On Fri, 08 Oct 2004 09:57:07 -0400, Dave Hall
wrote in :

snip for brevity

I know, swing is not a technical term, but I don't know what else to
call it besides non-linear (distorted) modulation gain.


Good point.


A Class "C"
amplifier is biased below cutoff


.....my bad....


(Hence the improved efficiency) so if
your drive level is barely above that point, the region of gain there
is significantly non-linear.


Well, yeah, if by nonlinear you mean nonexistent.


At 2 watts of drive, the gain might be
6db. By the time drive hits 8 watts, it might be 10 db. The "swing
effect", then results in a radio with a 2 watt carrier modulating to
100% at 8 watts, feeding into an amp and coming out with an 8 watt
carrier with modulation peaks reaching 80 watts. It's dirty, it's
distorted, but it sure makes that wattmeter swing forward...


Now I see what you mean. I thought you were talking about the flywheel
effect. The train wreck must have been caused when my brain spilled on
the tracks.


But the point of my example was to illustrate that not all
non-linearity is a result of continual overdrive (Compression) or
momentary overdrive (Clipping).

It's non-linear, but it's not
compressed.


That's the point. It's -not- compressed. I see no reason to place that
label on a portion of a curve only because it loosly resembles one of
the characteristics of compression yet doesn't fall within its
definition. To me it's like using the word 'pill' to describe an RF
transistor, or 'swing' to describe modulation.

Don't shoot me, I'm only the messenger. Whether or not the term is
factually accurate, it is in common use. I adopted it from working
with those who used it, and many other support companies which offer
technical seminars on various conditions of amplification.


I realize that the term is used as you describe. My beef is that such
use is inaccurate and inappropriate.


And whether that nonlinear region is gradual or
sharp, it's still clipping because it's a limitation of the device. It
also causes distortion that is characteristic of clipping. And yes,
saturation is within the nonlinear region.

I differentiate the term "clipping" from "compression" by the
application in which the terms are used. In an application such as a
broadband amplifier with a broad spectrum of carriers (Such as a CATV
amp), is applied to the input, the term compression applies as you
reach the point of non-linear gain and the incidents of composite
second order beats rises disproportionately.

I look at "clipping" as a momentary chopping off of an otherwise
linear signal reproduction, such as what you would encounter with AM
modulation peaks which run out of headroom in an amplifier, which
would otherwise be still in the linear range when unmodulated. You can
still "clip" while not being fully into compression.

Maybe I'm over-analyzing these terms, but that's me.....


I see the image in your brain -- "clipping" suggests a straight-line
cut from the top of the wave. And I understand your definition of
'compression' as the transitional zone between linearity and clipping.
But clipping, by definition, is caused when the signal exceeds the
limits of the device.

Stop right there. You've got it. Clipping is when you exceed the
absolute limits of the device to amplify further. Compression is less
severe, the device can still amplify, but it is no longer linear db in
for db out.


Like I said, I understand what you mean by the 'compression region'. I
just think it's not the appropriate label, mainly because nothing is
compressed. I still think that 'clipping' is more appropriate, and I
could even accept 'limiting' or 'squaring'. But not 'compression'.


But no matter what you call it, the result is distortion.

No argument there.


I'll stick with my definition. And while I may take issue with some of
the more liberal definitions used by engineers these days, I should
point out that I worked for HP (Agilent) several years ago and I'm not
a big fan of their engineering department.

While the link I provided, was put out by Agilent, it was one of many
examples. I don't work for them either, but I do work with some
talented RF engineers, and we use the term compression frequently to
describe the point where linear gain ceases, and distortion products
increase disproportionately.


I've known many talented engineers who learned switch terminology
backwards: for example, they would describe a 6-position rotory switch
as a 6-pole single-throw. I've known some talented engineers who
didn't know the difference between 'active' and 'passive'. I've even
known a couple talented engineers who thought electrons flowed from
positive to negative. I guess I'm just anal (like THAT'S a suprise!).

That's the difference between the "hole" theory and the "electron"
theory. I admit that current flowing from positive to negative makes
more "sense", even if it has been shown to be the opposite.


You and I both recognize that current flow and electron flow are not
the same thing, and so do most engineers. But the engineers I
mentioned actually thought that -electron- flow was from positive to
negative. Maybe it is under certain circumstances, but I haven't been
to the anti-universe since high-school.


Absolutely. But even filters have limitations. Assuming Brian's amp
has 350 watt noise figures in the neighborhood of -18 to -24 dB (not
unrealistic), it would take a mighty stout filter to clean it up!

A 5 pole filter should bring it into line. All he would need is an
additional 30db of attenuation


Don't forget that as you increase the power you need to increase the
attenuation. This is because of the changes in the equipment rules
regarding harmonic emissions. The limit is now absolute whereas before
it was relative to signal strength.

That's only true above a certain power output. I believe it's 1 KW but
my mind's a little foggy on the exact level. Below those limits,
harmonic content is still rated relative to dbc. It makes little sense
to use absolute levels which would be higher than the -dbc ratings in
a "low" power amp. -40dbc from a 1 KW amp is100 mW. But if CB makers
were allowed to spec harmonic output at 100 mW, that would be much
worse than current specs.

I don't think Brian is playing in the big leagues yet. ;-)


Not if he can't jump into this discussion and defend his work.


Regardless, a LP filter won't
filter the splatter.

That's certainly true. That has to be handled in the biasing,
feedback, and by keeping drive levels in the linear range of the
devices.


If Brian appears and brings an open mind, maybe I'll show him how to
use predistortion in the bias regulator to linearize the output. But
that's a big 'if'.