MATLAB Examples

Finding Cascaded Gain, Noise Figure, and IP3

This example shows how to calculate the cascaded gain, noise figure, and 3rd order intercept (IP3) of a chain of RF stages. Each stage is represented by a frequency independent "black box", specified with it's own stage gain, stage noise figure, and either an output referred stage IP3 (OIP3) or an input referred IP3 (IIP3).

First, specify the system by defining a gain, noise figure, OIP3, and name for each stage. Second, create an rfchain object to represent the cascaded system. To add additonal stages to the chain, use the addstage function. To change or update any stage, use the setstage function. To view the calculated cascaded values of gain, noise figure, and IP3, use the functions plot and worksheet. To access these values, use the functions cumgain, cumnoisefig, cumoip3, and cumiip3.

Contents

Specify the System

In this example, we will analyze the six-stage receiver specified in [1]. First, we must create vectors to hold the stage gains, noise figures, OIP3s, and names.

g  = [-2.5 15.3 -2.5 14.5  -8  35];
nf = [ 2.5  1.4  2.5  7.8   8 6.4];
o3 = [ Inf   23  Inf   22 Inf   4];
nm = {'Duplexer','LNA','ImageFilt','Mix1','IF_Filt','Mix2'};

Create the rfchain object

Next, we use the rfchain object to represent the system.

sys = rfchain(g,nf,o3,'Name',nm);

View the Results

Use the plot function to view the calculated results graphically, or the worksheet function to see them in a spreadsheet-like format.

figure
plot(sys)
worksheet(sys)

Change Values Inside the rfchain

In the above example, it is possible that better IP3 performance in the 6th stage is needed. To see what the results would look like if one or more stages had different values, use the setstage function to update the values in a particular stage. To view the results after the 6th stage is re-specified with G = 25, NF = 2.3, and OIP3 = 18:

setstage(sys,6,25,2.3,18)

figure
plot(sys)
worksheet(sys)

References

[1] Les Besser and Rowan Gilmore. "Practical RF circuit design for modern wireless systems Vol. 1: Passive circuits and systems," Artech House Inc, 2003, p. 135.