[TenTec] Low Pass Filter

[Dr. Gerald N. Johnson]

On Mon, 2008-05-19 at 17:40 -0400, Bob Close wrote:
> a band pass filter is nothing but a low-pass combined with a high-pass 
> filter.

That is not necessarily true. Many a band pass filter is made of
parallel and series resonant circuits (at the center frequency of the
band pass) coupled together.

> 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?)

Yes, but you have to watch for resonances at known harmonics. Sometimes
the harmonic current can fry the components in the harmonic resonance.
The oldest filter designs called constant K have the fewest problems and
use the smallest capacitor values and so have the lowest shunt capacitor
currents. The filters with the fastest transition from pass to stop band
(Cauer) depend on resonant notches but the relatively unloaded resonant
circuits develop significantly large circulating currents when excited
at those notch frequencies.
> 
> 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) ,

In music the first harmonic is at 2x the fundamental. In radio the
"first harmonic" IS the fundamental. The the order of the radio harmonic
is the multiple of the fundamental.

> 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 at 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.
> 
73, Jerry, K0CQ