🚀 go-pugleaf

In article <1992Dec14.064445.9591@ips.oz.au> bruce@ips.oz.au (Bruce Paterson) writes:
>I need a VERY sharp notch filter (or actually 3, if sharp enough) to reject
>FM broadcast stations from an antenna signal. The requirements are tough:
>a. VERY sharp - I need to measure spectra of broadband noiselike signals and
>the present LC filters make a far too visible hole.
>b. No other holes (or even ripples) in the pass band - 57 to 180MHz.
>c. Low loss.
>
>LC type circuits are not sharp enough. Using a crystal is out as it is too
>lossy and has many other smaller holes. A carefully made cavity may be the
>answer, but while I can find many ways to use a cavity to reject one
>frequency and pass another (standard diplexers), I can't see how to use one
>to only reject a frequency, while passing all others. Any help?

Try searching for these:
	Monolithic crystal filters..several crystal pairs on a single
substrate to increase both the steepness and get rid of the worst ripples
	SAW (Surface Acoustic Wave) filters..similar to crystal filters,
but one of the variants may be what you need.

Or, try a different method. Mix the signal you are measuring down to say 10.7
MHz where sharper filters are available. Not only that, but you can
specifically exclude any frequency you want by not mixing it down. You could
mix directly to baseband, too, for that matter.

The only requirement is to build yourself a good PLL frequency synthesizer
circuit to generate the wide range of freqeuncies that you'll need.