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WHITE
PA P E R
Why Power Meters Can't Ensure
Wireless Devices Are Working Properly
Introduction
This paper describes the differences between the power measurements
made with a power meter and those made with Keithley's Model 2800
RF Power Analyzer. For example, unlike a power meter, which uses a
time domain measurement technique, the Model 2800 uses a frequency
domain measurement technique. Power meters use a sensor to convert an
RF waveform to a low frequency voltage, but the Model 2800 performs a
frequency translation of the RF waveform, then determines RF power by
digitally processing the individual frequency components of the signal.
A power meter can't measure the center frequency, the power in a
specific bandwidth, or the spurious power outside a specific bandwidth.
Although a power meter can describe how much power is being generated, the
Model 2800 can determine whether the power is being generated at the correct
frequency, within the correct bandwidth, and with spurious power within
acceptable limits.
For testing mobile phones and RFIC power amplifiers, a power
meter can't provide all the information the Model 2800 can. For example, a
power meter can't verify a mobile phone or a power amplifier is transmitting
at the correct channel and within the channel bandwidth, and that it isn't
generating excess spurious power. In production, a power meter can pass a
defective phone that's generating the correct power, but in which the power
isn't contained within the appropriate channel. As a result, a power meter is
inadequate for mobile phone and RFIC power amplifier testing because it can't
determine whether the device is transmitting in the correct channel and if the
appropriate amount of power is contained within the channel. A Model 2800,
or an instrument like it, is the only type of instrument that can test a mobile
phone power amplifier and transmitter properly.
Keithley Instruments, Inc.
28775 Aurora Road
Cleveland, Ohio 44139
(440) 248-0400
Fax: (440) 248-6168
www.keithley.com
A G r e a t e r M e a s u r e o f C o n f i d e n c e
Section I describes how a power sensor and power meter work. Section II explains
how the Model 2800 operates. Section III compares the two measurement techniques. Each
instrument's ability to test wireless devices is discussed in Section IV. Appendix A explains
the power sensor term "video bandwidth."
I. Power Meters
Figure 1. Block diagram of power meter and power sensor
Today's power sensor/power meter converts RF power to DC or a low frequency equivalent
signal, as shown in Figure 1. Consequently, both carrier frequency and digitally modulated
data are lost. The detected signal represents the envelope of the RF signal. Figure 2 illustrates
one configuration, showing how the power sensor and power meter work together to make
wideband power measurements. This configuration uses a RF diode sensor. A typical diode-
based RF sensor can detect power over a very wide frequency band, such as 10MHz to
18GHz. Representative signals are shown throughout the schematic.
The sensed DC signal is very low (approximately 160nV for a thermocouple sensor
output with 1mW/0dBm applied power or about 50nV for a diode sensor output with
100pW/