a fundamental part of testing most RF components and systems.

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