I am old but not quit that old. The amps we had were all solid state but the
handbook that I was referring to was from 1968.
I did see some of those old tube amps around but I don't remember much about
them.
73
Gary K4FMX
> -----Original Message-----
> From: Carl [mailto:km1h@jeremy.mv.com]
> Sent: Friday, April 16, 2010 5:57 PM
> To: garyschafer@comcast.net; 'Roger'; 'AMPS'
> Subject: Re: [Amps] A tale of two IMs What happens?
>
>
> Tube amps? You really must be ancient (-;
>
> The ones I worked with were SS, 5-450 mHz, and around the start of the
> broadband Class A brick output module typically driven by the highly
> linear
> 2N5109's. I sort of remember IMD approaching -50dB.
>
> In some cases we had to go with a pricier feedforward design to reduce
> IM so
> we could cascade a few more amps and not have the expense of adding more
> hardline cable runs.
>
> I know I have those study papers somewhere, probaly still in the attic
> since
> the 1989 move.
>
> Carl
> KM1H
>
>
> >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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