Cascaded NF and IP3

One of the calculations that is required when designing a receiver or transmitter line-up of circuit blocks is cascaded Noise Figure (NF) and cascaded Third Order Intercept Point (IP3).  The formulas shown above can be simply programmed into an Excel spreadsheet to find the cumulative performance from the beginning up to a specific circuit stage.

From the cascaded NF formula, it can be observed that the first stage, usually the LNA in a receiver, dominates the overall contribution to the cascaded noise figure. The second stage NF is reduced by the gain of the LNA, and so on.  It is therefore common practice to choose a LNA with sufficient NF and gain in order to maintain a low system noise figure and thus improve receiver sensitivity.

The system IP3 is dominated by the intercept points of the latter stages.  The IP3 of earlier stages in the line-up are reduced by the following gain and thus have less effect on the system IP3. Input IP3 can be calculated by subtracting the gain (in dB) from the output intercept point.  The formula can be used for both third order intercept point (IP3) and second order intercept point (IP2).