[AMPS] Low Pass Filters and Amplifiers

Peter Chadwick Peter.Chadwick@zarlink.com
Mon, 3 Dec 2001 07:58:59 -0000

Bob asked:

	>better high frequency suppression and or additional power handling
	>if two identical low pass filters are placed in series at the output of
the amplifier?  

All things being equal, the attenuations will add. The power handling
requirement of the second filter will be less than the first by the insertion
loss of the first, but that should be negligible. How well the filters cascade
depends on the filter and its iterative impedance  (i.e. the input impedance
when terminated by the characteristic impedance) and perhaps more importantly,
when terminated by the real load that it sees. In the cascade state, that's the
input impedance of filter#2 when terminated in its load. Question is, do you
really need the additional attenuation?

As far as tuners are concerned, I still believe in measuring the impedance to be
matched and designing a dedicated matching circuit for each band/antenna. You
can then frequently combine them into one box. For example, you may well be able
to use a straight L network, depending on the impedances. There was a review of
various tuners in QST some years back, which is worth looking up - can't
remember when, and I have neither the indexes or the CD ROM. The commercial
tuner has to try and match almost anything to 50 ohms, and frequently ends up
with certain compromises as a result. Especially with ferrite baluns at the
output ...............see Rich's web page for his approach to getting round that


Peter G3RZP

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