In article , Paul Burridge
writes:

Since you don't specify anything important like IP3,

Can someone kindly confirm what the hell "IP3" is?? I can only think
of "3rd order intermodulation products" which might easily be
completely wrong. It would be helpful if whoever first uses an
uncommon abbreviation would have the courtesy to state what it meant!
(as is customary first time around)
snarl, hiss, etc

Heh heh heh, can't say I blame you for being a bit confused.

The "3rd Order" label is due to the mix products of F_a +/- 2F_b
with F_b being twice its original frequency. "Order" comes from
the constant multipliers, 1 for F_a and 2 for F_b so "1" + "2" equals
3 or the 3rd Order.

What happens is that the slope of the "3rd Order" mix product is
steeper than the normal F_a +/- F_b (order of 2) equation's power
slope. The "3rd Order Intercept Point" is where the two slopes
intersect.

It is much much easier to show this graphically than in words.

The reason for defining intermodulation distortion that way is in
that it is simpler and more accurate to test on both the production
line and in any test lab of a finished product. The old way had been
to just increase the input level and VERY carefully note the output
level, especially when the output level slope departed from a straight
line (indicating some compression, the start of intermodulation
distortion). That old system being very prone to instrument
calibration errors led to some arguments between makers and
buyers, not to mention non-standard notations of "1 db points" and
"3 db points" of distortion relative to input levels.

The Mini-Circuits website used to have some mention of the above
plus the Avantek website and, of course, Agilent when the T&M
division was the original Hewlett-Packard. You might find something
on a search for "3rd Order IP" on the 'net that will also have graphs.

Len Anderson
retired (from regular hours) electronic engineering person