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RF intermodulation distortion measurement basics

This page covers RF two tone intermodulation distortion measurement. It includes test tools, test equipments and test procedure for the measurement of RF two tone intermodulation distortion.

When two frequency signals (say f1 and f2) are fed as input to any nonlinear device such as amplifier, it will generate amplified output at f1 and f2. In addition it will generate IMD (Intermodulation distortion) products at other unwanted frequencies (+/-n*f1+/-m*f2).

Frequencies at f1-f2, f2-f1, 2f1 and 2f2 are referred as second order IMD products while frequencies at 2f2-f1 and 2f1-f2, 3f1 and 3f2 are referred as third order IMD products. Frequencies at 2f2-f1 and 2f1-f2 are predominant and hence are required to be measured.

Measurement method of these intermodulation distortion products at 2f1-f2 and 2f2-f1 is known as two tone test method. Refer Second VS Third Order Intercept point basics.

Test Tools or Test Equipments Needed

Following test equipments are needed for the intermodulation distortion measurement setup.
• RF Spectrum analyzer
• RF Isolators
• RF Power Combiner
• RF Power Meter
• Device Under Test (DUT)
• Two RF signal generators or CW sources
• RF Attenuator (Fixed or variable)
• RF Directional Coupler

Test Procedure for RF intermodulation distortion measurement

two tone intermodulation distortion measurement

Following test procedure can be used for the measurement of intermodulation distortion:

➨Do the setup as shown in the figure-1.

➨Generate continuous wave frequency signals at f1 and f2 using two signal generators. f1 and f2 is vary closeby frequencies which will have about 1MHz of difference between them.

➨These frequencies are passed through RF isolators individually and are combined using RF power combiner.

➨The combined output is fed as input to the nonlinear DUT.

➨The output of DUT will have f1, f2 and other unwanted intermodulation distortion products as explained. Output is applied to directional coupler via RF attenuator.

➨The directional coupler output is connected with RF Input of Spectrum analyzer. In spectrum analyzers markers are set at f1, f2 and at the frequencies of 2f1-f2 and 2f2-f1 positions.

➨The difference between f1 or f2 and 2f1-f2 or 2f2-f1 positions is referred as intermodulation rejection. It is measured in dBc. Higher the rejection value, better is the system.

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