July 19,2018 | Edit by:Leo Xu
We had meet a lot of question about - what is the linearity?
Whenever we meet this question, we always can't find a better way to explain what it is and what linearity effect on in a few words, but it is genuinely as an essential parameter for a power amplifier(PA) that can't avoid by any of the RF engineers. So we write down this article to give a simple concept for the readers and want to explain what linearity is as well as to help them to know how to choose the right RF PA when they are building an RF system.
In brief, the different working purpose will require for suitable RF power amplifiers with specified linearity, which will directly impact on the amp, and engaged if the amplifier can works properly into the system.
First of all, you should know there have three kinds of output power to describe an amplifier, and they are P1dB, Psat and Po, the definition of them each as follows:
P1dB: the output power at the amplifier's 1dB point of compression.
When inputting a small signal into the power amplifier step by step, the output power should increase the same value in the meanwhile when it is linear. Then keep going with the same method until the output power shows increased (N-1)dBm when it had been input NdBm, and the output power at this time should be the P1dB power of this amplifier.
Psat: Means saturation power.
Generally speaking, keep inputting 5dBm power more when PA output reached its P1dB compression point, the output power of this time is the saturation power. Usually, the value will higher than P1dB 1-3dBm.
Po: linear output power.
It is the actual maxim power of the amplifier and is usually lower than P1dB 3dBm.
HOW TO CHOOSE THE LINEARITY OF A RF POWER AMPLIFIER?
According to the practical purpose or the actual project application, there are several parameters for the users to help them evaluate the amplifier's linearity:
1- IMD3: (About 5% of linear relative bandwidth) When input two CW signal f1, f2 at the same carrier level, calculate with formula Pf1-P(2*f1-f2) and Pf2-P(2*f2-f1) (P means output power), the bigger one between the two values is IMD3.
For the systems like Tetra, GSM and DCS which have 1-4 narrow channels and low data transmit rate:
Basic require: 36dBc to 40dBc
For micro BTS: 45dBc to 50dBc
2- ACPR - Adjacent Channel Power Ratio: (About 5% of linear relative bandwidth) For the Broad channel systems like 3G, 4G, LTE, OFD and Wifi which with high data transmit rate:
Basic require: 32dBc
For a higher standard: 40dBc to 45dBc
3- Broadband working system:
For the broadband system such as 20-500MHz and 500-2500MHz bands, linearity usually means harmonics. And reference values are:
Odd harmonics >12dB
4- Pulse working system, especially for radars:
Typically, without specific requirements we will only consider the rising and falling time, reference value: <100ns