I worked in the cable TV industry for a short time a long while ago. I dug
out my old CATV handbook and they give a pretty good explanation on intermod
although they call it "cross modulation". I quickly looked over the part on
IM and this is what I came away with:
They say that the easiest way to look at it is to use percentages of
distortion rather than db. In other words if we have two cascaded amps of
the same type and they each have .1% IM products then the result out of the
second amp will be .2%. Adding a 3rd amp of the same type also with .1% IM
would yield a total of .3% IM out of the 3rd amp.
So it seems that the IM products directly add together.
They go on to say that with 2 amplifiers that gives an increase of 3 db in
total IM over a single amp.
Now the amps in this case were tube amps of the "distributed" type, which
makes them very broad band and they hold their phase shift constant. They
refer to them as "well behaved" amplifiers.
With "not so well behaved" amplifiers the total IM output is a different
story. They show a typical dip in IM products in the mid to 3/4 output
range. So IM products are higher at around 1/3 power, dip down at around 3/4
power and are highest at full output.
I suspect that most of that would come from phase shifts in the amps being
"not well behaved amplifiers".
The not so well behaved amplifier scenario is probably similar to amateur
exciter/amp setups. In a post I made on this a few days ago I mentioned that
in most amplifiers there is a dip in the IM products mid way in 3rd order
and a dip in 5th order at a higher level and then the IM products increasing
again at full output.
According to Orr these IM products can be pretty closely predicted
mathematically from the tube curves.
I think that Collins was trying to reduce the total IM products from the
30S1/KWM2 system when they recommended a specific length interconnect cable
for the RF input to the amp. They make mention of the concern for the phase
of the signal at the plate of the driver and the phase of the signal at the
plate of the amp. I am guessing that the attempt was to have some amount of
cancellation of the total IM products by this.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com]
> On Behalf Of Roger
> Sent: Thursday, April 15, 2010 2:58 PM
> To: 'AMPS'
> Subject: Re: [Amps] A tale of two IMs What happens?
>
>
>
> Dr. David Kirkby wrote:
> > Roger wrote:
> >
> >> No one has touched the question on how reducing power without
> returning
> >> affects IM I now have another one.
> >>
> >> Given that most of todays transceivers have an IM or roughly -35db
> "so
> >> I've been told", and we put a amp behind it that also has an IM of
> >> -35db, what is the resultant IM? What if the amp has an IM of -55db?
> >> Do they add, subtract, or go with the lowest number? IOW it is the
> amps
> >> job to "faithfully" reproduce the input signal, but that really only
> >> happens when running class A, if the user is lucky.
> >>
> >> Intuitively, "I would think" that the two figures would add, but if
> that
> >> were the case the amp with -35db and the exciter with -35db would
> have a
> >> pretty ratty signal. OTOH in the case of the -55db amp behind the -35
> db
> >> exciter does the amp "clean up" the exciter signal? Doesn't seem
> likely.
> >>
> >
> > It is not likely to clean it up, but in theory if the phase
> relationships were
> > just right, you could completely cancel the products. But doing that
> at multiple
> > frequencies, for multiple amplitudes is just not going to be
> practical.
> >
> It'd probably be like hitting the big winner in the lottery...three
> times in a row.
> > The first thing to note is that even if the exciter only generated 3rd
> order
> > products, and the amplifier only generated 3rd order projects, the
> combination
> > would (relative to the presumed perfect input, produce both 3rd and
> 5th order
> > products.
> >
> >
> I'll try not to stumble on the numbers, but I seem to almost always make
> a simple mistake when I end up with enough numbers.
> Sticking with some simple numbers, let's say 36 and 56 db. A so-so
> number and one that's pretty good, while remembering that db is nothing
> more than a ratio of two number of the same type that only have a
> specific meaning when so defined, such as dbm.
> Going though the numbers as if they added in real life and also
> sticking with a nice even numbers of 100 watts and 1 KW as a starting
> point, 30 db from 1KW would be 1 watt. (10 db down = 100 watts, 20 db
> down = 10 watts, and 30 db down = 1 watt) That's a lot of garbage for
> your neighbors. 6 more db means we are down to a quarter watt. Carrying
> the string a bit farther 40 db would be 0.1 watt and 50 db down would be
> 0.01 watt or 10 milliwatts while another 6 db = 2.5 milliwatts.
>
> So we have an exciter that is creating 25 milliwatts of distortion (36
> db started with 100 watts) and an amp creating 2.5 milliwatts of
> distortion. (56 db from 1000 watts) BUT if the amp faithfully
> reproduces the input signal that becomes 250 milliwatts of distortion
> from the original signal and another 2.5 milliwatts from the amp. If it
> were additive then we have a total of 252.5 milliwatts.
>
> Now going with 36 db for both the 100 watt exciter and 1000 watt amp we
> have 250 milliwatts distortion from the amp and 25 milliwatts from the
> exciter. Going through the amp we now have the exciter's 25 milliwatts
> increased to 250 milliwatts along with the amps 250 milliwatts for a
> total of 500 milliwatts or 1/2 watt which is 33 db down. That's an
> increase of 3 db in the IM products. This does not take into account the
> distortion of the distortion and what it does to the overall output.
>
> I don't think this is all that far out of line with what Jim's
> experiments showed.
> Of course there were best and worst cases as well which to me would
> indicate the complexity of those signals and as you say below, the
> difficulty of doing an exact analysis of such complex signals.
>
> I think the so called, "sweet spot" when checking IM with a two tone
> test indicates there is some cancellation in real life, but real life is
> also dynamic as has been pointed out already. So I would think the
> resultant would be a summation of every thing between the best and worst
> cases. "I would think" How much the result would be weighted toward best
> or worst case would depend on the individual's voice characteristics as
> well the characteristics of the exciter and amp as well as their tuning.
> > I believe an exact analysis of this would be very difficult, as the
> phases of
> > signals matters here - not just the amplitudes. So you can't just
> add/subtract
> > real numbers and expect to get exact answers. Also, the fact the
> exciter has
> > produced undesirable products, the amplifier will amplify those, as
> well as
> > generating others.
> >
> >
> If you want a clean signal, both the exciter's and amp's IM products are
> important, but It appears the exciter is the most critical of the two as
> the amp in general multiplies the exciters mess by roughly 15 times.
> (going from a 100 watts out to 1500 watts out) OTOH if you are using one
> of the old sweep tube amps with 30 db IM it's not going to be pretty.
>
> If anything this exercise makes me realize why there are so many crappy
> signals out there, but OTOH I wonder why there aren't more. <:-))
>
>
> Now we'll see how far I strayed in my math when I hit [send]
>
> 73
>
> Roger (K8RI)
>
> >
> >> It'd be interesting to see the IM figures for today's exciters, (and
> >> amplifiers) rather than than adds that just say "Provides amazingly
> low IM".
> >>
> >
> > Get onto the ARRL, convince them of it, then perhaps when they give
> reviews,
> > they can point out this stupidity. A suitable comment on an amplifier
> might be:
> >
> > "The amplifier is clearly aimed at the amateur market, as the
> manufacturer says
> > it provides amazing low IM. Any amplifier aimed at the professional
> market would
> > specify the worst case
> >
> >
> Amen!
> >> I also see I need a refresher on working with logs.<:-))
> >>
> >
> > I believe others have given you that, but it's not quite as simple as
> just
> > adding powers directly.
> >
> > I think a reasonable answer would be that the resultant signal would
> be
> > approximately the *worst* of the exciter and amplifier. So if the
> exciter is -20
> > (very poor) dB
> Even -30db isn't all that great.
> > and the amplifier is -60 dB (excellent), the resultant is still
> > going to be -20 (very poor). Likewise if you have an excellent exciter
> (-60),
> > but put a poor amplifier (-20) after it, the result will be -20
> (poor).
> >
> > I suspect when the exciter and amplifier have very close specs (-35
> dB) to use
> > your example, the resultant would be a little bit worst than -35 dB,
> but not by
> > very much.
> >
> > It is an interesting question, and one that I believe exact answer's
> can't be
> > obtained just by knowing just the magnitude of the IM products, as
> given by the
> > specifications. An exact analysis would have to consider the phases
> too, so you
> > would get into the realms of imaginary numbers.
> >
> > Dave
> >
> > A: Because it messes up the order in which people normally read text.
> > Q: Why is top-posting such a bad thing?
> > A: Top-posting.
> > Q: What is the most annoying thing in e-mail?
> >
> >
> >
> >
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> >
> >
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