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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 21^st, 2015, 11:43pm

Title: Questions about 《Accurate and Rapid Measurement of IP2 and IP3》
Post by DiegoFan on Sep 21^st, 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 23^rd, 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 24^th, 2015, 1:45am

Ken Kundert wrote on Sep 23^rd, 2015, 10:39pm:

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 24^th, 2015, 2:49pm

1. No, it implies that c is small. In the regime where you measure IP₃ the 3^rd order component is always small compared to the first order component.

-Ken