Jim,
But is it in an RF amplifier? Really?
Spend a bit of time on any ham band with a spectrum analyzer that has
good frequency resolution and you'll learn in the first 30 minutes that
this is true.
Unfortunately that test isn't able to check what I want. My question
was, specifically, whether waveform distortion such as clipping a
two-tone signal at the zero line, which certainly causes intense
harmonics, will or will not cause objectionable IMD.
Or take a look at pictures from mine looking at CW and SSB signals.
http://k9yc.com/P3_Spectrum_Measurements.pdf
Nice and interesting tests, but not applicable to my fundamental question!
The splatter MUST be created in the power amp -- AF harmonics outside
the SSB filter will be suppressed by that filter.
Yes, that's crystal-clear. Nonlinearity before a good SSB filter will
create distortion within the passband of the transmitter, but not
outside it. But still doesn't answer my question! :-)
So, given that nobody clearly answered my question, I had to do a
supreme effort, and start thinking! :-) Actually, in addition to
thinking, I did some simulations and tests to understand the case. Here
is the result:
I first created a high quality two-tone signal, and put it through a
spectrum analyzer. The result, as expected, are the two peaks, and very
low levels of stuff around them. So far, so good.
Then I hard-clipped this two-tone signal at the zero line. This is the
same as passing it through a half-wave rectifier, or through a single
ended class-B amplifier that doesn't have any frequency-selective
filtering. I put this extremely distorted signal into the spectrum
analyzer. The result was this:
In addition to the original two tones I get the whole series of
harmonics of them with the second harmonic starting at just 6dB down
from the fundamental. And there are also strong IMD products, but these
are far away from the two tones! In fact the second order IMD product is
exactly as strong as the second harmonic - but it falls on the
difference frequency between the two tones, that is, in the audio range!
My two tones, distorted this way, create a strong IMD product on 1kHz.
But that's no problem in a practical amp, since audio is of course
stripped off!
Within the range of interest, the strongest IMD products are the 3rd
order ones, and interestingly they are very much weaker than the 3rd
harmonic: While the 3rd harmonic is only 20dB down, the 3rd IMD products
are way over 60dB down!
Then I started adding nonlinearity to the remaining half wave, thus
starting to distort the envelope. Even a small amount of envelope
distortion drives the close-in IMD way up, like to -20dB for a pretty
modest amount of nonlinearity.
So, the self-generated answer to my question is: Severely distorting the
waveform of a modulated RF signal does indeed created IMD, but the IMD
products falling within the bandwidth of the amplifier are weak enough
to ignore. The harmonics instead are so strong that they absolutely need
filtering. All strong IMD products fall on audio frequencies, far
outside the response of the amp.
This, folks, is what makes it possible to run single-ended class AB tube
amps without messing up the spectrum!
When designing amplifiers for ham radio, we need to keep the envelope
response highly linear, and we should also keep the phase response
linear. I don't know yet HOW linear we need to keep the phase
response... But the actual waveform of the RF signal at the active
devices doesn't matter, at least not by itself. Anything goes. We just
have to turn it back to a clean sine wave before it reaches the antenna,
by means of low pass filters, tank circuits, or whatever, so that we
don't put out the strong harmonics of a distorted waveform.
Thus I see no fundamental problem in using switchmode amplifiers for
linear amplification. That said, there sure are a lot of practical
problems, but they can be solved! A switching amplifier, pulse-width
modulated or supply-modulated, with envelope detectors before and after
it, a comparator and a high gain baseband error amplifier dynamically
controlling the gain, should produce high efficiency along with high
envelope linearity. If in addition the MOSFET capacitances are low
enough so that their voltage-induced variability results in low-enough
phase modulation, we should be all set.
It's a bit funny to do this analysis nowadays. It surely has been done
by others many decades ago, and likely even a century or two ago on a
more theoretical basis! But ham radio is a lot about self-instruction,
isn't it?
Manfred
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