XT2640 Operating Manual
13 July 2016
Page 42 of 187
For the LF setting the measurement period is widely variable; 1 second for frequencies above 1Hz and 1 cycle for
frequencies below 1Hz.
For the VLF setting the measurement period is widely variable; 5 seconds for frequencies above 0.2Hz and 1 cycle for
frequencies below 0.2Hz.
Setting this to a lower numbered VPA forces all measurements in both VPAs to be exactly synchronous (this is only
available for VPA2 or 3). This configures the VPA being configured to use the same LF/PERIOD setting of that VPA and also
use the fundamental frequency of that VPA. Note that if the VPA being configured contains W type channels then the lower
numbered VPA selected here must also contain W type channels, and similarly if the VPA being configured does not contain
W type channels then the lower numbered VPA selected here must also not contain W type channels. If configured in this
way then each VPA can still have independent WIRING, BANDWIDTH and HARMONICS settings but otherwise behave as if
configured in a single VPA.
Recommendations:
For typical applications, the 10Hz/0.3s or 20Hz/0.1s setting is recommended.
For low power measurements to EN50564 and with a rapidly changing load the 45Hz/20ms setting should be considered.
See section 17 for full details regarding EN50564 measurements.
12.4.2.2.2
Setting
the
Source
of
the
Fundamental
Frequency
The FUND button allows you to select the source of fundamental frequency for the selected VPA if MEAS MODE is not EN61000‐3‐2
or EN61000‐3‐12.
If you select NONE then no fundamental frequency is used and harmonic analysis will not be available.
If you select FIXED then a further screen allows you to enter the frequency to use.
If you select VOLTAGE or CURRENT then the fundamental frequency will be measured using the selected signals in this
VPA. With either of these selections a further screen allows you to enter the maximum frequency to measure.
If you select MAINS then the fundamental frequency will be measured using the voltage signals in this VPA. The measured
frequency must be in the range 45 to 65Hz, if the signal frequency cannot be measured (or is outside of this range) then the
XT2640 mains supply frequency is used.
If you select AVIONICS then the fundamental frequency will be measured using the voltage signals in this VPA. The
measured frequency must be in the range 300 to 900Hz.
If you select another VPA then the fundamental frequency used for this VPA is automatically obtained from that VPA in real
time.
If you select a GFUND (1, 2 or 3) then the fundamental frequency used for this VPA is automatically obtained from the
XT2640 VPA which has been programmed to provide that GFUND measurement for this multi‐unit group.
Recommendations:
For 50/60Hz applications the MAINS setting should be used.
For 400Hz applications the AVIONICS setting should be used.
When it is required to measure signals with a fundamental frequency close to or above the maximum measurable
frequency of the channels in this VPA then select NONE.
If mainly the DC amplitude is needed, or no suitable fundamental can be established from any signal and the AC content is
primarily over a few kHz, the NONE setting should be used.
When it is required to measure ripple on a DC supply at a specific frequency or harmonics of it then either the FIXED or
MAINS setting should be used. For example, if mains ripple is to be measured then set this to MAINS and set the
HARMONICS setting to at least 1 (typically 10 or more), harmonic analysis will then show the magnitude of the ripple at
each harmonic of mains.
If the VPA being configured is making DC measurements but the signals have significant ripple at a frequency related to
that being measured by another VPA, then setting that other VPA as the source of the fundamental for this VPA may be
used to reduce the effects of that ripple and also enables you to use harmonic analysis in the VPA being configured to
measure the ripple content of the DC signal.
In most other applications the VOLTAGE setting should be selected, however if the signals are a pulse width modulated
waveform (e.g. a motor drive) then the CURRENT setting should be selected as this typically has significantly less carrier
frequency content. The maximum frequency should be set as low as possible but high enough to allow the full range of
expected signal frequencies. See section 16 for full details regarding configuring measurements on PWM generated signals.
Note:
Lead/lag and all non‐harmonic measurements are still available when no fundamental can be measured; however the
measurement period cannot be automatically adjusted to be an integer number of cycles of the signal AC content (this has
minimal effect if the signal frequency is primarily over a few kHz).
12.4.2.2.3
Setting
a
High
Frequency
Bandwidth
Limit
The BANDWIDTH button allows you to select any bandwidth limiting to use within the VPA being configured if MEAS MODE is not
EN61000‐3‐2 or EN61000‐3‐12.
If you select AUTO‐TRACK then the XT2640 will automatically provide just enough bandwidth limitation to have an
insignificant effect on the measurements, as determined from the fundamental frequency and the HARMONICS settings in
real time.
If you select UNFILTERED there is no bandwidth limitation and the full capability of the channels is available.
If you select USER you are also prompted to enter the desired ‐3dB bandwidth limitation.
