Users of mobile phones and other wireless devices are always looking for longer battery life.
Measuring your mobile device’s overall power consumption is useful for optimizing
battery run time, but you also need to measure and analyze your device’s sub-circuits.
Some examples of important sub-circuits include: • Display illumination power controls
• Baseband microcontrollers for voltage-frequency scaling • RF power amplifiers when dynamic voltage control is used
All the various sub-circuits are usually powered by a power management unit.
The PMU provides independent, regulated bias voltages to each of the sub-circuits.
In this case, it is usually necessary to measure the respective voltage and current powering
each sub-circuit instead of directly powering it with an external DC source. Here is a setup
using conventional data acquisition equipment for measuring a battery input at the same
time as a sub-circuit voltage and current. High-speed digitizing of the current is essential,
as it provides many detailed insights about activities. Individual sub-circuits often
have much lower minimum currents and therefore may need even greater dynamic measurement
range than the overall device needs. The wide-ranging current drain usually exceeds the instrumentation’s
dynamic measurement range, limiting achievable accuracy. This becomes even more of a problem
when you consider that the current-shunt peak voltage drop should be kept below 50 milli-volts
so it does not affect the sub-circuit operation and test results. Conventional data acquisition
equipment usually falls short of providing sufficient gain, dynamic range, and accuracy
for good results. Additional independent measurement channels are also needed to log the battery
and sub-circuit voltages simultaneously with their currents. They provide the key insights
necessary for analyzing the battery input, sub-circuits, and PMU control activities of
your device. There is a better measurement solution from Agilent Technologies for making
these challenging measurements. The Agilent N6781A Source/Measure Unit installed in a
N6705B Mainframe has specialized features for testing battery powered mobile devices.
Here you see a test setup using two Agilent N6781A units to test a GPS device.
One of the units was connected to provide a battery input and the other was used to measure
the RF amplifier sub-circuit voltage and current. The N6781A has a number of unique features
that provide improved accuracy for this type of testing. The battery emulation source has
realistic input power characteristics and more accurate results. The zero-burden shunt
operating mode eliminates the voltage drop problem of conventional shunts. An independent
DVM input is provided for logging the sub-circuit bias voltage. Seamless measurement ranging
provides accurate current measurement of signal levels spanning from sub micro-amps to amps
in one continuous logging measurement. High-speed, high-resolution waveform capture and data
logging capabilities assure accurate measurements for most pulsed signals you are likely to encounter.
In this example, we measured the input power demands and turn-on timing
of the RF amplifier in a GPS device. To conserve power, the PMU should power up sub-circuits
only as needed. The test results, captured and displayed using the Agilent 14585A Control
and Analysis software, are shown here. Findings include: The battery input drew average and
peak currents of 0.290 amps and 0.822 amps respectively. The PMU powered up the RF amplifier
at the desired point of operation, as required. As you can see, evaluating sub-circuits with
this Agilent system configuration is a powerful way to gain key insights for optimizing your
mobile device’s battery run-time. For more information, visit the Agilent website at
www.agilent.com/find/N6781