BPFILT example 1

Design of a minimum loss bandpass filter for the LF band, with a center frequency of 137.7 kHz and a 26.3 kHz bandwidth at an attenuation of 60 dB. The filter has 3 resonators, with a Q of 350.

The bpfilt input screen will be:

    Enter Center Frequency [Hz]    : 137.7e03
    Enter Bandwidth [Hz]           : 26300
    Enter Stopband Attenuation [dB]: 60
    Enter Number of Resonators     : 3
    Enter Resonators Unloaded Q    : 350
    Enter Inductance L [H]         : 1.031e-03
    Enter Source Impedance [ohm]   : 50
    Enter Load Impedance [ohm]     : 50

Frequency response Here is a graph with the close-in frequency response of four different filters:

  1. capacitively coupled with parallel capacitor matching (Aplac file)
  2. inductively coupled with parallel capacitor matching (Aplac file)
  3. capacitively coupled with series capacitor matching (Aplac file)
  4. inductively coupled with series capacitor matching (Aplac file)

All the four types have (of course) the same passband, and start to deviate slowly going into the stopband; at -60 dB the bandwidth is about 26 kHz, as requested. The center band loss is about 2.4 dB.

Frequency response Here is a graph with the frequency response over a larger band.
On the low side the capacitively coupled filters roll of at -60 dB/decade (like a third-order high-pass filter) while the inductively coupled filters roll of at -140 dB/decade (like a seventh-order high-pass filter).
On the high side the capacitively coupled filter with parallel capacitor matching (#1 in the graps) rolls of at -20 dB/decade (like a first-order low-pass filter), the inductively coupled filter with parallel capacitor matching (#2 in the graps) rolls of at -100 dB/decade (like a fifth-order low-pass filter), the capacitively coupled filter with series capacitor matching (#3 in the graps) rolls of at -20 dB/decade (like a first-order low-pass filter), but only after about 10 MHz (off-screen), where there is a spurios resonance, while the inductively coupled with series capacitor matching (#4 in the graps)filters rolls of at -100 dB/decade (fifth-order) but only after about 10 MHz (off-screen), where there is a spurios resonance.

Another design example compares the minimum loss, equiripple and maximally flat filter types