Boonton 4500B Peak Power Meter - Manual (Page 328)

Boonton 4500B RF Peak Power Analyzer
Application Notes 6-18
The value to use for calibration level uncertainty depends upon the sensor calibration technique used. If AutoCal
was performed, the calibrator‘s uncertainty at the measurement power level should be used. For sensors calibrated
with FixedCal, the calibrator is only used as a single-level source, and you should use the calibrator‘s uncertainty at
the FixedCal level, (0dBm, for most sensors). This may make FixedCal seem more accurate than AutoCal at some
levels, but this is usually more than offset by the reduction in shaping error afforded by the AutoCal technique.
Calibrator Mismatch Uncertainty. This term is the mismatch error caused by impedance differences between the
calibrator output and the sensor‘s termination. It is calculated from the reflection coefficients of the calibrator (ρ
CAL
)
and sensor (ρ
SNSR
) at the calibration frequency with the following equation:
Calibrator Mismatch Uncertainty = ±2 × ρ
CAL
× ρ
SNSR
× 100 %
The calibrator reflection coefficient is a calibrator specification:
Internal 1 GHz Calibrator Reflection Coefficient (ρ
CAL
): 0.091 (at 1GHz)
The sensor reflection coefficient, ρ
SNSR
is frequency dependent, and may be looked up in the sensor datasheet or the
Boonton Electronics Power Sensor Manual.
Source Mismatch Uncertainty. This term is the mismatch error caused by impedance differences between the
measurement source output and the sensor‘s termination. It is calculated from the reflection coefficients of the
source (ρ
SRCE
) and sensor (ρ
SNSR
) at the measurement frequency with the following equation:
Source Mismatch Uncertainty = ±2 × ρ
SRCE
× ρ
SNSR
× 100 %
The source reflection coefficient is a characteristic of the RF source under test. If only the SWR of the source is
known, its reflection coefficient may be calculated from the source SWR using the following equation:
Source Reflection Coefficient (ρ
SRCE
) = (SWR - 1) / (SWR + 1)
The sensor reflection coefficient, ρ
SNSR
is frequency dependent, and may be looked up in the sensor datasheet or the
Boonton Electronics Power Sensor Manual. For most measurements, this is the single largest error term, and care
should be used to ensure the best possible match between source and sensor.
Sensor Shaping Error. This term is sometimes called ―linearity error‖, and is the residual non-linearity in the
measurement after an AutoCal has been performed to characterize the ―transfer function‖ of the sensor (the
relationship between applied RF power, and sensor output, or ―shaping‖). Calibration is performed at discrete level
steps and is extended to all levels. Generally, sensor shaping error is close to zero at the autocal points, and
increases in between due to imperfections in the curve-fitting algorithm.
An additional component of sensor shaping error is due to the fact that the sensor‘s transfer function may not be
identical at all frequencies. The published shaping error includes terms to account for these deviations. If your
measurement frequency is close to your AutoCal frequency, it is probably acceptable to use a value lower than the
published uncertainty in your calculations.
All peak power sensors use the AutoCal method only. The sensor shaping error for peak sensors is listed on the
sensor‘s datasheet or in the Boonton Electronics Power Sensor Manual.