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Choosing calibration power levels
If you are measuring an amplifier and the output
power level is higher than the input power level,
at what power level should you calibrate: at the
higher power level, the lower power level, some-
where in between the two levels, or at a different
level for each port? Calibrating and measuring
with the test port powers in the range of lowest
uncertainty, –10 to –50 dBm for the Agilent 8753E,
results in accurate measurements as shown above.
Calibrating at the input (lower) power level can
result in a power level at the output receiver that
is too low, because attenuation is required to pro-
tect the analyzer during measurement of the DUT.
Calibrating at the output (higher) power level
might not be possible since the gain of the DUT
can result in an output power level that the net-
work analyzer's source together with the booster
amplifier cannot generate. Therefore, you might
need to perform some calibrations at a power
level between the input power level and the
output power level. In general, to reduce noise
calibrate at the highest possible power below the
onset of receiver compression.
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Calibrating at one power level versus two power levels
A response calibration is done at only one power
level since only one forward sweep is made. Full
two-port calibration, on the other hand, can be
done at different power levels since both forward
and reverse sweeps are made. The analyzer can
calibrate one port using one power level and the
other port using another power level to more
accurately match the power levels present during
the measurement of a DUT.
As an example, the open, short, and load standards
for port one and the reverse through measurements
may be made at the lower power level. The open,
short, and load standards for port two and the
forward through measurements could then be
made at the higher power level. It is also possible
to perform a full two-port calibration at one power
level as in a response calibration. The power level
used would be determined from the dynamic
accuracy specification of the analyzer.
A final note on calibration: power-handling
capabilities of the calibration standards can
become an issue when calibrating at high power
levels. The common standards used for calibration
are the open, short, load, and through. The open,
short, and through standards are not a problem
since they do not dissipate any energy. The load,
how-ever, does dissipate energy so when you are
calibrating make sure the standards can handle
the power level.
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