Hi Mike,
Everyone with ability to reason through problems should see eye to
eye on this, because things work the way they work.
This is like beating a dead horse at this point, however, because
everything has been covered a half dozen times.
Your response may help make it more clear, since different
wording sometimes helps....
From: Michael Tope <W4EF@pacbell.net>
To: "Amps Reflector (E-mail)" <amps@contesting.com>
Copies to: "'W8JI@contesting.com'" <W8JI@contesting.com>
Subject: RE: [AMPS] Re: Ferrite Rod for 6M Amp
Date sent: Tue, 1 Jun 1999 20:40:01 -0700
> Hi Tom,
>
> Thanks for your comments. After considerable rumination, I think I
> have reconciled your arguments and mine along with some previously
> fuzzy thinking on my part.
> Yes, I would agree that mixers are non-linear devices - no question
> about it. I would also argue that they do produce close-in IMD and
> cross modulation. They do this to the degree that the input-to-output
> transfer characteristics are non-linear. If I take a mini-circuits SBL-1
> and drive the LO port with +7 dBm, apply a moderate signal, say -30dBm to
> the RF port, and then look at the IF port with a spectrum analyzer, I will
> see an IF signal at around -36 dBm. If I increase the the input signal to
> the mixer by 1dB, I would expect to see a 1dB increase in IF output power.
> The input/output relationship is reasonably linear, hence the IMD
> performance is good. If instead of increasing the input signal to the RF
> port by 1dB, I increase the LO drive by 1dB, then I will see little or no
> change in the level of the IF output power. The LO port of the mixer is
> very non-linear with respect to the IF port. If I put a multi-tone signal
> into the LO port, I will have lots of IMD at the IF port.
Bingo. It isn't the fact the mixers are non-linear over a fractional
cycle view that creates problems, it is the shape of the envelope
distortion caused by the signal input to signal output transfer
function that causes odd order IMD.
That's important, because if that shape is a sine-shaped non-
linearity there are only even-order products produced, and no in-
band IMD.
That's why amplifiers can be driven into the curved areas of
response, without objectionable IMD distortion. It's only when the
distortion "squares-up" that IMD becomes a problem.
>> Vacuum tubes or transistors in class AB are non-linear when
>> viewed at a fractional cycle rate, why do they often provide
>> acceptable IMD performance?
>
> This is a good question and admittedly is where my theoretical insight
> starts to falter. I guess this example illustrates the your argument about
> fractional versus envelope distortion rather well when I think about it. A
> fractionally conducting amplifier can still produce more RF output power
> when the drive level is increased implying a somewhat linear input to
> output relationship on an envelope basis even though a good fraction of
> each RF cycle is being chopped. From a mathematical point of view, the
> chopping (fractional distortion) is a even order effect which would be
> consistent with minimum IMD impact, since IMD is an odd order effect.
> Second order cross modulation would occur, but wouldn't be relevant since
> it occurs far from the passband (F2-F1, F2+F1).
> Okay, I have reconciled my uncomfortableness with the language of
> your argument. I think we are on the same page.
>
> It's incorrect to think fractional cycle distortion causes performance
> shortfalls in the application being discussed.
Amen.
I can't think of a single problem that would be caused by fractional-
cycle distortion in a ferrite material in the cathode choke of a GG
amplifier, except the problem that it wastes everyone's time to
worry about it.
But again I have to say any claim a ferrite material would work fine
at 10 MHz and suddenly distort at 14 MHz is obvious nonsense.
> Yes, I am sure that your are quite correct that IMD produced by a small
> perturbation of a sine wave would seem insignificant when set against the
> fractional conuduction angle of an RF amplifier. My only argument is that
> some level of finite IMD would accompany the fractional RF cycle
> distortion because distortion implies non-linearity which in turn implies
> some loss of linear independence. Okay, but I see where my thinking is
> incorrect. I was trying to argue that any non-linearity was a suffcient
> condition for IMD. This is not true, if the non-linearity is purely even
> order, then IMD will not occur, only even order cross modulation which
> isn't relevant in narrow band RF applications (this is why CATV amps have
> to be ultra-linear).
>
>> While the part about the core causing distortion at 14 MHz and
not
>> 10 MHz is fantasy enough, the most dazzling part of the sideways
>> thinking is the implied assumption a ferrite core carrying only a tiny
>> fraction of the applied RF current driving a tube that conducts just over
>> 1/2 a cycle will somehow be meaningful in distortion performance of that
>> PA.
>
> I can't really comment on the distortion that Rich measured (no pun
> intended). I wasn't there and I don't recall Rich going into much detail
> about the test setup or the exact nature of the waveform distortion. My
> point was this - if I really do have a core material that is subject to
> saturation, and I drive it hard enough with a sine wave, I would expect to
> see some harmonic distortion develope in the circuit. If the saturation is
> symmetric (flat topping of positive and negative peaks of the RF
> waveform), you will get odd harmonics. If there is some asymmetry in the
> resultant waveform (perhaps due to DC bias), then you will also get even
> harmonics. Now, if I take this same core material that is distortiing my
> sine wave and drive it to the same peak RF swing with two sine waves of
> different frequencies, I would expect to see some IMD (e.g. 2*F1-F2,
> 2F2-F1, etc) or cross mod (F2-F1, F2+F1) products develop. In other words,
> the minute some non-linearity is introduced into the circuit, IMD or cross
> modulation will appear since the two sine waves are no longer linearly
> independent.
That's right. Distortion is primarily caused by loss of permeability
as flux density increases. What happens is suddenly the cores
effective permeability suddenly slopes off, and additional
magnetizing force increases flux density as it would in air.
For all practical purposes, the core vanishes for additional
magnetizing forces and flux increases at a much slower rate than
at lower magnetizing forces. This has nothing to do with frequency,
with the exception that the change becomes less as frequency
increases because ui generally decreases with increasing
frequency. Cores tend to behave more like sources of dissipation
(resistors) than inductors as frequency is increased. Magnetic
effects decrease, and so the slope of permeability change with
magnetizing force becomes less because the ui isn't as high at
higher frequencies.
The real-world effect tends to be, if anything, the exact opposite of
what Rich claims he measured.
Of all the parameters that govern non-linearity in a core, frequency
is at the bottom of the list. Temperature and flux density are at the
top.
When the core is in a choke, the end-effect of saturation is the
inductance of the choke drops at high flux densities. That doesn't
imply significant distortion.
As a matter of fact in radio frequency SSB and CW medium or high
power applications, core overheating would generally occur long
before before saturation!!
> I would argue that while even order fractional cycle distortion won't
> produce IMD, odd order fractional cycle distortion (symmetric crossover
> distortion, slew rate limiting, etc) would cause some IMD.
Almost everything in the world causes some IMD. The question is if
the effect is important in the grand scheme of things.
73, Tom W8JI
w8ji@contesting.com
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