XT2640 Operating Manual
13 July 2016
Page 78 of 187
16
PERFORMING
MEASUREMENTS
OF
PWM
GENERATED
SIGNALS
Because of its’ wide bandwidth and spread‐spectrum sampling method the XT2640 can make high accuracy measurements of PWM
based signals however the user should take some special precautions when wiring and configuring the XT2640 specifically for these
measurements. This section details those precautions.
It is assumed in this section that you are conversant with the general operating methods used in the XT2640 as described in section
12.
Note:
In PWM applications the voltage signals contain significant signal content at two or more frequencies, typically –
The “carrier frequency”. This is sometimes called the PWM switching frequency, also sometimes called the PWM
Frequency. In some sources this may be at a fixed frequency, while in others it may vary to reduce interference, improve
efficiency, and/or improve waveform shape of the modulation signal content. In most cases this is a ‘square‐wave’ voltage
whose peaks are fixed by the internal DC voltage from which the source is generated (sometimes called the “bus voltage”)
and the duty cycle is continuously varied to control the modulation signal component. The carrier frequency component
often the largest component of the voltage signals.
The “modulation frequency”. This is the frequency at which the carrier frequency duty cycle is varied to cause signal
content at a frequency much lower than the carrier frequency, including DC in some cases. In the case where the load is a
motor this is sometimes called the drive frequency and for a synchronous motor this frequency sets the motor speed. For
an inductive load (e.g. a motor) the current signal generally includes very little of a carrier frequency component, being
primarily only at the modulation frequency.
16.1
CHOICE
OF
CHANNELS
When considering which channels to use in PWM applications it is important to consider the type of channel (the 1
st
letter of the two
letter channel option). The 2
nd
letter is only important with regards to the current level and the type of transducer if one is being
used.
A and S channel types are not recommended for high accuracy applications where the carrier frequency is above 80kHz (10kHz if
harmonics accuracy is needed) and W channel types are generally not recommended above a 400kHz carrier frequency (50kHz if
harmonics accuracy is needed).
In many PWM applications (e.g. motor drives) the duty cycle of the waveforms can range from a few percent up to almost
100%. This means that for best accuracy the signal integrity must typically be maintained up to a frequency of >10 times
the carrier frequency for non‐harmonic measurements (e.g. RMS voltage, RMS current and total power) and >80 times the
carrier frequency for harmonic measurements (e.g. individual harmonics and THD). If the range of duty cycles is known to
be less than this and is nominally centered close to 50% then higher carrier frequencies can be accommodated with
accuracy.
16.2
WIRING
RECOMMENDATIONS
You should keep all wiring between the source, the XT2640, and the load as short as possible. If using the 3ø3w(3ch) method you
should ensure that the inter‐connection between the three V‐ terminals is kept as short as possible and is left floating.
Generally PWM generated voltage signals have very short rise and fall times on their switching edges. If the wiring is long
then these signals can have very significant overshoot and ringing caused by the inductance of the wiring. Such overshoots
and ringing can cause interference which may affect nearby electronic equipment, and in severe cases may affect the
measurement accuracy and the overshoot voltages may be higher than the XT2640 is able to withstand.
16.2.1
RECOMMENDED
3
‐
PHASE,
3
‐
WIRE
WIRING
METHOD
SELECTION
This section is only applicable if the source and load are 3‐phase with no neutral connection (i.e. 3‐phase, 3‐wire). For single phase,
2‐phase (2ø3w), and 3‐phase with a neutral connection (3ø4w) applications use the normally applicable wiring method.
If the accuracy of harmonics results is not of significant importance to you then either 3ø3w(2ch) or 3ø3w(3ch) wiring methods can
be used.
If the accuracy of harmonics results is of significant importance to you then it is recommended to use the 3ø3w(3ch) wiring method.
Generally the accuracy of non‐harmonic results are unaffected by this choice.
In the XT2640 there is very little difference between these choices in regards to the available measurement results. For both
choices the majority of inter‐phase non‐harmonic and harmonic measurements are available and inter‐phase voltage scope
signals are available.
The individual phase voltages have significantly less high frequency content than the inter‐phase voltages in multi‐phase
PWM applications. At near zero crossings of the inter‐phase voltage signal the two phase signals have nearly the same PWM
duty cycle, so the inter‐phase signal consists of one or two (depending on the switching method) very slim pulses having full
DC bus voltage amplitude and a very low duty cycle (significantly less than 1%). These pulses have very significant content at
high frequencies well above the specified frequency range of the power analyzer so any high frequency non‐linearity or
sampling artifacts within the power analyzer may become significant and cause the appearance of harmonics in those results,
possibly up to a few percent at typically 3
rd
harmonic.
Because the XT2640 is not designed solely for mains power applications but has a wide bandwidth extending up to several
MHz, the presence of such signals well above the specified bandwidth may affect it slightly. By using the 3‐channel method
this effect is eliminated as extremely slim pulse widths will only occur at the very extremes of output amplitude in each phase
and generally PWM switching systems are designed to not achieve such slim pulses since these also have a significant impact
on the PWM drive itself.
