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ACPR (Adjacent Channel Power Ratio) measurement basics

This page covers ACPR (Adjacent Channel Power Ratio) measurement. It includes test tools, test equipments and test procedure for the measurement of ACPR (Adjacent Channel Power Ratio).

The term ACPR is used to refer Adjacent Channel Power Ratio. It is basically the ratio of adjacent channel power to the reference signal power. One can take frequency points at some distance away in frequency axis with reference to center frequency of test as adjacent channels on both lower as well as upper sidebands. Refer ACPR basics.

ACPR is specific to the wireless device under test(DUT). DUT can be compliant to GSM, CDMA or WiMAX etc. Based this, IQ file specific to respective wireless standard can be used in the measurement setup.

Let us assume ACPR measurement of GSM Device:
It has been specified as follows:
ACPR < -65 dBc at 400 kHz with RBW (Resolution Bandwidth) of 30KHz

Test Tools or Test Equipments Needed

Following test equipments are needed for the ACPR measurement setup.
• Spectrum analyzer
• Power Meter
• Device Under Test (DUT)
• RF Vector Signal Generator (RF VSG) (Built in IQ file or external IQ interface)
• RF Attenuator (Fixed or variable)
• RF Directional Coupler

Test Procedure for ACPR measurement

ACPR measurement setup

Following test procedure can be used for the measurement of ACPR:

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

➨Generate modulated RF signal as per GSM standard compliant I/Q baseband vector. The same can be done either playing IQ file in the RF VSG itself or externally providing IQ vector signal using IQ signal generators.

➨This IQ generated signal is converted to RF vector inside the RF VSG which is later provided as input to the DUT.

➨DUT output is connected to RF directional coupler via RF attenuator.

➨One output of RF coupler is connected to RF power meter and the other output is connected to the RF spectrum analyzer.

➨RF Power meter measures the absolute signal power in dBm.

➨As shown in the figure, two markers are set in the RF spectrum analyzer, one at the center (say 'X') and the other at 400KHz away((say 'Y') from the center frequency(i.e. reference frequency). The difference between X and Y is taken as ACPR measurement. This is measured in dBc.

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