Dr. OM,
Both Harry, PA0LQ, and I, PA0SE, for reception of the 137 kHz band use a
passive audio filter with a bandwidth of 35Hz, centered on 1000Hz. Harry
designed it as a linear phase filter with four tuned circuits using high-Q
coils. Due to the linear phase characteristic the filter does not ring.
Harry applied the well known 88 mH pupin coils and I found suitable ferrite
coil assemblies in my junkbox.
Unfortunately suitable coils are not easily obtained.
To make duplication easier Harry has come up with an alternative active filter
using four operational amplifiers. Results are just as good as with the passive
filter.
Harry's description of the filters can be found in the two attachments; number
1 in WordPerfect 5.1.; number 2 in ASCII.
73 from Dick, PA0SE
E-mail: [email protected] or [email protected]
FILTER.1
Description: Binary data
FILTER.2
Description: Binary data
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