9500B
Calibration Manual
60
Calculation of Acceptance Limits
1.
Introduction
The first part of the verification procedure in Verifying the Leveled Sine
Function: LF Gain deals with verification of amplitude at frequencies below
50 kHz, into input impedance of 50
Ω
. A DMM in ACV function is used here as
the calibration standard. Unfortunately, the frequency response of DMMs falls
off at frequencies which must be used to verify HF flatness above 1 MHz, and so
a commonly-used technique employs an RF Power Meter. Flatness is normally
expressed as a voltage relative to that at a reference frequency of 50 kHz, and in
our procedure, Tables 12, 13, 14 and 15 are used to register the values at this
reference frequency.
For the flatness verification, each output voltage is measured as power into 50
Ω
in an RF Power Meter, and converting power to pk-to-pk voltage using a formula
given in the procedure. This voltage is compared against the power meter reading
at 50 kHz, by checking that it is within Acceptance tolerance limits about the 50
kHz value.
To calculate the Acceptance tolerance limits at each verification point, we must
take into account the Total Measurement Uncertainty and the specified 9500B
flatness, with respect to 5 0kHz. These are combined using an RSS calculation.
2.
Example: Calculation of Acceptance Limits at 10MHz
•
From the Power Meter Specification, let us say that its Total Measurement
Uncertainty (Power) at 10 MHz, including the Sensor uncertainty, is: ±1.4 %.
•
But this is a power uncertainty, and the pk-pk voltage uncertainty will be
half: ±0.7 %.
•
With this we must combine (by RSS Method) the 9500B pk-pk voltage
flatness specification. At 10 MHz, the 9500B specification relative to 50 kHz
is ±2.0 %.
Acceptance Limits =
√
[(0.007)
2
+ (.020
2
)]
= ±0.0212 = ±2.12 %
•
We must now multiply this by the Reference value at 50 kHz, and obtain the
higher limit at 10 MHz by adding the Acceptance Limits to the 50 kHz
Reference value. The lower limit at 10 MHz is found by subtracting the
Acceptance Limit from the 50 kHz Reference value.