Tom Bruhns wrote:
(ForNewsPost) wrote in message
...
Hi All,

I have been very curious if IP3 matters for class A receiver amplifiers used
for FM or FSK applications. Since linearity doesn't matter with frequency
modulated RF signals, does a high IP3 matter at all?

Sure it does! Imagine that you want to listen to 146.52MHz and
there's a weak signal there. Imagine that you have a couple local
repeaters on 146.62 and 146.72MHz. Where are the third-order products
from the repeaters? One lands right on your weak signal. This sort
of thing is specifically a problem at repeater sites where there may
be many transmitters sharing the same mountain top or tower.

The "capture effect" of FM means that in general you either don't have a
problem, because the wanted signal is the stronger one... or else you
have a big problem.

One of the problems with FM is that because everything's channelized,
IMD products are sure to fall on another channel.

With a shared site, you can also get third-order IMD problems that only
arise when a particular combination of three transmitters are activated
at the same time. It can even happen if one or even two of the
transmitters aren't actually on that site. Such intermittent problems
can be a pig to track down - especially when you don't have authority to
do it the easy way, which is by turning off other people's transmitters!

If this doesn't happen often, and you're not up there at the site to
monitor the audio on the digital link, you may not even be aware of the
increased retry count. On the other hand, that probably also means you
can live with it.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek