EMRFD Message Archive 155

Message Date From Subject
155 2006-09-14 16:15:01 topossibilities An interesting brick-wall low pass filter
In this month's EDN, the Design Ideas section features a somewhat
novel 'brick-wall' (i.e. shape factor approaches 1:1) low pass audio

To obtain a brick-wall transition with nearly 60dB of stop-band
rejection, a seventh-order elliptic LC filter was chosen.

To get the same stop-band shape and attenuation would require a 27th
order Butterworth filter!!

The reason for starting with a LC design is that normalized filter
design tables are readily available.

The article outlines an active-filter-synthesis approach that enables
construction with inexpensive and commonly available parts.

This is accomplished by transforming the impedance of each passive
component (e.g. R into C, L into R and C into a superC).

Since you can't buy super-capacitors off-the-shelf a 'gyrator'
circuit (e.g. op-amp with resistors and capacitors) is used to
synthesize them.

The final circuit only requires 2 quad op-amps, medium tolerance
resistors and 8 capacitors.

The link is:



P.S. Years back I built the R1 DC receiver with an eight-order
elliptic low pass filter....it was phenomenal.

Lot's of books, parts, kits and test equipment for sale at: