Boonton 4530 Peak Power Meter User Manual - Manual (Page 158)

Chapter 5 Boonton Electronics
Making Measurements 4530 Series RF Power Meter
5-16
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 (
,
SRCE
) and sensor (
,
SNSR
) at the measurement frequency with the following equation:
Source Mismatch Uncertainty = ±2
0 ,
SRCE
0 ,
SNSR
0 100 %
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 (
,
SRCE
) = (SWR - 1) / (SWR + 1)
The sensor reflection coefficient,
,
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.
For CW sensors using the fixed-cal method of calibrating, the shaping error is higher because it relies upon
stored “shaping coefficients” from a factory calibration to describe the shape of the transfer function, rather
than a transfer calibration using a precision power reference at the current time and temperature. For this
reason, use of the AutoCal 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 in the Boonton Electronics
Power Sensor Manual as “Power Linearity Uncertainty”, and depends upon signal level. If the AutoCal
calibration method is used with a CW sensor, a fixed value of 1.0% may be used for all signal levels.
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.
Sensor Temperature Coefficient. This term is the error which occurs when the sensor’s temperature has
changed significantly from the temperature at which the sensor was AutoCal’d. This condition is detected by
the Model 4530 and a “temperature drift” message warns the operator to recalibrate the sensor for drift
exceeding ±4C on non-temperature compensated peak sensors. For these sensors, the typical temperature
effect 4 degrees from the AutoCal temperature is shown as a graph versus level on the sensor datasheet.
Temperature compensated peak sensors have a much smaller temperature coefficient, and a much larger
temperature deviation, ±30C is permitted before a warning is issued. For these sensors, the maximum uncertainty
due to temperature drift from the autocal temperature is:
Temperature Error = ± 0.04dB (0.93%) + 0.003dB (0.069%) /degreeC
Note that the first term of this equation is constant, while the second term (0.069%) must be multiplied by the
number of degrees that the sensor temperature has drifted from the AutoCal temperature.
CW sensors have no built-in temperature detectors, so it is up to the user to determine the temperature change
from AutoCal temperature. Temperature drift for CW sensors is determined by the temperature coefficient of
the sensor. This figure is 0.01dB (0.23%) per degreeC for the 51075 and many other CW sensors. Consult the
Boonton Electronics Power Sensor Manual for the exact figure to use. Sensor temperature drift uncertainty
may be assumed to be zero for sensors operating exactly at the calibration temperature.