What is linearity
We had meet a lot of question about - what is the linearity?
Everytime we meet this question we always can’t find a good way to explain what it is and what linearity effect on in a few words, but it is such an essential parameter for a power amplifier(PA) which the RF engineer can't avoid, so we write down this article in order to give a simple concept for the readers, and want to explain what linearity is and help them to know how to choose the right RF PA when they are building a RF system.
In brief, linearity have a direct impact on PA and decide if it can works properly in system, what's more, linear require is different for different working purpose.
Ok, here we go
First of all, you should know there have three kinds of output power to discribe an amplifier, they are P1dB, Psat and Po, definition of them each as following:
P1dB: the output power at amplifier's 1dB point of compression - When input a small signal into the power amplifier step by step, meanwhile the output power should increase a same value 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 adding 5dBm input power when PA output at P1dB compression point, the output power of this time is the Psat, by the way, Psat usually greater than P1dB 1-3dBm.
Po: The linearly output power - It is the actual maximum output power of the power amplifier, and it might lower than P1dB 3dBm. After that, the value is depends on the parameters.
HOW TO CHOOSE THE LINEARITY OF RF POWER AMPLIFIER?
According to the working purpose or the actual application, there are several parameters for the users that can help them to evaluate amplifier's linearity:
1- IMD3: (About 5% of linear relative bandwidth) When input two CW signal f1, f2 with same carrier level, calculate with formula Pf1-P(2*f1-f2) and Pf2-P(2*f2-f1) (P means output power), the biggest one between the two values is IMD3.
For the systems like Tetra, GSM and DCS which with 1-4 narrow channels and low data transmit rate:
Normally 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 systems:
Basic require: 32dBc
Higher standard: 40dBc to 45dBc
3- Broadband working system:
For the broadband system like: 20-500MHz and 500-2500MHz frequency bands, linearity usually means harmonics, reference value:
Odd harmonics >12dB
4- Pulse working system, especially for radars:
Normally, without the specific require, we will only consider the rising and falling time, reference value: <100ns