Frequency/Period Measurements

This tutorial recommends tips and techniques for using a National Instruments digital multimter (DMM) to build the most accurate measurement system possible. In this tutorial, you learn how the NI 4070 can operate as both a 6½ digit digital multimeter and a fully isolated, high-voltage digitizer, capable of acquiring waveforms at sample rates up to 1.8 MS/s at ±300 V input. This section of the tutorial covers NI 4070 frequency/period measurements.

For more information return to the Complete Digital Multimeter Measurement Tutorial.

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The NI 4070 Digital Multimeter can measure periodic signal frequencies, up to 500 kHz, from a variety of signal sources, ranging from millivolts to 300 V_rms.

The NI 4070 measures frequency by counting the number of zero-crossing rising edges of the input signal, using an onboard 28.8 MHz timebase.

Frequency (Hz) = timebase (Hz) x Ns / Nt

Period = 1 / Frequency (Hz)

where

timebase = 28.8 x 10⁶ Hz

Ns = number of rising edges detected in the measurement window

Nt = number of timebase clock periods between the first and the last rising edge

To measure a frequency signal of f Hz, the digital multimeter needs a minimum aperture of (2/f) s. The resolution of the frequency measurement is independent of the input signal frequency and depends only on how long the NI 4070 Digital Multimeter is able to look at the signal. A longer aperture enables a finer frequency resolution.

Frequency Resolution (ppm) = 10⁶ (ppm) x 4 / [timebase (Hz) x aperture (s)]

where timebase = 28.8 x 10⁶ Hz.

For example, the aperture setting of 100 ms allows you to measure a signal of a minimum frequency of 20 Hz with a resolution of 1.4 ppm.

The accuracy of the frequency measurement is directly related to the absolute accuracy of the 28.8 MHz oscillator used on the NI 4070 Digital Multimeter. The accuracy of frequency measurement is 25 ppm including temperature and time drift. In addition, the amount of noise that couples with your signal can affect the accuracy. The NI 4070 Digital Multimeter frequency measurement circuit has a hysteresis circuit that rejects noise up to 5% of the AC V range being used.

For example, if you are measuring TTL level signals, you would use the 5 V AC range (10 V peak-peak). The frequency measurement circuit offers hysteresis of 250 mV in this case, which means you can have up to ±125 mV noise glitches superimposed on top of your signal and still be able to measure frequency accurately before it reaches the frequency measurement comparator circuit.

The minimum peak-peak signal amplitude required to measure frequency is 10% of the AC V range being used. In the previous example, a minimum of 500 mV (peak-peak) is required by the frequency measurement circuit to work correctly. Notice that for the 300 V AC range, the hysteresis is 8% of range (25 V), while the minimum peak-peak signal amplitude required is 17% of range (50 V).

As signals approach the 500 kHz bandwidth of the AC V path, the minimum peak-peak signal amplitude requirement increases by approximately a factor of three.

ACV Range	Maximum Peak-Peak Signal Voltage Allowed	Minimum Peak-Peak Signal Amplitude Required	Hysteresis
50 mV	200 mV	5 mV	2.5 mV
500 mV	2 V	50 mV	25 mV
5 V	20 V	500 mV	250 mV
50 V	200 V	5 V	2.5 V
300 V	450 V peak and <300 Vrms	50 V	25 V

Another way to measure frequency is to perform waveform acquisitions and then to use the signal processing functions in the ADE to extract the frequency. This method of measuring frequency also allows you to extract frequency information based off of level and hysteresis.

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