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

On Fri, 08 Oct 2004 11:54:22 -0400, Dave Hall
wrote in :

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


Absolutely. One method is to tap the input signal to a small RF power
transistor with similar nonlinear characteristics and use it to shunt
the input to the final. That method also eliminates the need for a
base resistor and improves bias regulation. Overall, the amp requires
a little more drive power, but the benefits are well worth the costs.
There are other methods that work even better such as high-frequency
OP-amps with nonlinear feedback, inductors with nonlinear saturation
characteristics... just about any part of the circuit can be tailored
to compensate for a nonlinear power device.






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On Fri, 08 Oct 2004 17:26:07 -0700, Frank Gilliland
wrote:

On Fri, 08 Oct 2004 11:54:22 -0400, Dave Hall
wrote in :

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


Absolutely. One method is to tap the input signal to a small RF power
transistor with similar nonlinear characteristics and use it to shunt
the input to the final. That method also eliminates the need for a
base resistor and improves bias regulation.

That's sounds more like adaptive active bias. It's a bit tough to find
a biasing device that tracks the precise non-linear characteristics of
the amplification device, and you have to be especially watchful of
thermal runaway.



Overall, the amp requires
a little more drive power, but the benefits are well worth the costs.

This would not be a problem in the CB area, where the tendency is to
overdrive them anyway.....


There are other methods that work even better such as high-frequency
OP-amps with nonlinear feedback, inductors with nonlinear saturation
characteristics... just about any part of the circuit can be tailored
to compensate for a nonlinear power device.

I guess it all boils down to how much you want to invest in a good
design. At some point, you reach that magical point of diminishing
returns.

I wonder why more ham and commercial two-way radio amps aren't using
better designs than the simple basic stuff.

Dave
"Sandbagger"

On Mon, 11 Oct 2004 06:57:51 -0400, Dave Hall
wrote in :

On Fri, 08 Oct 2004 17:26:07 -0700, Frank Gilliland
wrote:

On Fri, 08 Oct 2004 11:54:22 -0400, Dave Hall
wrote in :

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


Absolutely. One method is to tap the input signal to a small RF power
transistor with similar nonlinear characteristics and use it to shunt
the input to the final. That method also eliminates the need for a
base resistor and improves bias regulation.

That's sounds more like adaptive active bias. It's a bit tough to find
a biasing device that tracks the precise non-linear characteristics of
the amplification device, and you have to be especially watchful of
thermal runaway.


What I described is kind of a psuedo-Darlington connection. It has
less gain than the classic Darlington, but has a higher frequency
response and a lower VBEsat. It also has the Darlington's high
linearity. And while the final will still saturate, it will do so with
more difficulty. The problem of thermal runaway is avoided by mounting
both transistors on the same heatsink. The real problem comes from the
collector capacitances (the devices being RF power transistors), so
the pairs must be carefully chosen and matched for phase using a few
extra reactive components. Lot's of math. Needless to say, it's not a
popular design except in some very high power amplifiers where other
methods would be more expensive.


Overall, the amp requires
a little more drive power, but the benefits are well worth the costs.

This would not be a problem in the CB area, where the tendency is to
overdrive them anyway.....


I'm sure they would find a way.


There are other methods that work even better such as high-frequency
OP-amps with nonlinear feedback, inductors with nonlinear saturation
characteristics... just about any part of the circuit can be tailored
to compensate for a nonlinear power device.

I guess it all boils down to how much you want to invest in a good
design. At some point, you reach that magical point of diminishing
returns.


The real expense is paid in the initial design of a custom component,
and that's just an one-time expense. After the design is finalized
even custom components are relatively inexpensive, especially when
ordered in large quantities. (For custom inductors, Micrometals has
been a real good company to work with over the years. They even have
composite cores that would be ideal for applications just like this.)


I wonder why more ham and commercial two-way radio amps aren't using
better designs than the simple basic stuff.


Well, many do. I'm sure you've looked at component lists and seen
inductors that are listed only as in-house numbers. Sometimes that
applies to semiconductors, too (such as the Peavey dual-diode I was
talking about in another thread). Sometimes capacitors are chosen not
for their linearity but for their -non-linearity. And everything I
said here about linearity also applies to other factors -- seemingly
run-of-the-mill components are sometimes carefully selected for
specific temperature coefficients, equivalent series resistances,
breakdown characteristics, etc. Even tubes and transistors are often
selected for gain figures that fall within a range smaller than the
defined tolerances of the part specification. What often appears to be
an off-the-shelf part may not have an off-the-shelf replacement. An
example is the use of sequential mic/keying relays used on many older
tranceivers -- the modification to the relay is rarely documented, and
replacement with a generic (synchronous) relay can quickly burn out
the screens in the finals. Been there, done that, and the T-shirt is
now a shop-rag.

A good design requires a LOT of engineering, not just big heatsinks
and pretty PCB artwork. The specs for Brian's amp don't demonstrate
much engineering at all. Heck, even a little negative feedback would
help that amp immensely but he won't even to -that- much. Oh well.

I didn't notice, but is he still using a single relay to switch both
the input and output?






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