Boonton 4541, Instruction Manual

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Boonton 4540 Series RF Power Meter
Application Notes
6-23
Step 2:
The Calibrator Level Uncertainty for the Model 2530 External 1GHz calibrator may be calculated from the
calibrator’s specification. The 0dBm uncertainty is 0.065dB, or 1.51%. To this figure, we must add 0.03dB or 0.69% per
5dB step from 0dBm. 13dBm is 2.6 5dB steps (13/5) away from 0dBm. Any fraction must always be rounded to the next
highest whole number, so we’re 3 steps away.
U
CalLevel
= ±(1.51% + (3 × 0.69%))
= ±3.11%
Step 3:
The Calibrator Mismatch Uncertainty is calculated using the formula in the previous section, using the 1 GHz
calibrator’s published figure for ρ
CAL
and calculating the value ρ
SNSR
from the SWR specification on the 57518’s datasheet.
ρ
CAL
= 0.091 (internal 1 GHz calibrator’s reflection coefficient)
ρ
SNSR
= (1.15 - 1) / (1.15 + 1) = 0.070 (calculate reflection coefficient of 57518, max SWR = 1.15 at 1 GHz)
U
CalMismatch
= ±2 × ρ
CAL
× ρ
SNSR
× 100 %
= ±2 × 0.091 × 0.070 × 100 %
= ±1.27%
Step 4:
The Source Mismatch Uncertainty is calculated using the formula in the previous section, using the DUT’s
specification for ρ
SRCE
and calculating the value ρ
SNSR
from the SWR specification on the 57518’s datasheet.
ρ
SRCE
= 0.057 (source reflection coefficient at 900 MHz)
ρ
SNSR
= (1.15 - 1) / (1.15 + 1) = 0.070 (calculate reflection coefficient of 57518, max SWR = 1.15 at 0.9 GHz)
U
SourceMismatch
= ±2 × ρ
SRCE
× ρ
SNSR
× 100 %
= ±2 × 0.057 × 0.070 × 100 %
= ±0.80%
Step 5:
The uncertainty caused by Sensor Shaping Error for a 57518 peak sensor is 4% at all levels, from the sensor’s
datasheet. But since we’re measuring at 900MHz, which is very close to the 1GHz AutoCal frequency, we’ll assume that the
frequency-dependent portion of the shaping error becomes very small, and we’ll estimate that 2% remains.
U
ShapingError
= ±2.0 %
Step 6:
The Sensor Temperature Drift Error depends on how far the temperature has drifted from the sensor calibration
temperature, and the temperature coefficient of the sensor. In our case, we are using a temperature compensated sensor, and
the temperature has drifted by 11 degrees C (49C - 38C) from the AutoCal temperature. We will use the equation in the
previous section to calculate sensor temperature drift uncertainty.
U
SnsrTempDrift
= ± (0.93% + 0.069% /degreeC)
= ± (0.93 + (0.069 11.0)) %
= ± 1.69%
Step 7:
This is a relatively high-level measurement, so the noise contribution of the sensor is probably negligible, but we’ll
calculate it anyway. We’ll assume modulate mode with default filtering. The signal level is 13dBm, or 20mW. The ―noise
and drif
t‖
specification for the 57518 sensor is 50nW, from the sensor’s datasheet. Noise uncertainty is the ratio of these two
figures.
U
Noise&Drift
= ± Sensor Noise (in watts) / Signal Power (in watts)
= ±50.0e-9 / 20.0e-3 100 %
= ±0.0003%
Step 8:
A separate Sensor Zero Drift calculation does not need to be performed for peak sensors, since ―noise and drift
‖
are
combined into one specification, so we’ll just skip this step.