Frequency/Period
The NI 4070/4071/4072 can measure periodic signal frequencies, up to 500 kHz, from a variety of signal sources, ranging from millivolts to 300 Vrms (NI 4070/4072) or 700 Vrms (NI 4071).
The NI 4070/4071/4072 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 signal of frequency f in Hz, the DMM 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 4070/4071/4072 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 4070/4071/4072. The accuracy of frequency measurement is 25 ppm including temperature and time drift. In addition, the amount of noise that couples with the signal can affect the accuracy. The NI 4070/4071/4072 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, use the 5 V AC range (10 Vpk). 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 the signal and still be able to measure frequency accurately before it reaches the frequency measurement comparator circuit.
The minimum peak-to-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-to-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-to-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-to-peak signal amplitude requirement increases by approximately a factor of three.
| AC V Range | Maximum Peak-to-Peak Signal Voltage Allowed | Minimum Peak-to-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 (NI 4070/4072 only) | 450 V peak and <300 Vrms | 50 V | 25 V |
| 700 V (NI 4071 only) | 1000 V peak and <700 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 on amplitude and hysteresis.