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RF LNA Measurements

This page of RF LNA test procedure describes how to test RF LNA.It covers RF LNA measurements such as gain,noise figure and S parameters. This will help RF engineers in how to test and measure RF LNA using spectrum analyzer, vector network analyzer(VNA), signal generator and noise source.

As we know RF LNA is used at the receiver of any wireless communication system. The main function is to boost the received weak signal from distant transmitting station. It does this by amplifying the desired signal and maintaining signal to noise ratio so that noise is not amplified much. Hence the name Low Noise Amplifier. Following table mentions useful RF LNA measurements for C band application.

It's location is kept just after the RF antenna to avoid noise being further degraded by any 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

Following RF LNA test procedure describes how to test RF LNA measurements outlined above.

RF LNA Test Procedure-How to test RF LNA

LNA S-parameters measurement:
For this measurement VNA (Vector Network Analyzer) is used. Spectrum analyzer can also be used for such measurement. As there are two ports port 1 and port 2. There are four S-parameters, S11, S22, S12 and S21. Here S21 is the gain of the RF LNA.

Before the measurements VNA is calibrated for the entire frequency band of operation.

LNA Gain measurement:
For this either VNA is used or signal generator with power meter or spectrum analyzer is used. Input power of certain value is fed to the input port of RF LNA and output power is measured. The difference between input and output power is taken as LNA gain.

LNA P1dB Measurement:
Feed the power at different values for the same frequency band as desired. Note down the power output for the above input values (-40dBm, -30,-20, -10, -5, -3, 0, +3, +5dBm). Refer RF measurement tutorial for more about 1dB compression point calculation. From the tabular result determine P1dB point.

IP3 measurement of RF LNA:
When two input frequencies of identical amplitude is being applied to nonlinear circuit, it produces intermodulation products. The closest among these are third order products such as 2f1-f2 and 2f2-f1. Frequency spacing of 1MHz between the two tones has been selected for our measurements.


LNA IP3 measurement
Input Tone Power(dBm) Fundamental Tone Power Intermodulation product tone power
-30
-25

Repeat the above experiment for different power values such as -30, -25, -20, -15, -10, -8, -7, -6, -5, -4, -3, -2, 0, 2 and 4 dBm. Record the fundamental tone power for desired frequency value say (f1 or f2) and Intermodulation product tone power at 2f2-f1 or 2f1-f2. The difference between this two power values is referred as IP3.


LNA Noise figure measurement

LNA Noise Figure Measurement:
The measurement is carried out using HP8970B or HP N8973A noise figure meter. As shown in the figure, first the noise figure is calibrated using noise source. After the above is done DUT i.e. RF LNA is inserted in the path which will provide noise figure of the DUT.

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