PPA5xx/15xx user manual
7-6
When Speed: Window is set, the smoothing time constant
is nominally 4.5 times (normal) and 18 times (slow) the
window size (synchronised to the frequency).
The smoothing response is usually set to ‘auto reset’
where the filtering is reset in response to a significant
change in data. This speeds up the response of the
instrument to changing conditions. This function can be
disabled so that the filtering has a fixed time constant,
which would have an exponential response to a step
change.
The frequency may be measured from the voltage or
current. On a multi-phase instrument, any channel may be
selected for the frequency measurement.
Phase angle measurements have to be made with
reference to a specific input – normally phase 1 voltage.
The phase angle reference can be set to current which is
useful if operating the instrument with only current inputs,
or with low level voltage inputs. In multi-wattmeter wiring
modes, phase 1 is always used for the phase angle
reference; phase 2 or phase 3 is used when in single
phase 2 wiring or single phase 3 wiring.
A frequency filter, 4kHz low-pass, may be selected to filter
out the hf carrier component of a PWM waveform ensuring
measurements are carried out on the fundamental
frequency.
Normal frequency measurement is from 5Hz upwards so
that there is not a very long delay if measuring dc. There
is a low frequency option that extends the frequency
measurement down to 20mHz. This low frequency option
also applies a digital filter, which can be useful when
measuring in a low frequency, noisy environment.
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