Basic RF Technic and Laboratory Manual

Basic RF Technic and Laboratory Manual Dr. Haim Matzner&Shimshon Levy April 2002 2 CONTENTS I Experiment-4 Power Meter and Power Measurement 5 1Introduction 7 1.1 Prelab Exercise .............................. 7 1.2BackgroundTheory............................ 7 1.3dBanddBmTerminology........................ 7 1.4FundamentalsofRFPowerMeasurement................ 8 1.5MicrowavePowerMeter-HP-E4418................... 9 1.5.1 TheoryofOperation....................... 9 1.6TypesofPowerMeasurements...................... 10 1.7AverageandInstantenousPower.................... 10 1.8PowerofModulatedSinusoidalSignal.................. 11 1.9PulsePower................................ 12 1.10PowerSensingMethod.......................... 13 1.10.1 Thermocouple as a sensor of Power meter. ......... 13 1.10.2DiodeasaSensorofPowerMeter................ 14 1.10.3DirectionalPowerSensor..................... 16 2 Experiment Procedure 17 2.1RequiredEquipment........................... 17 2.2TurningOnthePowerMeter...................... 17 2.3 Front Panel Tour ............................. 17 2.4PowerMeterOperation.......................... 19 2.4.1 ZeroingthePowerMeter..................... 19 2.4.2 CalibratingthePowerMeter................... 19 2.5 Average Power .............................. 20 2.6 PowerofaModulatedSinusoidalSignal................ 21 2.7PulsePower................................ 21 2.8 Diode Detector .............................. 22 2.9FinalReport................................ 23 2.10Appendix-1................................ 23 2.10.1Tophaselocktwofunctiongenerator............... 23 2.10.2Settingazerophasereferenceattheendofthecable...... 24 4 CONTENTS Part I Experiment-4 Power Meter and Power Measurement 5 Chapter 1 INTRODUCTION 1.1 Prelab Exercise 1. Define average power, Instantaneous power, PEP, thermocouple principle, square law region of diode. 2. Describe how you intend to measure incident and reflected power using directional coupler. 1.2 Background Theory At low frequencies, the strength of the signal is calculated by measuring the voltage or current. Voltage and current are related by Ohm’s law (current= voltage impedance), and power defined as the product of voltage and current. At microwave÷ frequencies theequivalentstovoltageiselectricfield, and magnetic field to current. It is not an easy task to measure accurately magnetic and electric fields. Power is the quantity that is measured and the magnetic and electric fields are derived from the measured power. Power is the amplitude of the electromagnetic wave, and is measured in units of watts, which related to mechanics units as watt (W)=1joule/sec. At microwave frequencies, the reference level of power is not 1 W,but1mW . The reason is that a milliwatts of power is enough to operate microwave devices and components, and even wireless products. Techniques for power measurements depend on frequency. Below 100kHz, voltage and current are practically measured. At frequencies of tens hundreds of MHz, power measurement is more accurate then the power calculated by measuring voltage and current. Above 1GHz, power measurement is dominant and current and voltage measurements are not practical. 1.3 dB and dBm Terminology If we look at table one, we see that all the zeroes, before the decimal point for high powers ( more than 1watt), and the zeroes for low powers make the calculation cumbersome. For convenience and the cases we perform relative power measurement, for example, to compare the output power coming out of amplifier, relative to that going into it. the dB system of units is used and expressed as : 8 Introduction P P [dB]=10log10( ) (1.1) Pref The dB number system can also be used to express absolute value of microwave power as dBm and defined as: P P [dBm] =10log ( ) (1.2) 10 1mW Watts dBm designation 1,000,000 90 1megawatt 1,000 50 1kilowatt 1 30 1watt 0.001 0 1 milliwatt 0.000,001 -30 1microwatt 0.000,000,001 -60 1nanowatt 0.000,000,000,001 -90 1picowatt 0.000,000,000,000,001 -120 1 femtowatt Table-1 Prefix used to specify microwave power 1.4 Fundamentals of RF Power Measurement The measurement of power in RF and microwave applications has the same signif- icance as voltage measurements in electrical engineering. Power meters are used for a wide variety of measurement tasks. In comparison with spectrum or network analyzers, they are relatively cheap and unsophisticated instruments. The development of carrier-based telecommunications at the beginning of this century derived a parallel development in the field of power measurements. The ma- jority of methods were based on converting electrical energy into heat(Thermistor and thermocouple devices). For a long time, this was the only way of making accurate measurements at practically any frequency. In the meantime, direct voltage and current measurements can be made up into the GHz range assuming matched system without having to convert electrical energy into heat. Nevertheless, the in- tensity of RF and microwave signals is still given in terms of power. Apart from the high accuracy of thermal power meters, there are other important reasons for using power. Any signal transmission by waves, for example sound propagation, involves thetransferofenergy.Onlytherateofenergyflow, power, is an absolute measure of waveintensity.IntheRFandmicrowaveranges,thewavepropertiesoftheelectro- magnetic field play an important role because the dimensions of the lines used are of the same order of magnitude as the wavelength used. This fact has to be taken into account when the quantity to be measured . Voltage and current are less appropri- ate because they depend on the physical characteristics of the transmission medium (dimensions, dielectric constant, permeability) and field strength. Consider, for example, two matched coaxial cables with characteristic impedance of 50Ω and 75Ω. Microwave Power Meter -HP-E4418 9 Diode Sensor Power Meter RF Input Matching Signal BPF cocditioni ADC Network ng EE Temperature PROM Chopper DSP Sensor driver Micro Calibrator Display Bus Processor Figure 1 Block diagram of Power Meter with Diode Sensor For the same transmitted power, the voltage and current for the two impedance differ by a factor of 1.22. here are further reasons for selecting power as the quantity to be measured. There is no direct way of measuring voltage and current in waveguides, and when standing waves occur, there are large measurement errors. 1.5 Microwave Power Meter -HP-E4418 1.5.1 Theory of Operation Digital signal processing and microwave semiconductor technology have now advanced to the point where dramatically-improved performance and capabilities are available for diode power sensing and metering power sensors are now capable of measuring overawidedynamic-70to+20dBm,rangeof90dB.Thispermitsthenewsensors to be used for CW applications which previously required two sensors. The new HP ECP-E18A power sensor features a frequency range 10 MHz to 18 GHz. A simplified block-diagram of the sensor is shown in Figure-1 . The front end construction is combines the matching input pad ( low value Attenuator), diodes, FET choppers, integrated RF filter capacitors, a driving pre-amplifier. All of those components operate at such low levels that it was necessary to integrate them into a single thermal space on a surface-mount-technology PC board. To achieve the expanded dynamic range of 90-dB, the sensor/meter architecture depends on a data compensation algorithm, which is calibrated and stored in an individual EEPROM in each sensor. The data algorithm stores information of three parameters, input power level versus frequency versus temperature for the range 10 MHz to 18 and - 70 to +20 dBm and 0 to 55 ◦C. At the time of sensor power-up, the power meter interrogates the attached sensor, using an industry-standard serial bus format, and in turn, receives the upload of sensor calibration data. An internal temperature sensor supplies the diode’s temperature data for the temperature-compensation algorithm in the power meter. The new sensor store cal-factor tables for two different input power levels to 10 Introduction Average Power θ Amplitude t Voltage Power Current Amplitude of sinusoidal power (solid line), voltage (dashed line), current (dash- dotted line), and average power (dotted line) as function of time. θ is the phase difference between current and voltage. Figure 2 Average and Instantenous power improve accuracy of the correction routines. Figure -1 shows a simplified schematic of the HP EPM-4418A meter. The pre-amplified sensor output signal receives some early amplification, followed by some signal conditioning and filtering. The signal is then applied to a dual ADC. A serial output from the ADC takes the sampled signals to the digital signal processor, which is controlled by the main microprocessor. A differential drive signal, synchronized to the ADC sampling clock, is output to the sensor for its chopping function.The ADC provides a 20-bit data stream to the digital signal processor, which is under control of the main microprocessor. Even the synchronous detection is performed by the ADC and DSP rather than use of a traditional synchronous detector. Experiment-3 Power Meter and Power Measurement 1.6 Types of Power Measurements The main types of power measurements are: average power, pulse power and peak envelope power. The first is suitable for energy transfer considerations, the second type deals with square shape power pulses as function of time, and the last with a more complicated shapes of power as function of time. The pulse power measurement

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