Topband: In-Band IMD Test

[Tree]

> Hi Tree,
> 
> N6TR wrote:
> >IMHO (getting really brave jumping in here - so emphasis on the H),
> the "in band" IMD is a very important feature when using a receiver
> that will be dealing with lots of noise (like Topband).
> 
> >Think of noise as a bunch of little signals in your passband.  Think
> about how the signal you are trying to receive interacts with that
> noise (which is about the same strength - right?).
> 
>         I've never paid much attention to that test since it is mainly
> intended as a test for internally-generated SSB splatter.  I think there
> could be  2 flaws in your assumption that the test may be a good
> indicator for noise floor CW interference on 160.

Well, here is where we start getting into a debate more similar to 
the golden ears of high end audio.

There are many people that I know who feel that some of the specific
radios out there sound very poor when they are being called by 
hundreds of stations (typically while operating at a DX location).

When enough signals fill up their passband - the audion sounds "muddy".

There are radios that seem to work better than others in this regard.
The TS930 is a good one, so is the TS50 of all things.  Some of the
higher end radios are not so good - the MPs, TS850s are two examples.

So - I think there is an audio quality component to this situation as
there is when when receiving a SSB signal.  

Perhaps someone smarter than me can explain this from a mathematical 
perspective - where we have all of these little signals 35 db down
filling up the passband when you have 100 signals in it - and all
of them are about the same strength.  The case of two signals is 
pretty simple to imagine, but when you have many of them - all beating
against each other, each generating 6 or so products, it is much 
harder to comprehend.

Tree