In connection with my interests in short-wave radio reception, the need has arisen for a 30 MHz low-pass filter.
Years ago, long before the advent of the personal computer, when I needed to design such things like this it meant recourse to pencil, paper, formulae and doing lots of heavyweight mathematics. But these days, PCs remove all that number crunching: my favourite for such work is the AADE filter design software - which I am sure some of us here will be familiar with.
I used that to design this filter: a 9th. order Chebychev 0.5 dB ripple LPF.
Like many of my home-brew projects, the end result will not win any constructor's competition, (think of it as a 'design prototype') but it meets the need and all the bits (except the mica caps. and copper tape) came out of the 'junk box' - so the cost was almost zero. This, to me, is an important factor!
Description, etc.
The coils are salvaged items from an old Philips broadcast radio: they were re-wound as required. The trimmers (3-30 pF) came from . . . . I can't remember where - but they are USA in origin - probably salvaged from something many, many years ago. The 'chassis' is made from re-cycled off-cuts of copper laminate board, single and double-sided. Plus copper tape to ensure screening integrity and improve appearance.
The voltage transfer function is a very close match to that predicted by AADE: from 30 to 35 MHz, 6dB per MHz; from 35 to 45 MHz, 3.5 dB per MHz (figures are approx.) The operating impedance is 50 Ω.
Coil design and measurement was aided by an HP LCR meter. Alignment was facilitated with an HP 8640B sig. gen. and an Advantest TR1132 spectrum analyzer.
[attachment=8850] [attachment=8851] [attachment=8852]
Al. / Aug. 8, 2013 //
Years ago, long before the advent of the personal computer, when I needed to design such things like this it meant recourse to pencil, paper, formulae and doing lots of heavyweight mathematics. But these days, PCs remove all that number crunching: my favourite for such work is the AADE filter design software - which I am sure some of us here will be familiar with.
I used that to design this filter: a 9th. order Chebychev 0.5 dB ripple LPF.
Like many of my home-brew projects, the end result will not win any constructor's competition, (think of it as a 'design prototype') but it meets the need and all the bits (except the mica caps. and copper tape) came out of the 'junk box' - so the cost was almost zero. This, to me, is an important factor!
Description, etc.
The coils are salvaged items from an old Philips broadcast radio: they were re-wound as required. The trimmers (3-30 pF) came from . . . . I can't remember where - but they are USA in origin - probably salvaged from something many, many years ago. The 'chassis' is made from re-cycled off-cuts of copper laminate board, single and double-sided. Plus copper tape to ensure screening integrity and improve appearance.
The voltage transfer function is a very close match to that predicted by AADE: from 30 to 35 MHz, 6dB per MHz; from 35 to 45 MHz, 3.5 dB per MHz (figures are approx.) The operating impedance is 50 Ω.
Coil design and measurement was aided by an HP LCR meter. Alignment was facilitated with an HP 8640B sig. gen. and an Advantest TR1132 spectrum analyzer.
[attachment=8850] [attachment=8851] [attachment=8852]
Al. / Aug. 8, 2013 //