On Tue, 08 Feb 2005 19:50:19 GMT, Bruce in Alaska
wrote:

In article ,
Wayne.B wrote:

On Mon, 07 Feb 2005 19:40:33 GMT, Bruce in Alaska
wrote:

This has absolulty nothing to do with the IMD Spec for Type Acceptance.
IMD is not Frequency Stability.

=====================================

Bruce, does the IMD spec speak to audio clarity, off channel
interference or both?


IMD = Intermodulation Distortion
A Spec for Transmitters, and it is a very HARD Spec to design for, and
PROVE, via Type Acceptance Testing for General Coverage (1.8 - 30 Mhz)
transmitters. There is a significant variation in parts used in the
post Power Amplifier Filtering Systems, of these radios, and to get
the required IMD Spec, for all these various filters and bands, for
all the production radios isn't an easy trick, especially for the radios
that you MUST submit to the FCC Lab for Type Acceptance.


Bruce in alaska

A little clarification:
Intermod distortion is usually refered to as unwanted inband products
produced by the mixing of different rf frequencies that are closely
spaced (audio produced). Mixing together that produce unwanted signals
within the audio pass band and in adjacent channels. The main
contributor to this type of distortion is non linearity in one or more
RF power amplifier stages.

This type of distortion can cause rough sounding audio on a signal.
The bigger problem is that it causes splatter on adjacent channels.

The post amplifier filters do nothing for this type of intermod
reduction. They do however reduce second and third harmonics of the
wanted signal which is another strict requirement of commercial type
accepted radios.

There can also be other unwanted mixing products produced in a radio
as the result of other types of intermod. These are commonly referred
to as spurious signals. These usually result from non linear stages
mixing signals to produce signals significantly removed from the
wanted signal. The output filter may or may not help with this type of
unwanted product depending on where it falls.
These can be tough to get rid of at times. The spec for commercial
type accepted radios is tougher on this also.

Regards
Gary