Low-frequency measurements require the selection of a suitable multimeter
Most modern multimeters can measure AC signals with frequencies as low as 20Hz. However, some applications require the measurement of signals with lower frequencies. In order to make such measurements, you need to select a suitable multimeter with an appropriate configuration. See these examples below:
1. It is very important to set the correct AC filter. The filter is used to smooth the output of the true RMS converter. The correct setting is LOW when the frequency is below 20 Hz. At the LOW filter setting, stabilisation of the multimeter is ensured by inserting a delay of 2,5s. Set the LOW filter with the following command.
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 stabilisation times for each low-frequency measurement will significantly slow down autoranging.
We recommend that you set the manual range.
3. The 34401A uses an isolated DC capacitor to block the ACRMS converter from measuring DC signals. This allows the AC component to be measured in the range of a multimeter. When measuring a source with a high output impedance, it is necessary to ensure that there is sufficient time for the DC isolation capacitor to stabilise. The stabilisation time is not affected by the frequency of the AC signal, but will be affected by any variations in the DC signal.
The Agilent 3458A has three methods of measuring ACRMS voltages; its synchronous sampling mode is capable of measuring signals as low as 1 Hz. To configure the multimeter to make low-frequency measurements:
1. Select the synchronous sampling mode:
SETACV: SYNC
2. When you use the synchronous sampling mode, for the ACV and ACDCV functions, the input signal is DC-coupled. In the case of the ACV function, the DC component is mathematically subtracted from the reading. This is an important consideration because the combined AC and DC voltage levels may cause an overload condition, even if the AC voltage itself is not overloaded.
3, Selecting the appropriate range speeds up measurements because the autoranging characteristic causes delays when you measure low-frequency signals.
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. Synchronous sampling mode triggers the synchronous signal with a level. However, noise on the input signal may cause false level triggering and get no readings. It is important to select a level that provides a reliable trigger source. For example, avoid sine wave peaks, because the signal changes slowly, while the noise can easily cause false triggering.
6. To get good readings, make sure your surroundings are electrically "quiet" and use shielded test leads. Enable level filtering, LFILTERON, to reduce sensitivity to noise.
