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

To: <>, "David Thompson" <>
Subject: Re: [TowerTalk] Are All Low-Pass Filters Alike? -
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Tue, 16 Aug 2005 18:49:59 -0400
List-post: <>
> Why don't hams place the LP filter between the Xcvr and
the Amp?

Probably because it won't help reduce TVI. It's virtually
always a waste of a filter.

> way, the amount of harmonics reaching the Amp will be
> rendering their output levels relatively the same, if not
lower, than if
> the LP filter was AFTER the Amp.

The amp won't amplify harmonics anyway if it is properly
designed Don.

We have to remember what causes harmonics. Harmonics are
caused by fractional cycle non-linearity or distortion, NOT
by gain compression or envelope non-linearity. The effect
causing splatter is entirely different than the effect
causing harmonics.

The waveshape over every RF cycle before filtering causes
harmonics. Most PA's conduuct over a fraction of the RF
cycle, and have a large portion of each RF cycle where the
slope of the sine wave is changed to something other than a
perfect sine wave. Many PA's actually only have just over
180 degrees of linear conduction in each RF cycle, so they
generate strong harmonics.

The very same PA can have a very good transfer function.
Output power can track input power closely, so the Pa will
have good envelope linearity and low splatter or IM

If a low-pass between the exciter and PA helps things, it
means the exciter or PA is poorly shielded or very poorly
designed or constructed.

73 Tom


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