=?iso-8859-1?B?Rm9y52FDZWx0YQ==?= ) writes:
Hello,
I am designing a radio receiver and I need a narrow filter in the FI
stage,
at 10.7 MHz, in order to filter a single carrier.
Anybody could tell me where to get information about designing ladder
filters?
Thanks

It depends on what you need. Don't forget that the first single signal
selectivity came to receivers in the thirties, via a single crystal filter.
I'm suddenly blank about the name, but it was a balanced transformer
feeding a crystal on one side and a trimmer capacitor on the other. You'd
trim out the crystal holder's capacitance with the trimmer. Not perfect
skirt, but really great selectivity. Later, variants would appear where
load resistors were added to broaden the selectivity for voice.

Much later, you'd see them cascaded, to improve the skirt.

Their big benefit is that you don't have to have more than one crystal
frequency, up till ladder filters started making a mark some years back,
crystal filters all tended to use crystals separated in frequency by
about the required bandwidth. So you'd see these cascaded filters
as add-ons, and in the age of solid state, the balanced transformer
was replaced with a transistor, with the collector and emitter acting
as the two sides of the output winding to drive the crystal and variable
capacitor.

For sloppy selectivity, there has been lots about just putting a crystal
or ceramic resonator in the cathode of a tube or emitter of a transistor.
I'm sure for many applications, that would work fine, and doesn't requre
any fussing.

Michael VE2BVW