Designing a 30kHz Bandpass: Passive or Active?

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SUMMARY

The discussion centers on designing a 30kHz bandpass filter with a preference for passive designs, although active options are considered to avoid inductors with ferrite cores due to stray magnetic fields. A suggested solution involves using a crystal ladder filter with 32.768 kHz watch crystals, which can achieve a bandwidth of approximately 1 Hz with minimal cost. However, challenges were encountered due to high equivalent series resistance of 35kOhm, impacting the filter's performance. The conversation also references a book by Pat Hawker for further guidance on filter design.

PREREQUISITES
  • Understanding of bandpass filter design principles
  • Familiarity with crystal ladder filters and their specifications
  • Knowledge of equivalent series resistance in electronic components
  • Experience with MHz crystal software for filter design
NEXT STEPS
  • Research the design of crystal ladder filters using 32.768 kHz watch crystals
  • Explore active filter design techniques to minimize component size
  • Investigate methods to reduce equivalent series resistance in crystal filters
  • Read "Technical Topics" by Pat Hawker for advanced filter design strategies
USEFUL FOR

Electronics engineers, audio engineers, and hobbyists involved in RF design or filter development will benefit from this discussion.

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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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