Boonton 4240 RF Power Meter Manual do Utilizador descarregar pdf (Página 114)

Boonton 4240 Series RF Power Meter

al his reason, the uncertainties are more

commonly combined using the RSS method. RSS is an abbreviation for “root-sum-of-squares”, a technique in which

each

“n

st normalized in this

way, term

Three main e of

he worst case approach is a very conservative method in which the extreme conditions of each of the individual

ncertainties are added together. If the individual uncertainties are all independent of one another, the probability of

l being at their worst-case conditions simultaneously is extremely small. For t

uncertainty is squared, the squares are summed, and the square root of the summation is calculated.

efore the RSS calculation can be performed, however, the worst-case uncertainty values must be scaled, or

ormalized” to adjust for differences in each term’s probability distribution or “shape”. The distribution shape is a

atistical description of how the actual error values are likely to vary from the ideal value. Once

s with different distribution shapes can be combined freely using the RSS method.

types of distributions are Normal (Gaussian), Rectangular, and U-shaped. The multipliers for each typ

istribution are as follows:

Distribution Multiplier “K”

Normal 0.500

Rectangular sqrt(1/3) = 0.577

U-shaped sqrt(1/2) = 0.707

The formula for calculating RSS measurement uncertainty from worst-case values and scale factors is:

‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗‗

RSS

= √ (U

)

+ (U

)

+ (U

)

+ (U

)

+ ... (U

)

This calculation yields what is commonly referred to as the combined standard uncertainty, or U

, with a level of

confidence of approximately 68%. To gain higher levels of confidence an Expanded Uncertainty is often employed.

6.2.2

is very small, since absolute errors in the circuitry

are maintained in calibrated condition. The figure is a calibrator specification which depends upon

+20 to -39 dBm: ±0.075 dB (1.74%)

-40 to -60 dBm: ±0.105 dB (2.45%)

here U

through U

represent each of the worst-case uncertainty terms, and K

through K

represent the normalizing

ultipliers for each term based on its distribution shape.

sing a coverage factor of 2 (U = 2U

) will provide an Expanded Uncertainty with a confidence level of

proximately 95%.

Discussion of Uncertainty Terms.

ing is a discussion of each term, its definition, and how it is calculated.

Instrument Uncertainty. This term represents the amplification and digitization uncertainty in the power meter, as

well as internal component temperature drift. In most cases, this

are calibrated out by the AutoCal process. The instrument uncertainty is 0.23% for the 4240 Series.

Calibrator Level Uncertainty. This term is the uncertainty in the calibrator’s output level for a given setting for

calibrators that

the output level:

50MHz Calibrator Level Uncertainty:

At 0 dBm: ±0.055 dB (1.27%)

Application Notes

6-6

1 2 ... 109 110 111 112 113 114 115 116 117 118 119 ... 127 128

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