EMRFD Message Archive 13882

Message Date From Subject
13882 2017-05-17 20:27:43 xb6zmeyanbsxhnbt2... IMD intercepts for JFETs
How do we specify IMD for JFETs? In playing around with a common-gate JFET amp in LTSpice, I found that 3rd-order IMD seems to rise more slowly than 2nd-order with increasing drive. Is this a real effect? If so, it seems like 3rd-order intercepts might be somewhat meaningless for JFET amps.

-Tom N1OOQ

13883 2017-05-17 23:58:42 Dana Myers Re: IMD intercepts for JFETs
13886 2017-05-18 13:13:31 Graham / KE9H Re: IMD intercepts for JFETs
By definition, third order products are the multiplication of three signals. If all of the signals are the same signal, then it will rise as the cube of the level.  If it is the mix of one signal and the second twice (as in the math is (2a+b, or 2a-b) then the product will rise as the level of 'a' squared.

A second order product will be approximately linear function of either signal if two different signals multiplied (math of a+b or a-b, as in superhet mixer) or as a square of the level if the same signal is used twice ( math of 2a) as in second harmonic generator.

So, in general, yes, the third order products are supposed to rise faster than the second order products. The exact details are dependent on the math involved in the mixing process.

--- Graham