Voltage and Power Measurements Fundamentals, Definitions, Products 60 Years of Competence in Voltage and Power Measurements RF measurements go hand in hand with the name of Rohde & Schwarz. This company was one of the founders of this discipline in the thirties and has ever since been strongly influencing it. Voltmeters and power meters have been an integral part of the company‘s product line right from the very early days and are setting stand- ards worldwide to this day. Rohde & Schwarz produces voltmeters and power meters for all relevant fre- quency bands and power classes cov- ering a wide range of applications. This brochure presents the current line of products and explains associated fundamentals and definitions. WF 40802-2 Contents RF Voltage and Power Measurements using Rohde & Schwarz Instruments 3 RF Millivoltmeters 6 Terminating Power Meters 7 Power Sensors for URV/NRV Family 8 Voltage Sensors for URV/NRV Family 9 Directional Power Meters 10 RMS/Peak Voltmeters 11 Application: PEP Measurement 12 Peak Power Sensors for Digital Mobile Radio 13 Fundamentals of RF Power Measurement 14 Definitions of Voltage and Power Measurements 34 References 38 2 Voltage and Power Measurements RF Voltage and Power Measurements The main quality characteristics of a parison with another instrument is The frequency range extends from DC voltmeter or power meter are high hampered by the effect of mismatch. to 40 GHz. Several sensors with differ- measurement accuracy and short Rohde & Schwarz resorts to a series of ent frequency and power ratings are measurement time. Both can be measures to ensure that the user can required to cover the entire measure- achieved through utmost care in the fully rely on the voltmeters and power ment range. The Rohde & Schwarz design of the probe or sensor and meters supplied: sensors feature built-in calibration through the use of microprocessors for data memories and temperature sen- computed correction of frequency – Preaging of basic units and sensors in sors to ensure that the measuring response, temperature effect and line- temperature tests lasting several days instruments are calibrated and ready arity errors. with monitoring of the drift and aging for use immediately after the sensor is to identify unstable components plugged in. Manual calibration, a – High-quality RF connectors to ensure a potential error source, is thus avoided. Measurement accuracy constantly low SWR Two factors are decisive for the accu- – In-depth self-testing upon switch-on The calibration-data memory contains racy of power measurements: the pre- and during measurements all the relevant information required to cision of sensor calibration and the produce accurate results, plus the sen- degree of sensor matching to the sor-specific data like serial number, device under test. A great amount of Operation type and calibration date as well as hardware and software is involved In many applications, operating the the permissible measurement and fre- especially in calibration. To ensure Rohde & Schwarz voltmeters and quency ranges. It is merely the fre- that the calibration standards for the power meters just means connecting quency of the current measurement URV/NRV sensors (see pages 8/9) the DUT and selecting the display that has to be entered on the meter comply with the stringent require- mode for the result. Fast autoranging which will then automatically scan the ments, many of them are compared and a digital averaging filter matched data memory for the relevant calibra- directly with the primary standards at to the measurement range ensure opti- tion factors and perform any required the German Standards Laboratory mally worked out results. Individual interpolation (see also page 21). (PTB). settings can be made either via the menu or by direct key entry. All infor- Each sensor can be used with any of Measurement errors due to mismatch mation required on the instrument sta- the basic units of the URV/NRV series, are in practice the main source of tus (range hold, zero, etc) is displayed ie voltage and power measurements error (see page17 ff). Therefore the in plain text or using easy-to-under- are possible with one and the same power sensors of the NRV-Z series are stand symbols. instrument – with equally high accu- not only carefully calibrated but also racy. optimized for minimum SWR. The displays with digital reading fea- ture in addition a quasi-analog bar- graph indicator with selectable scale Reliability to immediately show the user instanta- Even a top-quality measuring instru- neously the trend in signal variations. ment may fail, either due to obvious Level Meter URV 35 combines a functional faults or – with severe con- pointer instrument with an LCD scale. sequences – out-of-tolerance condi- tions that remain unnoticed. An increase in the measurement uncer- Individually calibrated and sensors tainty is very difficult to detect in par- The sensors of the URV5/NRV-Z family ticular with power meters, since there permit direct measurement of voltages are no reference instruments which between 200 µV and 1000 V and of are much more accurate and any com- powers between 100 pW and 30 W. Voltage and Power Measurements 3 Power Meters Depending on the application, there Diode sensors feature a higher sensi- Directional power meters are two types of power meters: tivity; their power measurement range starts at about 100 pW. Since they ... are inserted into an RF line and – Terminating power meters are con- are able to measure true RMS power measure the magnitude of the forward nected to the output of a source, down to 10 µW, they may even be and reverse wave separately with the absorb the wave incident on the used for signals with harmonic con- aid of a directional coupler. Direc- sensor and indicate the power of tents, noisy or modulated signals. tional Power Meters NRT and NAS this wave. can then be used to determine the – Directional power meters are con- Diode sensors with an integral 20 dB transmitter output power and the nected between source and load attenuator fill the gap between pure antenna or load matching in all stand- and measure − practically with no diode sensors and thermal sensors. ard communication bands up to loss − the power of the forward and They provide true RMS measurements 4 GHz and for transmitter powers up reflected wave. in a range up to 1 mW and satisfy the to the kilowatt range. Due to the low most exacting requirements on meas- insertion loss of the sensors, the power urement speed even for levels between meter can remain connected to the Terminating power meters 10 and 100 µW, where the use of transmitter for continuous monitoring. thermal sensors is limited. Special sensors with peak weighting Depending on the principle of opera- are available for measuring pulsed tion, the RF power is either converted Peak power sensors contain a peak signals such as in modern digital radio into heat or measured with the aid of hold circuit for measuring the peak networks (see page 10). diode rectifiers. envelope power (PEP). They enable direct measurement of pulsed signals NRT-Z43 and NRT-Z44 take a special The thermal sensors from R&S open up with a pulse width from 2 µs, eg of the place among the directional power a wide power range from 1 µW to TV sync pulse peak power or the sensors from Rohde & Schwarz. These 30 W. Irrespective of the waveform, power of TDMA radio signals. In the sensors can be operated even without they measure with extremely high range from 1 µW to 20 W these sen- basic power meter on any PC under a accuracy the RMS value over the full sors are a value-for-money alternative Windows user interface. They can be range and are thus easy to use. Sig- to special peak power meters. connected via a standard serial (RS- nals with harmonic contents or modu- 232) or PC Card interface adapter. lated and complex signals do not cause any additional measurement errors. Moreover, the R&S models feature an unrivalled linearity. Evolution in power measurements: Directional Power Sensor NRT-Z44 as a self-contained measuring instrument that can be connected to every PC WF 42665 4 Voltage and Power Measurements Voltmeters The sensors for the RF voltmeters from Which is the best? WF 39821 Rohde & Schwarz function similarly like diode power sensors. A diode Both methods – RF voltage and power rectifier in the sensors changes the measurement – have their merits and unknown AC voltage into DC voltage. drawbacks. Users of Rohde & This DC voltage is processed in the Schwarz Voltmeters URV 35 and basic unit (the rectifier characteristic URV 55 or Terminating Power Meters being linearized by the microproces- NRVS and NRVD however do not sor through correction) and indicated. have to make a decision: as pointed Voltmeters operating on this principle out before, all voltage and power sen- use the square weighting of the diode sors of the URV5-Z and NRV series can rectifier to measure the RMS value of be connected to any of the available small voltages up to about 30 mV (or meters with no loss in accuracy. This higher with divider) and the peak or – unique concept allows universal use of if a full-wave rectifier is used – the any model for the whole range of RF peak-to-peak value of voltages above measurements. 1 V. Due to linearization the meter reads the RMS value of a sinewave in the entire measurement range. The RF probe is indispensable for measurements on non-coaxial circuits and components. Small input capaci- tance and low losses enable low-load measurements directly in the circuit. Dividers increase the voltage measure- ment range and minimize the loading Probes and insertion units being used in RF voltage measurements of the DUT.