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?
>
>
>
>
> _______________________________________________
> 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
|