The Designer's Guide Community Forum https://designers-guide.org/forum/YaBB.pl Measurements >> RF Measurements >> Questions about 《Accurate and Rapid Measurement of IP2 and IP3》 https://designers-guide.org/forum/YaBB.pl?num=1442904197 Message started by DiegoFan on Sep 21st, 2015, 11:43pm

 Title: Questions about 《Accurate and Rapid Measurement of IP2 and IP3》 Post by DiegoFan on Sep 21st, 2015, 11:43pm Thanks for sharing this good doc. It's very useful to me and I found this forum through this doc. I have questions about it.1. Page7, it shows "Define po1 = (aα)^2 to be the output power at ω1," Why?  As I see, it should be " （aα+3cα^3/4+3cαβ^2/2)^2 showed in (25) :-?2.  Page5, Supposed, In(3), we only consider two-order terms(bu^2). In(4), we suppose α=β=A. So we can get all second order IM2 products: DC: b*(A^2)f1+f2:  b*(A^2)cos((w1+w2)t)f1-f2: b*(A^2)cos((w1-w2)t).My question is:  IM2 products are distributed against total IM2 power as:   50%(-3dB) at DC,  25%(-6dB) at f1+f2,  25%(-6dB) at f1-f2.   WHY?

 Title: Re: Questions about 《Accurate and Rapid Measurement of IP2 and IP3》 Post by Ken Kundert on Sep 23rd, 2015, 10:39pm 1. you are correct, but the cubic terms will be small and are considered negligible compared to the linear term.2. I don't understand your question. You went through the mathematics to get the distribution, and then ask why. The why is explained by the mathematics.-Ken

Title: Re: Questions about 《Accurate and Rapid Measurement of IP2 and IP3》
Post by DiegoFan on Sep 24th, 2015, 1:45am

Ken Kundert wrote on Sep 23rd, 2015, 10:39pm:
 1. you are correct, but the cubic terms will be small and are considered negligible compared to the linear term.2. I don't understand your question. You went through the mathematics to get the distribution, and then ask why. The why is explained by the mathematics.-Ken

Thanks. Based on the former 2 questions:
1. If we can skip the the cubic and squared items, it means α,β<<1. Based on this premise, we can eliminate α,β and get the final formula which shows indepedence between iIP3 and α,β. Conversely, if α,β>1, α,β cannot be eliminated in the final formula so it means  iIP3 has some relation with α,β.

However I know my conclusion above should be wrong because iIP3 is the non-linearity parameter of circuit and it should be only decided by the circuit itself. Anyway I don't know in which derivation part I make a mistake?

2. I know how to get the mathematics. But I don't know what' the reason to get the conclusion(50%,25%,25%) based on these mathematics.

Sorry for my long questions. Thanks a lot!

 Title: Re: Questions about 《Accurate and Rapid Measurement of IP2 and IP3》 Post by Ken Kundert on Sep 24th, 2015, 2:49pm 1. No, it implies that c is small. In the regime where you measure IP3 the 3rd order component is always small compared to the first order component.-Ken