Hi Manfred,
Inter modulation is just what the term implies. Two or more signals modulate
each other in the signal path. For this to happen there needs to be a non
linear device in the path to cause modulation to occur.
A single signal by itself passing thru a non linear amp will generate
harmonics but not IMD. If a second signal is present it will modulate the
first signal and the first signal will modulate the second.
Any time you want to transmit more than one signal thru an amplifier the
amplifier must be linear. Of course SSB contains more than one signal
(frequency) at any given time as does a two tone signal.
If those signals are pre-distorted before they reach the amplifier that in
itself will not cause IMD. But if the distortion causes new and more
frequencies to be generated then the linear amplifier will have more signals
to deal with and can more easily be driven into overload which will cause
the amplifier to operate farther from linear and that will cause IMD.
A class B amplifier doesn't cause IMD because it cuts off one half of the
cycle provided that it remains linear over that half cycle. It does however
generate high second harmonic energy.
Class B audio amplifiers are usually run in push pull to cancel the harmonic
distortion. But if they generated IMD because they were a class B amplifier
they wouldn't be very useful as an audio amplifier.
Although a distortion free class B amplifier is hard to obtain, in reality
what is called class B is usually AB, which is not totally cutoff when the
lower half of the signal is present. This increases the linearity and allows
close to class B operation.
Maybe I am missing something here?
73
Gary K4FMX
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Manfred
> Mornhinweg
> Sent: Wednesday, December 14, 2016 10:53 AM
> To: amps@contesting.com
> Subject: Re: [Amps] MOSFET amp filtering - was: auto-tune
>
> 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
>
>
> ========================
> Visit my hobby homepage!
> http://ludens.cl
> ========================
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|