technology and measuring techniques advance, the time eventually comes
when continuing to use old methods not only doesn't make sense, but
can actually harm your reputation by providing data that cannot be duplicated
by customers who long ago adopted the new ways. Orwill Hawkins, of LadyBug
Technologies, has written a white paper describing how to make accurate,
repeatable peak and pulse power measurements on waveforms using the
precision of modern instruments that provide a graphical view of the
entity being investigated. USB power meters provide an inexpensive means
of obtaining high quality measurements and the benefits of a graphical
display environment (on your computer) at a relatively low cost. I particularly
like the use of the word 'mesial' in describing the midpoint of a pulse's
rising and falling edges.
Measuring Peak and Pulse Power with USB Power Sensors
LadyBug Technologies, LLC
computing and measurement technology have increased the capabilities
of USB RF Power
Sensors. These advances have made measurement capabilities possible
at modest costs that were previously out of reach. Capabilities like
direct measurement of pulse power can now be done with low cost USB
power sensors, such as those used in the example below. This paper will
compare Pulse Power measurement using new, accurate USB Sensor technology
to traditional measurement methods.
Pulse Power has been measured using average power and applying the pulse’s
duty cycle using the long time accepted formula:
= Average Power / Duty Cycle.
This method uses total measured
power over time and produces a computed Pulse Power result based on
assumed pulse information. Unfortunately, additional information such
as peak power, droop, and crest factor may remain unknown, rendering
the measurement limited in value. Significant errors are also possible
with this method due to errors in the assumed pulse width and duty cycle.
Errors caused by discrepancies between actual and assumed pulse shape
or malfunctioning equipment may also occur.
the rapid advances in modulation technology, it is important to recognize
the need for the additional information and accuracy that is now available
with today’s Power Sensor technology.
Modern USB Power
Sensors utilize cutting-edge processing technology and are capable of
rapidly measuring and digitizing the demodulated waveform. These sensors
integrate the measurement data over time and provide the user with actual
pulse power, and provide additional measured parameters such as peak
power, crest factor and duty cycle, in addition to average power. This
information is often very useful for the engineer, designer and technician.
Figure 1 depicts a microwave pulse stream and indicates some of the
desirable information that can be measured with a USB Power Sensor.
Figure 1 - Pulse Waveform Detail
the necessary measurements, USB Peak and Pulse sensors use advanced
trigger schemes that accurately locate the pulse mesials and process
the digitized data from the measured power. These sensors provide accurate
Peak Power, Pulse Width, Pulse Power, Pulse Repetition Rate, Duty Cycle,
Crest Factor and more. As an alternative, some USB sensors, such as
the LadyBug LB480A PowerSensor+™ that was used in the example below,
support external triggering. This allows control of the measurement
timing and can be particularly useful with very low power measurements
where the signal might be near the noise floor. Averaging repetitive
signals improves measurement accuracy.
Pulse Power Example
For example purposes, pulse power was measured from a small test
source. The pulse modulating waveform has an 11 microsecond pulse
width and a pulse repetition time of 100 microseconds. The Pulse carrier
frequency is 1.9 GHz, and the on/off ratio for the test signal
is better than 80 dB. Pulse power is just under 6 dBm. The
test source was connected directly to the LadyBug LB479A Peak, Pulse
and Average Sensor. The LB479A operates from 10 MHz to 8 GHz
and over a dynamic range of - 60 dBm to +20 dBm.
A screen shot of the measurement is shown in Image 1. All of these measurements
were made simply by connecting the sensor to the signal source and entering
the carrier frequency. This greatly simplifies and speeds up the measurement
process weather it is in a bench, ATE, or service environment.
Image 1 - Power Measured with a LadyBug LB479A
The traditional method requires the user to
know the signal’s Duty Cycle in order to calculate Pulse Power. Because
Average only Sensors are not capable of measuring Duty Cycle, it must
be calculated using the formula below. In many cases the Duty Cycle
is given or assumed, not measured, adding suspicion to the measurements
accuracy. In this case the Duty Cycle is given as 11.0%
Cycle=Pulse Width / Pulse Repetition Rate.
Calculated Duty Cycle
is 11% (=11 us/100 us)
Note that the calculated
11% Duty Cycle is close to the 11.1% actual Duty Cycle measured by the
sensor. This method can lead to other inaccuracies in the measurement
caused by errors in the assumed pulse parameters, amplifier or modulator
distortion, failing components etc.
The sensor measured
-3.657 dBm average power. Prior to applying the traditional method
of determining Pulse Power, the measured average power must be converted
from dBm to a linear scale, in this case mW. The following equation
P mW=10^(PdBm/10) = 10^(-3.657/10) = 0.431 mW.
Now that linear power and Duty Cycle are known, Pulse Power can be calculated
using the standard traditional formula:
Pulse Power = Average
Power / Duty Cycle as mentioned earlier.
Plugging in the
numbers results in a pulse power of 3.92 mW or 0.431 mW/0.11.
This can now be converted back to dBm using
The result is 5.933 dBm, and is very close to the more precisely
measured 5.879 dBm.
For further comparison, the Sensor was
replaced with a LadyBug LB480A with pulse profiling time domain analysis
capability as shown in Image 2.
Image 2 - Pulse Profile Measurement
Zoom was utilized
to expand the data and three markers were placed to determine the pulse
width and repetition rate. Start time is shown by Marker #1 at 199.950 us,
Marker #2 marks the pulse’s end time at 210.950 us, and Marker
#3 marks the end of the cycle at 311.100 us. The results visually
and mathematically confirm previous measurements.
USB Peak, Pulse and Average Power Sensors, provide a lot of the
valuable information that a Time Domain, Pulse Profiling sensor provides,
at less cost. These sensors accurately measure Peak, Pulse & Average
Power plus Duty Cycle along with analysis such as Crest Factor. The
Sensors can expose unexpected system faults that cannot be found using
an Average Power Sensor. These facts make them a very good alternative
to traditional methods that utilize average only Sensors and combine
the readings with the assumed pulse parameters. Today’s modern Peak,
Pulse and Average Power Sensors such as the LadyBug Technologies LB479A
that was used in the example above, are cost competitive solutions for
RF engineers and technicians.
Phone: 707-546-1050 x103
Posted October 24, 2013
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