Theory of Operation
Functional Description
4
4-13
4-11. Frequency Measurement
Frequency measurement is illustrated in Figure 4-7. The ac input signal is
divided by the voltage divider (Figure 4-4) and buffered by the ac rms
converter. The signal is then applied to a comparator in the MAC for
counting. The counter gate is controlled by the microcomputer, and the range
is automatically selected by the software in the microcomputer. For very low
frequency input signals, the counter actually measures the period of the input
signal which the microcomputer then inverts to derive the corresponding
frequency. The display update rate for all ranges is approximately one
second (except for frequencies between 12.2 and 16 Hz, which are updated
every 1 to 1.3s).
To
Counter
Comp.
Hysteresis
CM+
CM-
True RMS
AC
Converter
From
Voltage
Divider
Internal to the MAC
dx31f.eps
Figure 4-7. Frequency Measurement
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