Carl 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.
>>
>> 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.
>
>
> The CATV industry answered those questions in published papers in the 70's
> and early 80's using then available mainframe computers. The result showed
> the contributions of cascaded line amps (up on the poles) and how many could
> be cascaded and maintain FCC IMD specs. As the active devices in the amps
> improved the distribution legs became longer. The head end equipment could
> be considered as the "exciter" as it was all one way transmision.
You are basically saying what I thought - an exact analysis is non-trivial. You
don't surprise me this has been done before, but it needs more than the ability
to use logs and add/subtract a few numbers on a pocket calculator.
The use of the 'mainframe computers' suggests to me this was numerical
modeling.
Was that so? I could imagine Monte Carlo techniques could solve this sort of
problem.
Are any of the published papers available online free? I doubt many people are
going to want to pay $30 or so to download an IEEE paper, but perhaps some
would
read them if available free.
> When I was designing CATV component based bi-directional data networks in
> the mid 80's the IMD as well as phase delay even in passive components were
> of importance. At the time only C-Cor was able to provide consistent quality
> amps to the specs required.
>
> Carl
> KM1H
Whilst not claiming to know much about this, I suspect for amateur purposes to
assuming the worst of the exciter or the amplifier will not be too far from the
truth, though I could believe that approximation will be less precise if the
IMD
of both the exciter and amplifier are similar. Does that sound reasonable from
your understanding of it?
Dave
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