I think there are some nifty online java calculators that do multisection
filter synthesis, which is very handy for getting a feel for what's doable
and what's not. googling for java and filter synthesis might work.
Based on all our discussion over the last couple days, I'd guess that
filtering isn't the way to do Tx and Rx close together. For just the reason
Rick mentions.. You take a huge hit of loss in the Tx side (say you need 5
sections for 30 dB rejection) of 5 dB (baby, that filter's gonna get hot!)
Even if you do narrow band traps, you're still stuck. I tried doing some
quick back of the envelope synthesis for knocking down the carrier, and it's
ugly.
Consider that 2 m repeaters have a 600 kHz separation at 144 Mhz, which is
like 60 kHz at 14 MHz, and they use 6 or 8 cavities, which are fairly high
Q.
----- Original Message -----
From: "Richard (Rick) Karlquist (N6RK)" <richard@karlquist.com>
To: "Jim Brown" <jim@audiosystemsgroup.com>; "Towertalk"
<towertalk@contesting.com>
Sent: Monday, July 05, 2004 6:00 PM
Subject: RE: [TowerTalk] Re: narrowband filters
> 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
>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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