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IMS2022


ARTICLE | JUNE 1, 2022




HOW TO ELIMINATE COMMON POWER METER MISTAKES

Source: AR Modular RF

Modern wattmeters and power meters are simple to use and can provide digital
measurement data to several decimal places in dBm or watts. Despite this
accuracy, consistent power measurement remains elusive: different users
regularly take measurements using similar tools and methodologies, only to come
up with different numbers.

The most common reason for such disparity is user error, and correcting for the
mistakes described here will dramatically improve measurement accuracy. Most of
these issues are rooted in the fact that, when carrying out the most common
power meter application — measuring a signal’s power level — users tend to
forget that the power meter is not measuring exactly what they seek. Rather than
the signal’s power level, specifically, the power meter measures total power
over the sensor’s entire bandwidth.

Power Meters, Wattmeters, and Correction Factors

Most people recognize that a calibrated power meter is a superior measurement
instrument compared to a wattmeter, but they cannot explain why. Generally, a
wattmeter is similar to a power meter in that they both measure broadband power.
However, unless the user corrects the power meter for frequency, they are using
it as a wattmeter.

Power meter sensors that do not employ electronic calibration (e-cal) come with
a graph or tabular data showing the calibration factor and correction factors by
frequency (Fig. 1). Most users employ the reference calibration factor (CF)
percentage to set the power meter at the appropriate calibration reference
frequency, but what are the other numbers for?



Fig. 1 — Power meter stickers showing calibration factor settings according to
frequency. For frequencies not directly enumerated, users can apply
straight-line interpolation of adjacent data. If the measurement frequency is
not enumerated on the sensor or between data provided, you are using the wrong
sensor!

These numbers, in e-cal devices, are correction factors — similar to reference
CFs in older meters (99% and 99.4%, respectively, in Fig. 1) — and indicate the
response of the sensor to power as measured at those frequencies across the
entire measurement range of the sensor. The numbers are usually a percent of the
full-scale response and can vary between about 80% and 100%. For example, say
the sensor reference CF is 100% (not uncommon) and the CF of the frequency your
signal of interest occupies is 95%. If the user carefully performs a calibration
and then plugs the sensor into the signal port to take a measurement, the result
will be in error by at least 5%, or about 0.2 dB.

Since power sensors are available with correction factors as low as 90%,
measured power can be in error by as much as 10% — before other known
uncertainties even have been considered. Assuming the power to be measured is
within the sensor range, the signal-to-noise ratio is acceptable, and the VSWR
of the measurement port is acceptable, then the most important correction a user
can make to reduce the error is to account for the frequency of measurement.
Note, too, that measurement accuracy at higher power levels is more
significantly impacted by such errors than low-power operation: at -20 dBm, a
0.2 dB error only translates to about 0.5 uW, but at +55 dBm, the error is over
15 watts!

Correcting for Test Frequency

When operating a power sensor with a correction table by frequency (e.g., Fig.
1), the user must enter the percent correction, as shown on the sensor,
correlating to the frequency being measured. When operating a power sensor with
e-cal data, the user still must enter a frequency before the meter can apply a
correction. Users commonly err in assuming meters with e-cal data automatically
apply this correction; they do not. E-cal sensors’ key advantage is that users
do not have to interpolate the correction percentage by frequency from the data
table or graph. When the frequency is entered, the meter applies the appropriate
correction.

Still, be aware that CF uncertainty remains a factor when using power sensors
even when correction for frequency is applied. CF uncertainty increases with
frequency from about 1% to 3%, depending on the frequency range, but failing to
apply the correct CF to the measurement compounds this error.

Measurement System Slope and Offsets

Applying corrections by frequency to power measurements goes beyond simply
correcting the sensor. If the RF power level to be measured is not connected
directly to the power meter head — whether the RF path is just an attenuator or
an entire test bench setup — the user must account for the “slope” of the RF
path to correct the measurement.

A common method uses a measurement “offset” to add the attenuation of the path
loss to the power measurement displayed. Unfortunately, a single offset is
frequently used, as the meter may only retain a single value. The offset value
must be changed for each measurement frequency that has a different loss. Some
e-cal power meters allow users to input a table of offsets that will be
interpolated by the meter, but this feature only functions properly when the
user inputs the measurement frequency. High-quality attenuators generally come
with calibration data, or the slope can be measured with a network analyzer to
provide a reference.

The loads passing through an attenuator typically emerge relatively flat (i.e.,
they have the same amount of loss all the way across the beginning frequency of
the load to the end), but differences can exist, and misapplication of
attenuation can damage components. Accordingly, users will want to check the
load at each frequency using a load analyzer — gauging how much loss exists at
400 MHZ, 600 MHZ, or whatever frequency is being tested for that load. That loss
also is included in the user’s calibration offset.

Cables and Signal Generators

To ensure measurement accuracy, it is important not to take the signal
generator’s “Power Out” setting at face value. For example, a signal generator
set at 0 dB output may not, in fact be generating exactly 0 dB — thus, the
output of a device under test (DUT) may be flawed due to an erroneous input
signal. To alleviate this issue, use an inline power meter on the back of the
DUT. The meter will gauge input power to the system, ensuring the signal
generator’s output power matches its setting (Fig. 2). 



Fig. 2 — An input power sensor (R) is attached to a DUT, as well as a second,
output power sensor (L). This setup also uses the forward port of a directional
coupler to measure the DUT’s output power. This setup eliminates the possibility
of human error associated with accounting for cable losses.

 

Power meter users also must remember to calibrate based on any cables they use
with the system. Most cables, depending on their length, exhibit up to 0.5 dB of
loss. At 500 watts, a 0.5 dB loss via a cable would appear as about a 54-watt
loss in the system (if the cable loss was not included in the user’s calibration
setup*) 

Like calibration of the power meter sensor itself, calibration based on cable
loss is a simple-to-understand, but often-overlooked aspect of power meter use.

Bandwidth and Noise

Due to the high bandwidth of a power sensor, signal-to-noise ratio (S/N) is
critical when using a power meter to measure a signal’s power. Remember, total
thermal noise power (kTB) is a function of the Boltzmann constant (k), the
load’s absolute temperature (T), and the measurement bandwidth (B). Applying
this rule, a sensor with a 26 GHz bandwidth will exhibit B > 100 dB!
Accordingly, noise power can dominate the total power measured when signals fall
as low -50 dBm.

Adding a post-amplifier will not help S/N, as the noise will be amplified
alongside the signal. In this situation, a filter is warranted — ideally, a
band-pass filter at the frequency of interest. In the absence of a band-pass
filter, a high- or low-pass filter (or both) can help. Check the ON/OFF power
level of the system noise without the signal to see if kTB is a factor in your
measurement.

Additionally, when measuring signals at higher power levels (e.g., ~250 watts
and above), it is important to be aware of strong harmonics or other spurious
signals that may contribute too much to the total power measured. For example,
the receiver being used could pick up a harmonic and interpret it as transmitted
signal. Check the signal with a spectrum analyzer to be sure. In such scenarios,
filters again can be used to subtract that power before it reaches the sensor.
At lower power levels, a good spectrum analyzer (SA) may even record a superior
measurement over a broadband power meter, as the SA will employ detection and
filters that exclude the noise power.

VSWR Can Make a Difference

Another practical power meter measurement correction comprises minimizing
measurement uncertainty created by VSWR. When measuring with a sensor on an
unknown port, the user can quickly check with a 6 dB pad to see if the
measurement improves. Adding 6 dB of loss increases the return loss by 12 dB and
should improve poor VSWR by a substantial amount, allowing for a better
measurement.

The easiest way to accomplish this is to use the power meter “Relative”
measurement feature to “zero” the displayed power level. Then, add the pad and
see if the level changes by 6 dB. As long as the signal level remains in the
sensor’s range, if it changes more than the pad value, the VSWR is adversely
affecting the measurement and a pad should be utilized. It might require even
more attenuation than 6 dB. In any case, the user must enter the percent
correction (as shown on the attenuator) for the frequency being measured, or
they must input the measurement frequency.

To reduce the uncertainty of a relative measurement, try to keep the power
applied to the sensor at the low end of the power range. The relative
uncertainty can be as high as 6% at the high end, and as low as 1% at the low
end.

Final Thoughts

Many factors contribute to a successful power measurement, including using the
correct type of detector for the signal of interest, and making allowances for
gating and various forms of modulation. Most measurement errors are the result
of a poor setup. Avoiding the biggest power measurement mistake — by remembering
a power meter produces a broadband measurement, and does not just measure a
discrete signal — is a significant step toward more accurate and more consistent
results.

About AR Modular RF 

AR Modular RF is a technology leader in the design and manufacture of RF booster
amplifiers for use with military tactical radios to enhance radio performance by
providing more power, leading to greater range and strengthened communications.
AR Modular RF’s booster amplifiers are radio and waveform agnostic, providing
universal, turnkey, plug-and-play support with just an RF link to any
manufacturer’s host radio. Visit us at www.arww-modularrf.com.

 







AR MODULAR RF

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