Butterworth Filter Design Examples (14 - Passive Filters)

[@stefanopassiglia]

This entire series on Passive Filters is just amazing. Thanks for putting it together.

[@bob_mosavo]

Thanks, Prof. Danner 😁

[@abdalmonem4626]

شكرا كثيرا على هذا الشرح الرائع . اتمنى لك الخير والهداية

[@mikethompson7046]

Hello Professor Danner, I have tested the filter you proposed in this video. I put the Values calculated into LT-Spice and matched the source and Load impedance at 50 Ohms. But the resulting Filter looks nothing like a Butterworth Filter even in the simulation. I have also tried simulation without the Resistors still the same. It has very sharp roll off from the center frequency (That is correctly placed at 1MHz). It should be really flat though being a Butterworth Filter. Any Ideas why that is ?

[@phoenixrambles]

What you're really seeing with a band pass filter is the rolloff from both a low-pass and high-pass butterworth filter; you're only seeing the section of the butterworth filter that is attenuating the signal. The typical flat response from the butterworth filter is effectively hidden by the other filter. You can see this a little better if you split the bandpass filter into two separate filters; you will see the standard butterworth filter shape on each of the individual filters and the bandpass occurs where the bode plots of the two filters cross. In this case the lowpass and highpass filters' individual cutoff frequencies are approximately 2 decades below and 2 decades above the bandpass center frequency respectively.

[@mikethompson7046]

Thank you for your response. What you are saying is partially true. When I just split the filter with Butterworth' bandpass values it won't be the perfect BW LP and HP. My issue was that my lt spice installation was corrupted that's why it simulated wrong. And LT spice doesn't show you the s parameters it shows the voltage amplitudes and the simulated filter had an attenuation of 6dB in the passband. That resulted from the 50ohm input and output impedance. You essentially have a voltage divider that halves your voltage in the passband. Thus resulting in 6dB attenuation