> 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.
Correct. But, at some point it becomes pure Gaussian noise. Similar to 20M
in a SSB contest.....
>
> 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.
Most likely although I wasnt privy to the details.
>
> 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.
Ive no idea whats online, my involvement was before Al Gore invented the
Internet.
>
>> 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?
In a lab experiment yes. On the air when mixing with other signals most
anything is possible. Connect a SA to an antenna during a SSB contest and
look whats happening out of band.
Carl
KM1H
>
>
> Dave
>
>
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