Fluke 5790B, Руководство по обслуживанию

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5790B
Service Manual
2-2
Functional Block Diagram Discussion
Refer to part 1 of Figure 2-1, the functional block diagram. The ac signal to be
measured is applied to the FTS first through attenuators (precision resistor
networks switched in or out depending on range), the transfer switch, and
precision amplifiers (again depending on range). The A/D Amplifier (A15
assembly) measures the output of the FTS. In the block diagram this
measurement is called M1.
The next step in the transfer process is shown in part 2 of Figure 2-1. The system
takes another measurement, called M2. The CPU sets the precision DAC (digital
to analog converter) to approximately the same voltage as the output of the
divider network for M1. This voltage is converted to a 28 Hz square wave by the
precision chopper circuit and applied to the FTS through the transfer switch and
the same range amplifier. The output of the FTS is measured again to yield M2.
In Wideband mode, the Wideband option module takes over the function of the
Transfer assembly. The chopped reference from the A/D Assembly is 80 Hz for
Wideband mode. The Wideband assembly is ac-coupled, therefore does not
make ac-dc transfers.
Refer to the flowchart (part 3 of Figure 2-1) in the block diagram. After M1 and
M2 are taken, the CPU computes the value of the unknown ac voltage at the
input, called Vac, using the following formula:
( )
1 2 M M Vdc Vac
− + =
If Vac and Vdc closely agree, the results are displayed on the front panel and the
measurement is complete. If the difference between Vac and Vdc is too large,
the CPU readjusts the DAC based on the above formula and begins another
measurement cycle.
Calibration constants to correct for FTS and amplifier frequency response
variations are stored in memory and applied to measurements before they are
displayed. In order to apply the correct constants, a frequency counter measures
the frequency of the incoming signal.