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