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.
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 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.
> 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
> 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 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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