On Wed, 06 Oct 2004 11:47:29 -0700, Frank Gilliland
wrote:

On Wed, 06 Oct 2004 07:19:53 -0400, Dave Hall
wrote in :

snip
Geeze! Why don't you tell him what you REALLY think. ;-)

Actually the term "compression" refers to the condition where a
normally linear device, starts to lose that linearity. If the gain of
your amp is 10 db, then 5 watts in should give 50 watts out. 10 watts
in should give 100 watts out. If 15 watts in only results in 120 watts
out, you are now "in compression". You call that clipping, but
compression is also a valid term for this condition. We use this term
all the time where I work. Granted the amps I work with are not as
powerful, they are still governed by the same characteristics.
Usually once "compression" is reached the incidents of second order
harmonic generation increases disproportionately with the output,
usually at a 2:1 ratio.


Let's try and clarify a few terms here.....

Clipping -- distortion that occurs on the top of a waveform due to the
signal exceeding the limitations of the circuit.

No argument.


Limiting -- the result of intentionally preventing a signal from
exceeding a given level. This can be done by clipping, automatic gain
control, or both.

Also correct.



Compression -- a term usually applied to audio conditioning where the
amplification of a signal is varied inversely to it's input level. One
of the most common types of audio compressor is called "constant
volume amplifier".

That is but one type of compression. For a more detailed discussion of
RF amplifier compression, please refer to the following related link:

http://cp.literature.agilent.com/lit...5965-7710E.pdf

Generally speaking, amplifier compression occurs when the inpututput
power ratio no longer increases db for db.


But the problem here is that the term 'compression' has been adopted
by voodoo techs as a euphamism for 'clipping', making it sound as if
the distortion-causing effect is not only benign, but sometimes
preferred. It is neither.

After reading the link, you might want to revise your definition.
There are a bunch of very talented engineers here who would be a bit
insulted to find out that you refer to them as "voodoo techs".


But you are right about one thing. a -33dbc harmonic rating from a
single carrier signal is pretty poor. Perhaps a chebychev lowpass
filter on the output will fix it up.....


That would be nothing more than a kludge. The fault is in the design.
The response isn't even close to linear. That may be due to the bias
class, the bias regulator, the choice of active device, or just crappy
engineering overall. I suspect it's a little of everything.

Usually, if it is a push-pull design, device matching plays an
important part. Bias is also important, as is impedance matching. But
even a "good" design should have a follow-up low pass filter to
minimize any harmonic content.

Dave
"Sandbagger"
http://home.ptd.net/~n3cvj

On Thu, 07 Oct 2004 07:06:26 -0400, Dave Hall
wrote in :

On Wed, 06 Oct 2004 11:47:29 -0700, Frank Gilliland
wrote:

snip
Compression -- a term usually applied to audio conditioning where the
amplification of a signal is varied inversely to it's input level. One
of the most common types of audio compressor is called "constant
volume amplifier".

That is but one type of compression. For a more detailed discussion of
RF amplifier compression, please refer to the following related link:

http://cp.literature.agilent.com/lit...5965-7710E.pdf

Generally speaking, amplifier compression occurs when the inpututput
power ratio no longer increases db for db.


You should know me by now -- I just -have- to disagree. The use of the
term 'compression region' is really a misnomer. It's properly
described as a nonlinear region. 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. 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'll give him this one. This may be a matter of semantics, like noting
the difference between 'weight' and 'mass'. I had these things drilled
into my brain by my profs for the simple reason that using the wrong
term can cause a misunderstanding. And that's what I believe is
happening when people substitute the term 'compression' for when the
transistor has exceeded the limits of linearity and subsequently
clips, be it hard or soft.

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


But the problem here is that the term 'compression' has been adopted
by voodoo techs as a euphamism for 'clipping', making it sound as if
the distortion-causing effect is not only benign, but sometimes
preferred. It is neither.

After reading the link, you might want to revise your definition.
There are a bunch of very talented engineers here who would be a bit
insulted to find out that you refer to them as "voodoo techs".


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.


But you are right about one thing. a -33dbc harmonic rating from a
single carrier signal is pretty poor. Perhaps a chebychev lowpass
filter on the output will fix it up.....


That would be nothing more than a kludge. The fault is in the design.
The response isn't even close to linear. That may be due to the bias
class, the bias regulator, the choice of active device, or just crappy
engineering overall. I suspect it's a little of everything.

Usually, if it is a push-pull design, device matching plays an
important part. Bias is also important, as is impedance matching. But
even a "good" design should have a follow-up low pass filter to
minimize any harmonic content.


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!

On Thu, 07 Oct 2004 13:58:13 -0700, Frank Gilliland
wrote:

On Thu, 07 Oct 2004 07:06:26 -0400, Dave Hall
wrote in :

On Wed, 06 Oct 2004 11:47:29 -0700, Frank Gilliland
wrote:

snip
Compression -- a term usually applied to audio conditioning where the
amplification of a signal is varied inversely to it's input level. One
of the most common types of audio compressor is called "constant
volume amplifier".

That is but one type of compression. For a more detailed discussion of
RF amplifier compression, please refer to the following related link:

http://cp.literature.agilent.com/lit...5965-7710E.pdf

Generally speaking, amplifier compression occurs when the inpututput
power ratio no longer increases db for db.


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..... ;-)

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

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'll give him this one. This may be a matter of semantics, like noting
the difference between 'weight' and 'mass'. I had these things drilled
into my brain by my profs for the simple reason that using the wrong
term can cause a misunderstanding. And that's what I believe is
happening when people substitute the term 'compression' for when the
transistor has exceeded the limits of linearity and subsequently
clips, be it hard or soft.

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

No argument there.


But the problem here is that the term 'compression' has been adopted
by voodoo techs as a euphamism for 'clipping', making it sound as if
the distortion-causing effect is not only benign, but sometimes
preferred. It is neither.

After reading the link, you might want to revise your definition.
There are a bunch of very talented engineers here who would be a bit
insulted to find out that you refer to them as "voodoo techs".


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.




But you are right about one thing. a -33dbc harmonic rating from a
single carrier signal is pretty poor. Perhaps a chebychev lowpass
filter on the output will fix it up.....


That would be nothing more than a kludge. The fault is in the design.
The response isn't even close to linear. That may be due to the bias
class, the bias regulator, the choice of active device, or just crappy
engineering overall. I suspect it's a little of everything.

Usually, if it is a push-pull design, device matching plays an
important part. Bias is also important, as is impedance matching. But
even a "good" design should have a follow-up low pass filter to
minimize any harmonic content.


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

Dave
"Sandbagger"
http://home.ptd.net/~n3cvj

On Fri, 08 Oct 2004 07:08:10 -0400, Dave Hall
wrote in :

On Thu, 07 Oct 2004 13:58:13 -0700, Frank Gilliland
wrote:

On Thu, 07 Oct 2004 07:06:26 -0400, Dave Hall
wrote in :

On Wed, 06 Oct 2004 11:47:29 -0700, Frank Gilliland
wrote:

snip
Compression -- a term usually applied to audio conditioning where the
amplification of a signal is varied inversely to it's input level. One
of the most common types of audio compressor is called "constant
volume amplifier".

That is but one type of compression. For a more detailed discussion of
RF amplifier compression, please refer to the following related link:

http://cp.literature.agilent.com/lit...5965-7710E.pdf

Generally speaking, amplifier compression occurs when the inpututput
power ratio no longer increases db for db.


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.....


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.....


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.


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. And that's exactly what happens when the output
is pushed into the nonlinear region because it's operation is intended
to be limited to the linear region (otherwise it's not a linear amp).


I'll give him this one. This may be a matter of semantics, like noting
the difference between 'weight' and 'mass'. I had these things drilled
into my brain by my profs for the simple reason that using the wrong
term can cause a misunderstanding. And that's what I believe is
happening when people substitute the term 'compression' for when the
transistor has exceeded the limits of linearity and subsequently
clips, be it hard or soft.

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

No argument there.


But the problem here is that the term 'compression' has been adopted
by voodoo techs as a euphamism for 'clipping', making it sound as if
the distortion-causing effect is not only benign, but sometimes
preferred. It is neither.

After reading the link, you might want to revise your definition.
There are a bunch of very talented engineers here who would be a bit
insulted to find out that you refer to them as "voodoo techs".


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!).


But you are right about one thing. a -33dbc harmonic rating from a
single carrier signal is pretty poor. Perhaps a chebychev lowpass
filter on the output will fix it up.....


That would be nothing more than a kludge. The fault is in the design.
The response isn't even close to linear. That may be due to the bias
class, the bias regulator, the choice of active device, or just crappy
engineering overall. I suspect it's a little of everything.

Usually, if it is a push-pull design, device matching plays an
important part. Bias is also important, as is impedance matching. But
even a "good" design should have a follow-up low pass filter to
minimize any harmonic content.


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

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

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'.

On Fri, 08 Oct 2004 07:22:54 -0700, Frank Gilliland
wrote:

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

snip for brevity


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.

No, nothing that elaborate. Just simple basic stuff, but it's the
stuff that CB'ers seem to flock to, even if it isn't the cleanest use
of amplification devices.


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.

You will find this true in many cases. But unless I'm in a position to
correct the error, I just go with the flow, rather than calling
unwanted attention to myself for making (standing) waves....


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'.

Ok, try looking at it this way, in the audio sense of the term
"compression", a dynamic range of 90db, is often squashed into a range
of less than 70db. In the RF amplifier sense, the amount of change for
every 1 db of input signal changes from 1 db on the output to an
amount less than that. A large variation of change (within the region
of compression) on the input results in less of a range on the output.
That change is therefore "compressed".



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.

I actually knew a technician many years ago who thought that it would
take a few seconds for CATV signals several miles away to bleed off
after a line amp was disconnected. Hey, we're talking RF here not
water pressure! Sheesh!


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.

You noticed that too?


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'.

Can you do that for RF amps? I've heard of the technique, but thought
is was strictly for audio amps.


Dave
"Sandbagger"
http://home.ptd.net/~n3cvj