[Amps] A tale of two IMs What happens?

[Carl]

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 at contesting.com [mailto:amps-bounces at 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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