On Fri, 08 Oct 2004 05:57:51 -0700, Frank Gilliland
wrote:
You should know me by now -- I just -have- to disagree.
That's what makes discussion interesting. Unless, of course the other
party is nameless sociopath..... ;-)
Which one? There are so many.....
Does it matter? ;-)
The use of the
term 'compression region' is really a misnomer. It's properly
described as a nonlinear region.
A device can be non-linear without going into compression. The way I
learned it, compression is a specific term that applies when a
normally linear device loses linearity as it approaches it's maximum
drive level.
The problem is the application; i.e, the operational limits of the
device. More below.
The reason is that different devices
behave differently, and while a few devices (some transistors, a few
tubes, incandescent lightbulbs) have nonlinear characteristics similar
to compression, the fact is that nothing is being 'compressed', and
the vast majority of other devices have nonlinear regions that do not
resemble compression.
Exactly! That's why compression refers to a specific condition, so as
to differentiate it from other forms of non-linerarity. Certain class
"C" "Modulator" type amplifiers, for example, deliberately run in the
non-linear region just above device cutoff, to take advantage of the
"swing" effect of the device.
.....huh? That statement looks like a train-wreck. Assuming we're
talking Class C for both types, a device is usually pushed towards
-saturation-, and the result is clipping which forms a psuedo-square
wave that packs more energy than a sine wave, and making the stage
more efficient (a design that evolved into the Class E amplifier).
Also, Class C devices are -biased- above cutoff by definition. But
what is this "swing effect" you describe? I have never heard of a
-device- having such an effect unless it was a self-resonant device.
Last time I checked, most transistors don't tend to be self-resonant
below 30 MHz.....
I know, swing is not a technical term, but I don't know what else to
call it besides non-linear (distorted) modulation gain. A Class "C"
amplifier is biased below cutoff (Hence the improved efficiency) so if
your drive level is barely above that point, the region of gain there
is significantly non-linear. 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...
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.
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.
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.
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. ;-)
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.
Dave
"Sandbagger"
http://home.ptd.net/~n3cvj