[Amps] [TowerTalk] Are All Low-Pass Filters Alike? -

[Henry Pfizenmayer]

FWIW -- In the stop band - normal low pass filters (not of the type that 
terminate out of band harmonics instead of reflecting them ) will be 
anything but matched -  Further ,  especially on broad band PA's like tx 
coupled solid state - you can run into some unexpected problems (especially 
poor efficiency and gain flatness ) - when you terminate even or odd 
harmonics in un-determined impedances-that is why some of the solid state 
guys are designing and building filters that terminate rather than reflect 
harmonics-.( These are basically lowpass- highpass complementary filters 
with the highpass section feeding a resistive load to terminate the 
harmonics).

 Also , a finer point of conventional filters is that the "designed" 
attenuation will be different depending on whether the source looks like  a 
constant current or constant voltage generator - If you have a circuit 
simulator you can prove this to your self pretty quickly.

Low pass filters  with the first element as a series inductor  rather than a 
shunt capacitor will look entirely different at harmonic frequencies  - and 
affect the amplifier differently - of course the length of coax (if any) 
between the amp and the filter can rotate the filter out of band impedances 
all over the place as well.

 Lots of fun -- 

Hank K7HP


 >>>BUT -- the same cautions about impedance matching being necessary
>>>still apply, and that's the fly in the ointment. Not only must you
>>>be matched at the operating frequency for the filter specs to apply,
>>>you must also be matched at the harmonics! The chances of that
>>>occuring are slim to none unless you happen to be working into an
>>>antenna that is resonant at the harmonics. We DO use some of those,
>>>but it certainly isn't all that we use, and it would only apply to
>>>single band filters.
>>>
>>>Jim Brown K9YC
>
> http://www.minicircuits.com/appnote/filters8-2.pdf
>
> They are often used on the IF port of mixers, where it necessary to have
> constant impedance. The downsides are that they are pretty poor
> frequency response compared to filters designed without the requirement
> on constant impedance.
>
> I've never tried designing one, and know little about them, but I assume
> that since they don't reflect out of band signals and don't pass them
> either, then there must be resistive elements which absorb the unwanted
> power. There can not just be L's and C's, but it will need R's too.
>
> Without knowing the context in which the thread started, I can't make
> any further comment, but just wanted to point out you can make filters
> that present 50 Ohms over a wide range of frequencies.
>
> G8WRB
>
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