Keysight Technologies 3458A Guide D'utilisation page 92

3 Configuring for Measurements
Changing the reference frequency
For most operating conditions, the power-on reference frequency allows for
excellent NMR. However, for maximum NMR you should set the reference
frequency to the exact power line frequency. (If your power line frequency is
subject to drift, you may have to periodically correct the reference frequency.) The
following command measures the power line frequency and sets the reference
frequency to the exact measured value (for a
400 Hz line frequency. the multimeter divides the measured value by 8 and uses
that as the reference frequency).
OUTPUT 722;"LFREQ LINE"
You can also use the LFREQ command to directly specify the reference frequency.
This is particularly useful when the multimeter has a different power line
frequency than the device being measured. Suppose, for example, the multimeter
has a power line frequency of 60 Hz and the device being measured has a power
line frequency of 50 Hz. For this application you can achieve NMR by setting the
reference frequency to 50 Hz as follows:
OUTPUT 722;"LFREQ 50"
Remember that whenever power is cycled or the front panel Reset key is pressed,
the reference frequency returns to the rounded value of 50 or 60 Hz.
Setting the integration time
Integration time is the period of time that the A/D converter measures the input
signal. For DC or ohms measurements, the integration time determines the
measurement speed, accuracy, maximum digits of resolution, and the amount of
NMR for noise at the A/D converter's reference frequency, You can specify
integration time in terms of power line cycles (PLCs) using the NPLC command or
directly (in seconds) using the APER command. Since the NPLC and APER
commands both set the integration time, executing either will cancel the
integration time previously established by the other.
Specifying power line cycles
The multimeter achieves NMR for noise at the A/D converter's reference
frequency when the integration time is ≥1 power line cycles. You can specify
integration time in terms of power line cycles (PLCs) using the NPLC command.
The multimeter multiplies the specified number of PLCs by the period of the A/D
converter's reference frequency (LFREQ command) to determine the integration
time. For example, the period of a 50 Hz power line is 1/50 = 20 msec. If you
92 Keysight 3458A User's Guide