[Amps] A tale of two IMs What happens?

[Dr. David Kirkby]

Jim Thomson wrote:
> 
> ###  per ssb  systems and circuits, et all... the consensus was/is
> that if 2 x IDENTICAL.. IMD  circuits are in series...  say a TX.. with 
> -40db imd3... driving a linear amp also with  -40db imd3... the 
> author's claim the worse case you would get is -34db imd3.
> The best case you could get achieve is -37db  to -38db IMD3. 

There are lots of claims, but I have not seen anybody post anything to 
substantiate these claims.

I think it is a quite complex issue. I suspect an exact analysis is possible, 
which would put absolute limits on the parameters, but it would not be trivial.

The fact that the 3rd order products of one device makes higher order products 
in another, means you can't simply add powers.

> If you have to get a signal over a very long distance using cables, then the
> losses of the cables obviously become significant, and you might need to
> have many amplifiers in series. So the source is connected to the load like
> this:
> 
> source -> CL -> A -> CL -> A -> CL -> A -> CL -> A -> load
> 
> where:
> CL = 20 dB of cable loss
> A = 20 dB gain amplifier
> 
> ## IF I remember correctly, the gain of the CATV amps was much
> greater than the cable loss.  I THINK the input of each successive
> CATV amp was padded down.  Sorta like  25 db gain - 20 db CL - 
> then 5db padding - then CATV amp, then another 20db cable loss, etc. 
> The CATV amps all used equalizers too, since the cable attenuation
> was not uniform... but rises on the higher freqs.  IE: pre-emphasis
> employed, whereby higher freqs get amplified far mote than lower
> freqs.  Also known as slope equalizer's.  

Well, irrespective of whether there is a pad or cable loss, or pre-emphasis, the 
fact remains the situation is completely different from amateur radio, where the 
aim of the amplifier is to produce a bigger signal, rather than to just overcome 
losses. As such, the analysis from the CATV book seems totally inapplicable to me.


Dave