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Re: [TowerTalk] Are All Low-Pass Filters Alike?

Subject: Re: [TowerTalk] Are All Low-Pass Filters Alike?
From: Jim Lux <>
Date: Tue, 16 Aug 2005 08:20:07 -0700
List-post: <>
At 07:15 AM 8/16/2005, doc wrote:
>I want to use the switched filters at the output of a 1KW solid
>state amp to prevent harmonics from escaping to the antenna.
>I have 40 & 20M B&W low pass filters and a ParaDynamics 10M so
>far.  I have been seeing filters for 160, 80 and 15m when I came
>upon the AMECO text and it got me to thinking.
>Even and odd harmonics directly related to 15M would appear to
>be addressed by certain 10M low pass filters, so a 15M-specific
>filter would appear to be unnecessary.  True?

Depends on how much suppression you're looking for.

For most simple filter designs, the rejection is determined by the number 
of sections and the number of "bandwidths" away from the cutoff frequency 
you are.  Each section gives you 6dB/octave. So, if you had a 10 MHz LP 
filter with 3 sections, at 20 MHz, you'd be 18 dB down, at 40 MHz, you'd be 
36 dB down, etc.

A Chebyshev will give you a bit steeper rolloff closer in than a 
Butterworth, but the ultimate rolloff will tend to be the same.

You can do a bit better with an elliptic/Cauer, which has a zero right 
outside the pass band, but then "bounces" back up farther out, and actually 
has less attenuation than the simpler filters.

Your 15m harmonics are going to be at 42 and 63 MHz (give or take).  You 
need to look at that in the context of your likely 10m filter cutoff (30+ 
MHz).  The 42 MHz harmonic is pretty close to the cutoff, and may not be 
attenuated all that much.  Bear in mind that the 10m filter designer was 
looking for good attenuation at 56 and 84 MHz, and, particularly if they 
were using an elliptic, the stopband attenuation at 63 may not be very good 
(since it's in between the places were they would want to put the zeros)

This is one of those things were a filter designing program can be very 
revealing.  You can download free versions of various kinds of Spice, which 
will make running the curves pretty easy.  You can generate the component 
values by using cookbook values from the handbook, or from a table of 
standard filters.


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