As digital modulation techniques overtake analog AM and FM techniques, the complexity of accurately measuring signal power has increased significantly. Analog modulated signals are continuous, and their power can be measured at any time. But digitally modulated carriers must be examined at specified time intervals and for more than one parameter to determine their power characteristics.
The basic power-measurement principles still are the same for either modulation class. A signal to be measured is applied to a sensor connected to the power meter. Three types of sensors, thermistors, thermocouples, and microwave diodes, are currently in use. Thermistors and thermocouples are heated by the applied signal’s power and a voltage proportional to the temperature is generated which then is fed to the power-meter circuit.
Microwave diodes convert signal power directly to a voltage and simultaneously perform two functions: They demodulate the signal so that it can be processed and displayed. Secondly, when operated in their square law region, they perform the Pµ V2 conversion.
Thermal sensors provide very accurate average rms readings for continuous as well as AM and FM signals. They also can be used to determine the average rms and peak values of a pulse-modulated signal as long as the pulse repetition rate is known and the pulses themselves are not modulated. But burst and multiheight pulse signals present measurement obstacles.
Microwave diodes are much faster than thermal sensors, featuring rise times in the nanosecond range. Consequently, they can follow the signal envelope and provide power reading during any part of a pulse or complex modulated signal.
Commonly available sensors measure power from -70 dBm to +47 dBm, and some feature a dynamic range as high as 90 dB. The frequency coverage may extend from the low megahertz region to 18, 40, or 100 GHz.
Some companies offer sensors tailored to the needs of particular signal formats. “This provides the opportunity to use a sensor with a bandwidth and dynamic range combination that is optimum for the system being measured,” said Steve Reyes, product marketing manager at Giga-tronics.
“For example, for TDMA systems, an amplitude bandwidth of 400 kHz is adequate, and our sensors matching that bandwidth offer a dynamic range of up to 87 dB,” said Mr. Reyes. “However, the CDMA IS-95 cellular and PCS systems require an amplitude bandwidth of 1.2233 MHz, and we offer sensors with that bandwidth, but the dynamic range is decreased due to the increase in bandwidth.”
To achieve high measurement accuracies, linearity and temperature correction factors as well as frequency-related calibration factors are provided with most sensors. However, today’s sensors and microwave power meters make it unnecessary to consult correction tables or perform extra calculations. Instead, the meter reads out the calibration factors stored in the sensor’s EEPROM, performs the calculation, and applies the correction. Whenever the frequency or the sensor is changed, the meter automatically applies a new set of correction factors.
Many of today’s power meters digitize the sensor output voltage and apply digital signal processing to obtain a range of readings. These may include peak, average, maximum, and minimum power; rise and fall times; overshoot; duty cycle; pulse width, and frequency.
More computational and selective signal-acquisition facilities are required to measure specific parameters of complex digitally modulated signals. For instance, the burst-start and burst-stop exclusion feature of the Giga-tronics 8540C simplifies measuring burst-power during the appropriate portions of TDMA signals. Similarly, using the time-gating and the burst-average-power features helps perform TDMA/TDD signal power measurements.
When dealing with nonrepetitive or pseudorandom-modulated signals, statistical data processing can be useful for extracting and measuring critical parameters. The Boonton 4500 Digital Sampling Power Analyzer, for example, acquires signals at a 500,000 S/s rate and places power samples into 4,096 bins to derive histograms. The resultant data is processed to render a cumulative distribution function (CDF) or a probability density function (PDF) display.
The CDF display is a plot of the percentage of measured power levels vs total power. It offers a high-speed method for characterizing multiple carriers and spread spectrum signals as used in CDMA systems.
A PDF plot statistically represents the amount of time that a signal occurs at specific power levels. It helps analyze signals that have large peak to average ratios, multiple carrier signals that contain different forms of modulation, or amplitude-sensitive signals such as quadrature amplitude modulation.
While all microwave power meters perform a range of power-related measurements, ease of use and accuracy are very much determined by how well the meter can deal with the type of signal to be measured. But when making a selection, not only is it important to determine whether the meter provides the features you require, but also whether the sensors cover the type of signals that interest you.
RF/Microwave Test Equipment
Power meters are only one tool used by engineers to test and verify RF and microwave products. Other test instruments include spectrum analyzers, radio test sets, and multipath fading emulators. Technical descriptions of these instruments are provided in this section.
Peak Power Meter Measures
Signals From 1 MHz to 40 GHz
The Model 4500A RF Peak Power Meter provides analysis of RF signals over the frequency range of 1 MHz to 40 GHz and captures 500 kS/s in the single-channel mode. A second input channel enables you to perform measurements on two sources simultaneously at 250 kS/s. The peak power range extends from -40 dB to +20 dB, and the average power range is -50 dBm to +20 dBm with a resolution of 0.1 dB. A NIST-traceable precision calibrator is built in the meter. $15,000. Boonton Electronics, (973) 386-9696.
Universal Power Meter Performs
Radar and Communications Tests
The Series 8540C Power Meter measures the CW, peak, and true average power of complex modulated signals used in EW, radar, and communications systems. A time gating feature allows programming of measurement start- and duration-times, a facility needed to determine the average power of GSM, NADC, and other formats with controlled power on-off trajectories. An 80-dB dynamic range makes the 8540C suitable for open-loop power verification tests required for CDMA systems. Built-in power sweep and frequency calibration ensure high measurement accuracy. Starts at $3,095. Giga-tronics, (800) 726-GIGA (4442).
Meter Measures Digital- and
Analog-Modulated RF Signals
The Model APM-16 Advanced Power Measurement THRULINE® Wattmeter is designed for RF power measurement in PCS, cellular, ESMR, paging, and similar communications systems. It is equally effective for measuring RF power in conventional analog systems. The APM series of plug-in elements measures 1 W to 1 kW over a frequency range from 2 MHz to 2.3 GHz. Operation is identical to most of the company’s wattmeters. Forward and reflected power are measured by simply rotating the element. Call company for price. Bird Electronic, (440) 248-1200.
Power Meter Provides
Stable Readings At -70 dBm
The ML2430A Series of Power Meters combines the advantages of thermal-based, diode-based CW, and diode-based peak power instruments and performs measurements at a rate of 10,000 readings/s. Used in conjunction with the MA2470A series of sensors, the ML2430A provides a 90-dB dynamic range and achieves stable power readings at -70 dBm. The thermal and diode sensors are equipped with internal EEPROMs for storing calibration factor data vs frequency. A wide selection of triggering functions is provided. ML2430A: starts at $3,100; MA2470A: starts at $1,150. Anritsu Wiltron, (408) 776-8300.
Spectrum Analyzers Measure
Signals up to 40 GHz
The new FSEK Microwave Spectrum Analyzers, with a frequency range from 9 kHz to 40 GHz, are versatile, high-performance models designed for precision measurements in the millimeter range. The FSEK20 is a production test instrument, and the FSEK30 is aimed at advanced research and design applications. Both instruments have an accuracy figure of <3.5 dB. A fast sweep repetition rate ensures high manufacturing test throughput. An optional modulation/vector signal analyzer feature helps measure phase distortion. FSEK20: from 55,950; FSEK30: from $74,250. Tektronix, (800) 426-2200 (press 3, code 1036).
Dual-Mode Radio Test Set
Provides Multiple Features
The 2967 Radio Test Set combines digital and analog test capabilities in one instrument. It supports all protocols required to test GSM 900, DCS 1800, and PCS 1900 (GSM) subscriber units as well as analog cellular standards such as TACS, AMPS, and NMT. Transmitter tests measure power, peak and rms phase-error, frequency, and timing error. Receiver tests cover BER, RBER, and FER readings and sensitivity. The 2967 also includes standard RF instruments, such as a spectrum analyzer and frequency counter. $36,800. Marconi Instruments, (888) 369-2586, ext. 105.
Test Set for Multiport Devices
Boosts Production Throughput
The HP 87075C Multiport Test Set performs single-connection testing of 75-W multiport devices such as splitters, taps, and amplifiers. Used in conjunction with the HP 8711C Series Network Analyzer, it measures up to 1.3 GHz and eliminates the time-consuming process of connecting and reconnecting a device to measure all of its signal paths. Calibration time is reduced from about an hour to a few seconds. The HP 87075C is available in two-port, six-port, or 12-port configurations. From $4,000. Hewlett-Packard, (800) 452-4844, ext. 5551.
Multipath Fading Emulator
Suited for Mobile System Tests
The MP2700 Multipath Fading Emulator provides RF channel simulation for third-generation mobile phone systems. The emulator has the high dynamic range and wide bandwidth required for testing UMTS, IMT-2000, or wideband CDMA (W-CDMA) equipment. The standard GPIB interface facilitates remote control via software, such as the company’s CATS-98A package. Although targeted toward CDMA and W-CDMA application, the emulator and the software package can be used for other applications, such as TDMA and GSM system testing. Call company for price. NoiseCom, (201) 261-8797.
Single-Slot VXI C-Size Switch
Module Offers 4-GHz Bandwidth
The Model 1260-59 RF Switch features a 4-GHz bandwidth and provides the low-noise environment needed for switching signals used in modern communications systems. It is available in two versions: the 1260-59A provides four 1×4 relays and the 1260-59B version contains eight 1×4 multiplexers. The 1260-59 is part of the company’s 1260 Series of switching systems, which offers modules in configurations ranging from matrices to multiplexers and frequencies from DC to 26.5 GHz. All modules are message based. Starts at $4,750. Racal Instruments, 800 RACAL-ATE.
Copyright 1998 Nelson Publishing Inc.
February 1998