ACPR measurement | Adjacent Channel Power Ratio in RF

The term ACPR is used to refer Adjacent Channel Power Ratio and it is a critical parameter in RF (Radio Frequency) and wireless communication systems 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. Let us understand ACPR measurement in RF and how to measure Adjacent Channel Power Ratio using test set up block diagram and test tools or equipments.

ACPR is specific to the wireless device under test (DUT) which is compliant to GSM, LTE, WiFi, CDMA or WiMAX etc. Based this, IQ file specific to respective wireless standard can be used in the measurement setup. It measures the power of a transmitted signal relative to the power of signals in adjacent frequency channels. High ACPR values can lead to interference with neighboring channels and degrade overall system performance.

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 used for Adjacent Channel Power Ratio measurement

Following test tools or equipments are needed for the ACPR RF 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

To measure Adjacent Channel Power, you typically need a test setup as per block diagram, consisting of the following components.
• Signal Generator: This generates the modulated RF signal that you want to measure. It provides the primary signal along with adjacent channel signals.
• RF Spectrum Analyzer: This instrument is used to analyze the frequency spectrum of the generated signal. It displays the power level of the primary signal and adjacent channel signals.
• Power Meter: A power meter is used to measure the power levels of the signals accurately.
• Filters or couplers and Attenuators: These components are used to condition the signals and ensure that only the desired frequency bands are analyzed. Filters help in isolating the main channel and adjacent channels, while attenuators can adjust the power levels as needed for measurement accuracy. RF couplers are essential in ACPR measurements to isolate the signal path, split the signal for simultaneous transmission to the power meter and spectrum analyzer as shown. It ensures accurate power monitoring without signal interference.
• Test Cables and Connectors: Proper cables and connectors are essential to ensure signal integrity and minimize losses in the test setup.

ACPR measurement setup

Following test procedure is used for acpr measurement:
➨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.
➨One can also use following ACPR formula to calculate Adjacent Channel Power Ratio.
• ACPR = Power(main channel) - Power(adjacent channel)

Conclusion : In summary, measuring Adjacent Channel Power Ratio (ACPR) is crucial in RF communication systems to assess the interference potential of transmitted signals on adjacent frequency channels. Utilizing proper test setups with RF couplers, spectrum analyzers, and power meters ensures accurate characterization of ACPR, enabling the optimization of system performance and minimizing interference issues in wireless communication networks. There are many parameters which are measured other than ACPR in RF, Refer tutorial on RF Measurements for more information.

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