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Connection between IP3 and P1dB... - RF Cafe Forums

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New System Designer
 Post subject: Connection between IP3 and P1dB...
Posted: Sun Sep 25, 2005 2:20 pm 
Hello everyone,

Help needed:

What is the connection between IP3 and 1dB compression point in RF Amplifiers ? as I understood, when I deal with Amp. with high IP3 (for example: +30dBm) there is a problem to measure it with spectrum analyzer...

And I don't know what's the answer ?

I assume that for seeing the IP3 spurious, I need to inject very high power signal which might compress the Amp.

but I can't figure other way for measure the IP3 power without injecting very high signal and compressing the Amp. ??!!

 Post subject:
Posted: Mon Sep 26, 2005 8:13 am 
Site Admin
User avatar

Joined: Mon Jun 27, 2005 2:02 pm

Posts: 413

Location: Germany


There is no constant connection between IP3 and P1dB in amplifiers, and even the "rule of thumb" of: IP3=P1dB+10dB is not accurate and doesn't apply. These quantities are dependent by the technology and process of the amplifier. However, you can say that in amplifiers the IP3 is always higher than P1dB.

To measure the IP3 you need to inject 2 signals that are adjacent to each other, and measure the 3rd orfer products. The formula to measure the IP3 is:



P1 - The output power of the fundamental signal

IMR3- The power difference between third order products to fundamental product [dBc].

The IEEE definition is that the IMR3 should be equal or higher than 20dBc: IMR3 >=20dBc

In linear operation, if you increase the input signals by 1dB, the third order products will increase by 3dB. You should do the IP3 measurement in small signal (That is being covered by the IEEE definition) to obtain an accurate result.


Best regards,

- IR

New System Designer
 Post subject:
Posted: Wed Sep 28, 2005 3:59 pm 
Thanx for the reply IR. Your frequent postings on this forum are a great help - I've read through many of them. Keep up the great work!!!!

:D :D :D

 Post subject:
Posted: Thu Sep 29, 2005 2:12 pm 
Site Admin
User avatar

Joined: Mon Jun 27, 2005 2:02 pm

Posts: 413

Location: Germany

You are welcome! :)

One more clarification:

to measure the IP3 you need to inject 2 signals that are adjacent to each other

According to the IEEE the IP3 measurement is done under a narrwband condition, which is: ±5% of the center frequency. That gives you the range in which the adjacent signals can be located.

Good luck!


Best regards,

- IR

Posted  11/12/2012

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