[TowerTalk] Re: narrowband filters

[Richard (Rick) Karlquist (N6RK)]

 I'm looking
> for filters that are
> much narrower, more like 50-100 kHz wide. Specifically, I'm
> trying to learn two
> things.
>
> 1) How much attenuation can I hope to get with practical filters over how
> narrow a bandwidth?
>
> 2)  Where can I find designs for practical filters, suitable
> filters for sale, or
> both?
>
> Jim K9YC

100 kHz is less than 1% BW on 20 meters.  If you can
manage an unloaded Q of (quite difficult, but doable),
you will lose about 1 dB per resonator.  Let's say this filter
covers 14000 to 14100.  At 14150 (one bandpass octave out),
you will get about 6 dB suppression per pole.

Let's review that:  you lose 1 dB of desired signal for
each 6 dB of undesired signal suppression, assuming a Chebyshev
response.

Of course, you could separate the passband and stopband
somewhat, which would make things better at the expense
of losing the top of the CW band and the bottom of the phone
band.

Hopefully that gives you an idea of the difficulty of this
kind of filter.  Far beyond commercial whole band filters.

If you are interested in further details, refer to Zverev's
"Handbook of Filter Synthesis".

Rick N6RK