Agilent Technologies 34970A manual Frequency and Period Measurements

Chapter 8 Tutorial

Measurement Fundamentals

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.

			Reset
Analog	Signal	F/F	Counter	Latch
Input	Conditioning
	.01s .1s	1s
	Timebase		6 MHz
	and Divider

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.

1st Edge Last Edge

Input

Gate

Count

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