How Do the Line & Frequency Filters Affect Signals Close to the Filter Cutoff Frequency?

The circuit design for the line filters used on the WT & PZ series instruments are similar to Butterworth filters but have been redesigned. We redesigned the original filtering characteristic to obtain a sharper rolloff characteristic.
  • For the WT3000, the attenuation characteristic is similar to a 3rd or 4th order Butterworth Filter. 
  • For the WT1800, the attenuation characteristic is similar to a 2nd order, 2-pole Butterworth filter.
  • For the PZ4000, the attenuation characteristic is similar to a 4th order, 4-pole Butterworth filter.  
However, please keep in mind that only the attenuation characteristics are similar to a Butterworth Filter. The line filters on the WT and PZ series instruments are NOT ideal Butterworth Filters. Therefore, they will not have characteristics that behave exactly the same as an ideal filter circuit.

Other Related Information:
There are two types of filters that can be used by the WT & PZ series instruments, line filters and frequency filters.

Line Filters
The line filter is inserted into the voltage and current measurement input circuit and directly affects the voltage, current and power measurements. When the line filter is turned ON, the measured value does not contain high frequency components and the cutoff frequency can be selected from a range of Hz to MHz depending on the instrument used. The line filter can also be used as an anti-aliasing filter for harmonic measurements.

Frequency Filters
The frequency filter is inserted into the frequency measurement input circuit and directly affects frequency measurement. The frequency filter is not inserted into the voltage and current measurement input circuit. Therefore, the measured values include high frequency components even when the frequency filter is turned ON. In certain cases, depending on the measurement period, the frequency filter can be used as a filter for detecting the zero-crossing of the synchronization source signal. 

  • Please refer to the manuals  WT1800 (IM WT1801-01EN Page:2-19), WT3000 (IM760301-01E Page:2-12), and PZ4000 (IM 253710-01E Page:1-14) for more information on input filters.
  • Refer to the attached file for line filter frequency response example for a 20 kHz signal on the WT1800 and PZ4000.

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