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Re: [Amps] MOSFET amp filtering - was: auto-tune

To: amps@contesting.com
Subject: Re: [Amps] MOSFET amp filtering - was: auto-tune
From: Manfred Mornhinweg <manfred@ludens.cl>
Date: Wed, 14 Dec 2016 16:52:42 +0000
List-post: <amps@contesting.com">mailto:amps@contesting.com>
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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