A suitable multimeter must be used for low frequency measurements.
Most modern multimeters can measure AC signals with frequencies as low as 20Hz. However, some applications require the measurement of lower frequency signals. To make such measurements, you need to choose the right multimeter and configure it appropriately. Consider the following examples:
The Agilent 34410A and 34411A multimeters use digital sampling technology for true RMS measurements down to 3 Hz. It improves the settling time to 2.5s in the slow filter by digital method. For the measurement, you should pay attention to:
1. It is very important to set the correct AC filter. Filters are used to smooth the output of the rms converter. At frequencies below 20Hz, the correct setting is LOW. When the LOW filter is set, a 2.5s delay is inserted to ensure the stability of the multimeter. Set the low filter with the following command.
VOLTage:AC:BANDwidthMIN
2. If you know the level of the signal to be measured, you should set the manual range to help speed up the measurement. Longer settling times for each low frequency measurement will slow down autoranging significantly.
We recommend that you set manual range.
3. 34401A uses a DC blocking capacitor to block the ACRMS converter to measure the DC signal. This allows the range used by the multimeter to measure the AC component. When measuring sources with high output impedance, it is necessary to allow sufficient time for the DC blocking capacitor to stabilize. Settling time is not affected by the frequency of the AC signal, but is affected by any changes in the DC signal.
The Agilent 3458A has three methods for measuring ACRMS voltage; its simultaneous sampling mode can measure signals down to 1Hz. To configure the multimeter for low frequency measurements:
1. Select the synchronous sampling mode:
SET ACV: SYNC
2. When you use the synchronous sampling mode, for the ACV and ACDCV functions, the input signal is DC coupled. In the ACV function, the DC component is mathematically subtracted from the reading. This is an important consideration because combined AC and DC voltage levels may create an overload condition even if the AC voltage itself is not overloaded.
3. Selecting an appropriate range can speed up the measurement, because when you measure low-frequency signals, the automatic range feature will cause delay.
4. In order to sample the waveform, the multimeter needs to determine the signal period. Use the ACBAND command to determine the pause value. If you do not use the ACBAND command, the multimeter may pause before the waveform repeats.
5. The synchronous sampling mode uses the level to trigger the synchronous signal. However, noise on the input signal may cause false level triggers, resulting in incorrect readings. It is important to choose a level that provides a reliable trigger source. For example, to avoid the peak of the sine wave, because the signal changes slowly, but noise can easily cause false triggers.
6. To get the best readings, make sure your surroundings are electrically "quiet" and use shielded test leads. Enables level filtering, LFILTERON, to reduce sensitivity to noise.
To configure 34401A, you can use the same configuration method as 34410A and 34411A. 34401A
Use an analog circuit with a DC blocking capacitor to convert the rms voltage. It measures signals down to 3Hz. To achieve the measurement results, select a low-frequency filter, use manual ranging, and verify that the various DC biases are stable. When you use the slow filter, a delay of 7s is inserted, thus ensuring the stability of the multimeter.
