FEATURE TEST & MEASUREMENT
it is important to avoid exceeding the floating voltage rating of any of the supplies or subjecting any of the power supplies to negative voltages. Each power supply should be programmed independently to deliver an equal fraction of the total output voltage and limit current to the maximum that the load can safely handle. Connecting multiple power supplies in
parallel provides higher currents, but again there are limitations. One unit must operate in constant voltage (CV) mode and the rest in constant current (CC) mode. The output load must draw enough current to keep CC unit(s) in CC mode. In modern power supplies outputs can be grouped to create a single output with higher current and power capability.
TIP 7: SIMPLIFY BATTERY DRAIN ANALYSIS WITH ANALYSIS TOOLS To adequately specify the power source for devices that exhibit pulsed and dynamic current loading, it is necessary to evaluate both the peak and DC average current draws. A typical approach is to use an
oscilloscope to monitor a shunt or a current probe, but it is simpler and cheaper to use a power supply with built-in measurement capabilities. Units such as the Keysight 66300 mobile communications DC source store up to 4,096 data points at sample intervals from 15µs to 31,200s. Like oscilloscopes, they acquire pre- and post-trigger buffer data by crossing a user-set threshold. Device characterization software
works with DC sources that have battery emulation capabilities to accurately test designs for mobile, short-range radio, and wireless LAN access devices. Tests are facilitated by dynamic current characterization, data logging and
complementary cumulative distribution function (CCDF) measurements.
TIP 8: CHARACTERIZE INRUSH CURRENT WITH AN AC POWER SOURCE/ANALYSER Characterizing inrush current versus turn-on phase can uncover component stresses, test whether a product produces AC mains disturbances that interact with other products, and help designers to select proper fuses and circuit breakers. Traditionally this involves an AC
source with programmable phase capability and an output trigger port, a digital oscilloscope, and a current probe. Advanced AC power source/analysers with built-in generation, current waveform digitization, peak current measurement and synchronization capabilities can perform inrush current characterization without cabling and synchronizing separate instruments. Similar analysers are available for DC measurements.
TIP 9: USE A POWER SUPPLY TO MEASURE DUT SUPPLY CURRENT Accurately measuring DUT supply currents
Autoranging output characteristic
above 10A is beyond the range of typical DMMs in ammeter mode. One solution is to use an external shunt and the DMM’s voltage mode. Using the power supply itself is better. Many supplies boast an accurate measurement system, including a shunt, and can be started via a single command to the power supply. With typical accuracy around ±0.5% or better at full output levels, the advantages of using power sources to measure high currents is clear. Using them to measure low currents may not be so straightforward. Nevertheless, a power supply with multiple range readback caters for most requirements, offering full scale accuracy of 0.04% + 15µA at low range (100 mA) or 0.04% + 160µA at high range (3A).
TIP 10: CREATE DC POWER WAVEFORMS WITH LIST MODE Instead of using a DAC or arbitrary waveform generator to drive a power supply for creating DC power waveforms, there may be advantages to using a single power supply with list mode. List mode enables complex sequences
of output changes to be generated with rapid and precise timing, which can be synchronized with internal or external signals. Complex DC power waveforms can be produced including pulse trains, ramps, staircases, low frequency sinewaves with DC offset, arbitrary voltage and current waveforms. Once a list of commands is stored in the power supply, the entire list is executed by a single command. Example applications include power supply rejection ratio test, simulating automotive crank profiles, and generating pulse dropouts.
Microlease
www.microlease.com
SOLVING CHALLENGING RF DESIGN AND TEST APPLICATIONS
A second-generation vector signal transceiver (VST) has been introduced by National Instruments that is, the company explains, designed to solve the most challenging RF design and test applications. The NI PXIe-5840 module is the first 1GHz bandwidth VST, meaning it is suited to a wide range of applications including 802.11ac/ax device testing, mobile/Internet of Things device testing, 5G design and testing, RFIC testing, radar prototyping, etc. It combines a 6.5 GHz RF vector signal generator, 6.5 GHz vector
signal analyser, high-performance user-programmable FPGA and high-speed serial and parallel digital interfaces into a single 2-slot PXI Express module. In addition, its measurement accuracy enables the second-generation VST to measure 802.11ax Error Vector Magnitude (EVM) performance of -50 dB. It also offers measurement speeds up to 10x faster than traditional instrumentation using FPGA-based measurement acceleration and highly optimised measurement software, the company claims. “Although engineers can use the second-generation VST to solve many advanced RF test applications right out of the box, its software-designed
38 JULY/AUGUST 2016 | INSTRUMENTATION
architecture enables engineers to customise the user-programmable FPGA,” said Charles Schroeder, vice president of product marketing, RF, at NI. “Using the intuitive LabVIEW system design software, engineers can transform the VST into exactly what they need it to be at the firmware level and ultimately address the most demanding test and measurement challenges. This instrument is an unparalleled example of a product that delivers the RF performance required for traditional test and measurement and the flexibility of a software defined radio.”
National Instruments
www.ni.com/vst
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