RFRF LNA Measurements: Gain, Noise Figure, and S-Parameters
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This page describes how to test an RF LNA (Low Noise Amplifier). It covers RF LNA measurements such as gain, noise figure, and S-parameters. This guide will help RF engineers understand how to test and measure RF LNAs using a spectrum analyzer, vector network analyzer (VNA), signal generator, and noise source.
As we know, RF LNAs are used at the receiver of any wireless communication system. The main function is to boost the received weak signal from a distant transmitting station. It achieves this by amplifying the desired signal while maintaining the signal-to-noise ratio, ensuring that noise is not amplified significantly. Hence the name “Low Noise Amplifier.”
The following table mentions useful RF LNA measurements for C-band applications. The LNA is typically located directly after the RF antenna to prevent noise from being further degraded by other parts of the receiving system.
| RF LNA Specifications | Typical value |
|---|---|
| Frequency Range | 3.7 to 4.2 GHz |
| Gain | 60 dB |
| Gain Stability | less than +/-0.1 dB per Hour |
| less than +/-0.5 dB per Day | |
| Gain Frequency Response | 2dB peak-peak over 500MHz |
| Dynamic Range | 50 dB |
| Noise Temperature | 70 deg.K |
| Intermodulation distortion | Better than 50 dBc with two equal carriers of -68dBm each 1 MHz apart |
| Input/output VSWR | 1.25:1 |
The following RF LNA test procedure describes how to perform the RF LNA measurements outlined above.
RF LNA Test Procedure - How to Test RF LNA
LNA S-parameters measurement
For this measurement, a VNA (Vector Network Analyzer) is used. A spectrum analyzer can also be used for such measurements.
As there are two ports, port 1 and port 2, there are four S-parameters: S11, S22, S12, and S21. Here, S21 represents the gain of the RF LNA. Before the measurements, the VNA is calibrated for the entire frequency band of operation.
LNA Gain measurement
For this, either a VNA is used, or a signal generator with a power meter or spectrum analyzer. An input power of a certain value is fed to the input port of the RF LNA, and the output power is measured. The difference between the input and output power is taken as the LNA gain.
LNA P1dB Measurement
Feed power at different values for the same frequency band as desired. Note down the power output for the above input values (-40dBm, -30dBm, -20dBm, -10dBm, -5dBm, -3dBm, 0dBm, +3dBm, +5dBm).
IP3 measurement of RF LNA
When two input frequencies of identical amplitude are applied to a nonlinear circuit, it produces intermodulation products. The closest among these are third-order products such as 2f1-f2 and 2f2-f1. A frequency spacing of 1MHz between the two tones has been selected for our measurements.
| Input Tone Power(dBm) | Fundamental Tone Power | Intermodulation product tone power |
|---|---|---|
| -30 | -25 |
Repeat the above experiment for different power values such as -30dBm, -25dBm, -20dBm, -15dBm, -10dBm, -8dBm, -7dBm, -6dBm, -5dBm, -4dBm, -3dBm, -2dBm, 0dBm, 2dBm, and 4 dBm. Record the fundamental tone power for the desired frequency value (say f1 or f2) and the Intermodulation product tone power at 2f2-f1 or 2f1-f2. The difference between these two power values is referred to as IP3.
LNA Noise Figure Measurement
The measurement is carried out using an HP8970B or HP N8973A noise figure meter. As shown in the figure, first, the noise figure is calibrated using a noise source. After the above is done, the DUT (Device Under Test), i.e., the RF LNA, is inserted in the path, which will provide the noise figure of the DUT.
