Designing a 30kHz Bandpass: Passive or Active?

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Discussion Overview

The discussion revolves around designing a bandpass filter centered around 30kHz, with specific requirements for passband width, slope steepness, and a preference for passive designs. Participants explore various design options and share insights on potential components and resources.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant seeks recommendations for a bandpass filter design with a center frequency of 30kHz, emphasizing a preference for passive designs but openness to active solutions to avoid issues with inductors.
  • Another participant suggests using a crystal ladder filter with 32.768 kHz watch crystals, claiming it can achieve a bandwidth of around 1 Hz with low cost and minimal components.
  • A subsequent reply expresses enthusiasm for the crystal filter idea but raises concerns about achieving a wide and flat enough passband.
  • Another participant reports failure with the crystal filter design, attributing the issue to high equivalent series resistance and seeking alternative working designs.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the effectiveness of the proposed designs, with some expressing optimism about the crystal filter while others report challenges and failures in implementation.

Contextual Notes

Limitations include the unspecified impact of stray magnetic fields on passive components, the dependence on specific crystal characteristics, and unresolved issues regarding equivalent series resistance in filter designs.

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I need a bandpass with the following specs:

Center frequency around 30kHz fixed.
Passband width a few Hz
Steep slopes, phase doesn't matter
preferably passive but active would work too especially if it spares me from inductors with ferrite cores (we have stray magnetic fields...)

Can anyone recommend a design? I am somewhat flexible with the center frequency so if there is a commercial component for a few hundred dollars that would be great.

Also: What is the best book if I have to design it myself?
 
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Crystal ladder filter using 32.768 kHz watch crystals will give you around a 1 Hz bandwidth. 3 or 4 crystals and a few capacitors.. total cost = peanuts.

Featured in Technical Topics by Pat Hawker.

Scroll down..

http://www.rsgbshop.org/acatalog/GB2RS.html


ISBN 9781-9050-8639-9 Size 210x297mm, 176 pages plus CD
 
Last edited by a moderator:
Great idea! You never stop learning. Now I just need to see if I can get the passband wide and flat enough.
 
Turns out it didn't work. Probably because of the giant equivalent series resistance of 35kOhm. I used MHz crystal software to design the filter and those have 50 Ohm or so. If you have a working design please let me know.
 

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