Boonton 4240 RF Power Meter - Manual (Page 115)

Boonton 4240 Series RF Power Meter
The value to use for calibration level uncertainty depends upon the sensor calibration technique used. If AutoCal
is the mismatch error caused by impedance differences between the
is calculated from the reflection coefficients of the calibrator (ρ
CAL
)
quency
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
calibrator output and the sensor’s termination. It
and sensor (ρ
) at the calibr
SNSR
ation fre with the following equation:
a = ±2 × ρ
CAL
× ρ
SNSR
× 100 %
o
lection Coefficient (ρ
CAL
): 0.024 (at 50MHz)
oefficient, ρ
SNSR
is frequency dependent, and may be looked up in the sensor datasheet or the
Calibr tor Mismatch Uncertainty
The calibrator reflection c efficient is a calibrator specification:
50 MHz Calibrator Ref
The sensor reflection c
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
) and sensor (ρ ) at the measurement frequency
SRCE SNSR
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:
he
surements, this is the single largest error term, and care
ror”, and is the residual non-linearity in the
ize the “transfer function” of the sensor (the
be
your
measurement frequency is close to your AutoCal frequency, it is probably acceptable to use a value lower than the
ty in yo on
the shaping error is higher because it relies upon stored
the shape of the transfer function, rather than a transfer
the current time and temperature. For this reason, use of the AutoCal
ensor Characteristics" outlined in Section 2
tronic nsor Manual. If the AutoCal calibration method is used with a CW sensor, a
ay b all signal levels.
Source Reflection Coefficient (ρ
SRCE
) = (SWR - 1) / (SWR + 1)
ent, and may be looked up in the sensor datasheet or t The sensor reflection coefficient, ρ
SNSR
is frequency depend
Boonton Electronics Power Sensor Manual. For most mea
should be used to ensure the best possible match between source and sensor.
Sensor Shaping Error. This term is sometimes called “linearity er
measurement after an AutoCal has been performed to character
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.
A ponent of sensor shaping error is due to the fact that the sensor’s transfer function may not
identical at all frequencies. The published shaping error includes terms to account for these deviations. If
n additional com
published uncertain ur calculati s.
For CW sensors using the fixed-cal method of calibrating,
o describe "shaping coefficients" from a factory calibration t
calibration using a precision power reference at
method is recommended for CW sensors rather than simply performing a FixedCal. The shaping error for CW
sensors using the FixedCal calibration method is listed as part of the "S
of the Boonton Elec
s Power Se
fixed value of 1.0% m e used for
Application Notes
6-7