How to find SNR on input side of an LNA?

Hello, I am trying to determine the noise figure for the following system. The LNA has a gain of 20 dB, Input Impedance = Output Impedance = 50 ohms, and IP3 = 34.6 dBm. The carrier frequency is 2.4 GHz, and both the source impedance and load impedance are 50 ohms. I have simulated it for 1 ms. Now when I am checking output at Spectrum Analyzer 1, it is showing only noise, and the signal is not visible at all. Then how can I measure SNR (input)?

N.B.: If I look at output of Spectrum Analyzer 2, it is showing both noise and the amplified signal.

Answer 1

[Anmol]

Hi Koushik,

I understand that you are trying to determine the Noise Factor of an LNA(Low Noise Amplifier) circuit. You are facing difficulty in calculating the SNR(Signal Noise Ratio) for the input signal because of the noise in the signal.

These are the possible workarounds which might help:

1. MATLAB has “snr” function which provides different options for input formats to calculate signal to noise ratio, you can check the documentation link: https://www.mathworks.com/help/releases/R2021a/signal/ref/snr.html
2. This MATLAB answer post provides various workarounds for dealing with signals where the inside signal is not visible due to noise: https://www.mathworks.com/matlabcentral/answers/2018831-determining-signal-to-noise-ratio
3. You can temporarily disable the noise source. The input will now consist of only the signal, allowing you to measure its power (P_signal_in​) using Spectrum Analyzer 1. Then, you can re-enable the noise source and disable the signal source. The input will now consist of only noise. You can measure the average power of this noise (P_noise_in​) using Spectrum Analyzer. With these two values, you can then calculate the input SNR.

Hope, it helps.