That's exacltly what I was trying to say in my post. You obviously have more
knowledge about this subject so thanks for all.
I think it's fair to say then that the receivers noise floor and the IMD
point need to be included with the theoretical third order intercept point
to give a better overall picture of receiver performance.
----- Original Message -----
From: "Gary Schafer" <email@example.com>
To: "'Tony'" <firstname.lastname@example.org>
Sent: Friday, January 26, 2007 12:01 PM
Subject: RE: [TowerTalk] Third Order Intercept
> Hi Tony,
> The graph that you sent if you notice the A signal is the wanted signal
> B is the 3rd order IM signal. They are showing that with a -130 dbm
> sensitivity the 3rd order product starts to appear at -45 dbm. -130 to -45
> is the dynamic range of the receiver or the point at which im product just
> start to appear.
> If the signal levels are increased the 3rd order product increases 3 times
> faster than the wanted signal. Where you see the curve near the top of the
> trace is the compression point. Beyond that level the receiver is very non
> linear and may not even be able to amplify any more.
> The 3rd order intercept point is point C at the top which is a projected
> point obtained by projecting out the lines from the linear region below
> compression point.
> The other set of lines which derive intercept point C' is the result of
> adding the 20 db attenuator ahead of the receiver. Note that everything is
> shifted to the right with the attenuator. The sensitivity is now 20 db
> at -110 dbm and the 3rd order products start to appear now at -25 dbm (20
> better) and the 3rd order intercept point is now also 20 db better.
> The dynamic range remains the same but is now between -110 and -25 dbm.
> So the receiver with the attenuator will handle 20 db more signal with
> gain but at the expense of 20 db of sensitivity.
> So when talking about the 3rd order intercept point and the dynamic range
> a receiver you also must know what the sensitivity is in order for those
> items to be meaningful.
> The vertical scale on the graph is measuring the output level of the
> receiver stages being measured. So you see that the intercept output level
> remains the same.
> I hope I have all that right its been a long time!
> Gary K4FMX
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