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Frequency and Period Measurements
The internal DMM uses a reciprocal counting technique to measure
frequency and period. This method generates constant measurement
resolution for any input frequency. The internal
DMM’s ac voltage
measurement section performs input signal conditioning for frequency
and period measurements.
The timebase is divided to provide a gate signal. The gate signal and
input signal are combined to enable the counter. During the “on” time,
the counter counts the 6 MHz timebase signal. At the end of each gate
period, the total count is latched and the result is divided by the known
timebase frequency to determine the input frequency. The counter is
then reset before the next gate period. The resolution of the measurement
is tied to the timebase, and not the input frequency. This increases
measurement speed especially at low frequencies.
The reciprocal counter has the advantage of a constant number of digits
of display, no matter what the input frequency is. With a reciprocal
counter, the number of digits of resolution scales with the gate time. If a
1-second gate gives six digits of resolution, a 0.1-second gate gives five
digits, and so forth.
Analog
Input
Signal
Conditioning
F/F
.01s .1s 1s
6 MHz
Timebase
and Divider
Counter
Latch
Reset
1st Edge Last Edge
Input
Gate
Count
Chapter 8 Tutorial
Measurement Fundamentals
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