Created on 2012-11-08 21:49:00

A bandpass filter is designed to pass a band of frequencies narrower than the system bandwidth. This range is called the passband. The selectivity of the filter determines the width of the passband; a highly selective filter will have a narrow passband.

Bandpass filters are often used for noise measurements, restricting the measurement to the frequency range of interest.

In the APx Bandpass Level measurement you can choose from a number of bandpass filter widths using the Selectivity control. The list is ordered from narrowest at the top, to widest at the bottom.

• Window width
  This is the window width of the underlying FFT, typically only a few hertz wide. This selection has very steep skirts and a flat top.
• 1/24 octave
  This selection has –3 dB points that are 1/24 octave apart.
• 1/12 octave
  This selection has –3 dB points that are 1/12 octave apart.
• 1/9 octave
  This selection has –3 dB points that are 1/9 octave apart.
• 1/6 octave
  This selection has –3 dB points that are 1/6 octave apart.
• 1/3 octave
  This selection has –3 dB points that are 1/3 octave apart.
• 1 octave
  This selection has –3 dB points that are 1 octave apart.
• Rectangular Band
  This selection has very steep skirts and a flat top. The user defines the lower and upper frequency cutoff points.

All of the “x octave” filters are equivalent to second-order analog bandpass filters, with 6 dB/octave skirts. Q is chosen to achieve the specified bandwidth at the –3 dB points.

The “Window width” and “x octave” filters are centered at some user-defined frequency for each measurement. This can be

• Generator Frequency
  The current APx generator frequency.
• When the generator channels are outputting different frequencies (Split Frequency generation), the bandpass filter center is set to the Frequency A. This mode is not available when using a generator waveform file.
• Measured Frequency
  The current measured frequency.
  When the analyzer channels are receiving different frequencies, the bandpass filter for each channel is centered on the frequency in that channel.
• Fixed Frequency
  A fixed frequency selected by the user. When Fixed Frequency is selected, a Filter Freq: entry field becomes available beneath the Filter Tune Mode control

When Rectangular Band is the Selectivity choice, these fields become available for lower and upper frequency cutoff point entry.