[Amps] A tale of two IMs What happens?

[Carl]

> 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.

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