Sunshine Design Engineering Services
Joe Cahak, owner of Sunshine Design Engineering Services, has submitted another
fine article for posting here. Joe has many years of automated RF testing experience
to leverage when writing this paper on the basics of power measurement.
See list of all of Joe's articles at bottom of page.
Ponderings on Power Measurements
By Joseph L. Cahak
Copyright 2013 Sunshine Design Engineering Services
Pondering on Power Measurements
Figure 1 - Power Sensor type and Range courtesy Agilent
Figure 2 - Thermistor Power Meter Schematic courtesy Agilent
Figure 3 - Thermistor temperature response courtesy Agilent Technologies
Figure 4 - Diode Power Sensor courtesy Agilent Technologies
Figure 5 - Universal Power Sensor Range courtesy Anritsu
Figure 6 - Universal Wide Power Range Sensor courtesy Anritsu
Figure 7 - E4412/13 Sensor Architecture courtesy Agilent Technologies
Figure 8 - Sensor for E4412/13 Power Sensor courtesy Agilent
Figure 9 - Diode Sensor Error with harmonic signal present courtesy
Figure 10 - DSP Power Measurement courtesy Agilent Technologies
Figure 11 - Power Mismatch Curves for Anritsu Detector courtesy
A power measurement is a scalar quantity and is a measure of power detected.
These measurements can be made a variety of ways. Most of us are familiar with the
notion that voltage (volts) multiplied by current (amps) is power (watts) and power
multiplied by time is energy. At DC or low frequencies these power measurements
from the current or voltage is relatively easy and not very complicated. As we get
to higher frequencies the typical means of measuring voltage or current breakdown
and are not accurate. The power measurement inaccuracies are due to frequency response
of the detectors at high frequency and also the impedance match of the detectors
as well as the instantaneous frequency response of the detector network. All power
sensors are broadband sensors. They cannot discriminate between individual signals
in a multiple signal environment. These signals can add or subtract from the total
power as a combination of the power depending if the signals are in or out of phase.
Power measurement in the RF and Microwave frequency range are typically made
with thermistor, thermocouple or diode based instruments. The thermistor or thermocouple
based power sensors are most accurate for “true” or RMS power. True power is properly
integrated (modulation envelope) over time to give the' true' power no matter the
waveform shape. If the signal is a CW (continuous wave) signal that does not vary
in signal strength or frequency, the measurement is relatively easy and the RMS
value is easy to compute. In the case of more complicated modulated signals or complex
waveforms, computing or measuring True RMS power gets more difficult and complicated.
To better understand this we will review the methods of measuring RF Power.
Thermistor and Thermocouple Devices
The most accurate methods of measuring True RF power is with devices called a
thermistor or a thermocouple. These devices converts RF power to thermal power (heat)
and the thermal power is converted to a resistance or a voltage difference measurement
that can be measured and converted to the power measured. There are issues associated
with this method of measuring RF power.
The first issue with this method is a limited dynamic range that it will accurately
measure over. Most of the sensors in the market today that use thermistor based
sensors have a measurement range of -30 to +20 dBm. Some measure higher power
levels with an attached calibrated attenuator. Microprocessors and EEPROM calibration
tables are used to perform power correction for temperature and frequency response.
With RF power to thermal conversion, there is a small time lag for the thermal
response from the RF power. While this is a small sensor with a small thermal mass
in the sensor, nonetheless this equates to a small lag in the power response. This
property will affect accuracy of rapidly varying signals, and signals with complex
modulation. Finally, there is a frequency response associated with the sensor and
also the impedance match of the interface to the sensor. These responses can be
calibrated and removed using a cal factor for the sensor.
Another method of measuring power is with a diode sensor. These diode sensors
have a faster response time than thermistor based sensors, but due to the diode
characteristics, they have more impedance match issues than the thermistor. The
diode has low impedance compared to the 50 ohm characteristic Impedance of most
RF instruments and RF networks in use today. This means that some form of matching
network must be used to improve the match into the sensor and DC isolation (blocking).
These components have frequency sensitivity.
The diode is sensitive to VSWR and is more prone to measurement error due to
these issues. Another issue is the non-linearity of the diodes. What that means
is at higher power levels the diodes conduct and the current is no longer square
law proportional to the voltage of the detected signal. This has ramification with
measuring complex signal environments. Recall the power equation from voltage:
Power = V2/R = I2*R
This implies that while the diode is in the square law region the voltage output
from the rectification is directly proportional to the power in that region only.
Outside that region, the power is not directly linear to Voltage.
Making power measurements in the quasi and linear regions of the diode response
is less accurate when the signal input is modulated with wide bandwidth signals
or multiple tone signals. To make these measurements, the instrument must have the
dynamic measurement power range and the frequency response to be quantifiable, repeatable
and correctable. For the diode sensors, extensive EEPROM correction tables are used
for the frequency, signal levels and temperatures at which the power measurements
will be made. In many cases these corrections are not adequate for very wideband
devices such as Ultra Wideband USB or some of the other digital modulation formats.
Most sensors have an instantaneous bandwidth that they can respond to which typically
range from 10 MHz to 30 MHz for most power sensors available on the market.
This is not important for most measurement markets. With modulation formats wider
than this and higher in power than the square law region, pulsed power or sensor
instantaneous bandwidth can have varying amounts of error. Recall the comment above
regarding operation above the square law region. The trick that can be used to gain
some level of better power accuracy for modulated signals with diode sensors is
to keep the power within the square law region (-70 to -20 dBm).
Analog Devices has recently come out with a replacement for the Schottky diodes
to measure power. The ADL6010 is a coplanar input for measuring power from 500MHz
to 50GHz. It features built-in linearization for added accuracy.
One trend is communication power measurements is to use DSP (digital signal processing)
architecture to process the signals and get a better measure of power with complex
formats and frequency components. These can also provide the ability to measure
the peak or envelope power and crest power on multiple tones or modulated signal
measurements. They can also offer wider bandwidths than traditional sensors. Capability
is only limited by the sampling rate, bit depth and accuracy of the ADC's or Sigma-D
Peak Envelope Power and Peak or Crest Power
Other RF Power measurements are peak envelope power (PEP) and peak or crest power.
These are used to measure the power of multi tone and digitally modulated waveforms
to get the instantaneous power maximum of the system. There are many instances where
a power measurement that takes the peak power value of the envelope is needed. All
digitally modulated waveforms, AM and single sideband (SSB) use this measurement.
The peak measurement is also the crest power, which would be compared to the average
power to calculate the crest factor of the RF device, which is the ratio of the
peak power level above average power. These peaks can damage power amplifiers if
not contained in amplitude.
Instantaneous or Video Bandwidth
Instantaneous or video bandwidth (VBW) is the response after rectification of
the signal and the detection circuitry response and ability to integrate the RMS
power. This video modulated rectification result is used to calculate the power.
If the detection circuitry downstream of the rectification has poor frequency response,
the accuracy of the power measurement will degrade. Typical video bandwidth range
is 10 MHz up to 100 MHz video or instantaneous bandwidth. The user must be aware
of the signal measurement equipment requirements to account for this signal bandwidth
and to thereby ensure accurate power measurements of modulated signals. If measuring
pulsed power the Video or instantaneous bandwidth should be at least 5x the pulse
The quality or accuracy of the power measurement depends not only on the power
sensor calibration factors previously mentioned. Another significant source of measurement
error is the sensor impedance match and the match of the device port under test.
This mismatch error is computed with the formula Mismatch Error= 10log(1±ρgρl)2
. The + and – represent the max and minimum mismatch for the measurement mismatch
loss of power measured. ρg and ρl are the generator and
load reflection coefficient.
Agilent Application Notes:
Anritsu Product or Application Notes
- 4 Steps for Making Better Power Measurements App Note 64-4D 5965-8167E
- Fundamentals of RF and Microwave Power Measurements (Part 1) 1449_1_5988-9213EN
- Fundamentals of RF and Microwave Power Measurements (Part2) 1449_2_5988-9213EN
- Fundamentals of RF and Microwave Power Measurements (Part 3) 1449_3_5988-9213EN
- Fundamentals of RF and Microwave Power Measurements (Part 4) 1449_4_5988-9213EN
- Power Measurement Basics 5965-7919E
- ML2400A Series Power Meter -15000-00004 rev C
- Accurate Power Measurements on Modern Communication Systems
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Posted June 9, 2020
(updated from original post on 10/8/2013)