[TowerTalk] Third Order Intercept

[W5LT]

The Third Order Intercept concept is a theoretical point that is projected
as the intercept of the third order products from two equal level tones
(slope = 3:1) and the fundamental of the tones (slope = 1:1) as the total
level of the tones is varied. It has little to do with sensitivity.
The sensitivity is usually expressed as "Spurious Free dynamic Range", the
signal level range from the minimum detectible carrier level to the level
where the IM products just appear above the noise floor.
The above is in general terms, there are industry established standards for
measurement of these parameters, but the details escape me at the moment.

Bob, W5LT

-----Original Message-----
From: Tony [mailto:dxdx at optonline.net] 
Sent: Thursday, January 25, 2007 7:18 PM
To: towertalk at contesting.com
Subject: [TowerTalk] Third Order Intercept

Greetings all:

Have a question about third order intercept point. I understand the 
measurement takes sensitivity into account at the point were IMD products 
start to show at the noise floor; the greater the sensitivity is before 
distortion shows up the better.

If that's the case, then a good TOI figure alone doesn't tell the whole 
story. If the sensitivity isn't in the useable range for most HF work (-110 
~ 130dbm or so) then the receiver will have good IMD rejection, but would be

somewhat deaf compared to one with greater sensitivity.

It would be similar to adding attenuation to improve the TOI by making the 
rig less sensitive; a 10db attenuator might cause the TOI improve by 10db. 
Would this be an accurate interpretation of the TOI measurement?

Tony KT2Q