a band pass filter is nothing but a low-pass combined with a high-pass
filter. The net result is there is a band of frequencies passed in all
three designs. In the case of a low-pass filter, it is designed to roll off
at some design slope above a certain frequency. There are a bunch of names
of dead engineers attached to many of the designs, but they are essentially
variations on a theme: what is the knee frequency, and how fast do you want
to roll off". The rest of the stuff is to make the slope do what you want
it to do. I think. (Doctor?)
I agree, with good stuff NO filter should be needed. Close in spurs? I
think the Harmonics a low-pass filter impact are even and odd multiples of
the fundamental carrier frequency (is there a first harmonic? trick
question) , not the audio garbage ,generating "splatter" "buckshot", etc. in
close. Those would be caused by harmonics of the audio frequencies...a
whole different discussion. Both problems would go away with (1) good
design (which just about any commercial product exhibits to get FCC
approval), and good operating practice (not trying to get 1200 horsepower
out of a 1000 horsepower box). I hear so many signals that sound awful
because nobody looks at the ALC operation, their mic gain, their drive to
the amplifiers they run, etc.
Most 75 mtr conversations I hear are two hams located ten miles apart
running "texas kilowats" into humongus antennas telling each other: "well
Fred, ol buddy, you are the loowwdest durn signals ah can heah, 10-4" "But
, crank it up a bit, 'cause someone is complaining up the band about
splatter or some such thang, and botherin' me with their bit@hin'" .
"Roger, Jackson, I added a bit mo' power theah to hep you out--mah pair of
4-10,000s is runnin a dite warm, but ah think they is gonna hold ok" "Oh,
dat's good now--howz your gout?" "Still bothern me like every day last
month" . ...... .......... ......... .......
I usually hear them just fine on 40 mtrs 1500 miles away. Inevitably
they have "Extra" calls.
--------------------------------------------------
From: "Rick Denney" <rick@rickdenney.com>
Sent: Monday, May 19, 2008 3:50 PM
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Subject: Re: [TenTec] Low Pass Filter
Bob Close writes...
IF the transceiver had harmonics and those were transferred to the
linear,
would they not be amplified by a factor of ten?
Seems to me a low-pass filter is not a band-pass filter, and isn't
there for normal attenuation of close-in spurs. It seems to me there
for attenuating VHF spurs above 30 MHz so that they don't get into
nearby TV's or FM radios (or cordless phones, stereo speakers, etc.)
Those spurs should be pretty small to begin with for well-designed
stuff that isn't overdriven, and a single low-pass filter on the
output is enough to clean up any little bit that is there.
The only low-pass filter I have is the low-pass effect of my Ten Tec
229 tuner with its L network. We see no RFI on our TV. I doubt that
any more low-pass filtering would be of much use.
Rick "lucky to live in a rural area" Denney
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