Feature: RF design
Figure 5: OTA testing with a calibrated noise source
bandwidth of interest. Tus, the benefit of having two ENR levels is the ability to determine Y-factor noise figure of the DUT for radiated measurements. In fact, the chamber may be equipped with a precise positioner that can manipulate the DUT so the receiver antennas may be exposed to the calibrated output of the noise source. In this example a calibrated noise source outside the chamber is connected to a radiating reference antenna with a known gain and bandwidth, inside the chamber. Multiple receive antennas are positioned inside the chamber and connected to a spectrum analyser outside it. Capturing data from each antenna and comparing it with the reference signal generated by the noise source pro vides a quantified and calibrated model of the chamber and antennas in the test system. To overcome the lack of physical connections, engineers are
developing OTA testing techniques to quantify and analyse devices inside RF test chambers. Tese chambers allow devices to be remotely activated and subjected to a variety of tests with transmit and receive antennas inside the chamber. Tese antennas are typically connected to a variety of signal sources to stimulate the device, and measuring instruments like spectrum analysers, vector network analysers or power meters to capture and measure the responses. In order to make reliable and repeatable measurements inside
any chamber, the chamber and test system as a whole need to be calibrated and quantified with certification. Noise sources are the ideal device for this type of calibration process since they provide a known source with calibrated data points which can be used to determine cable loss, air path loss, antenna efficiency and total chamber response. Aſter the system is calibrated and quantified, the same noise sources
with known characteristics can be used as reference sources for the DUT to receive signals. Noise sources for OTA testing also act as a cost-effective alternative to expensive microwave and millimeter-wave signal generators. Te 5G mmWave devices operating above 24GHz incorporate
millimeter-sized patch antenna arrays or dipole antennas that become an integral part of the device module packaging, making OTA testing the only way to characterise and test their performance. However, a mmWave test chamber can introduce significant path loss, more than from cables and connectors. Understanding how to calculate the total link budget for OTA test is a critical step in 5G mmWave. Each patch antenna element in a 5G mobile or fixed- access device can transmit or receive electromagnetic waves with either vertical or horizontal polarisation. Unlike wireless products operating at sub-6 GHz frequencies,
mmWave products introduce a new test challenge related to OTA testing. Namely, over-the-air path losses, measured in dB, can be
22 November/December 2020
www.electronicsworld.co.uk
significant at mmWave frequencies relative to cable and connector contacted losses. For example, a 2.92mm connector-cable assembly can have a path loss of about 2.75dB/m at 40GHz, whereas the OTA path loss at the same frequency is about 64dB at 1m. Using the following equation calculates the OTA path losses when the distance R between the transmitting and receiving antennas is equal to or greater than the far-field (FF) region:
OTA-TestPATH LOSS = 10log10 (λ2 / (4πR)2 dB
where λ is the operating frequency wavelength in meters and R is equal to or greater than the FF region distance, which is the distance at which the spherical waves can be considered as a “plane” wave at the receiving antenna, thus fulfilling the following mathematical requirement:
R ≥ 2D2 / λ
where D is the largest dimension of the aperture (that is, the maximum effective antenna size) of either antenna. Calculating the total OTA test chamber link budget is critical for making
accurate DUT antenna measurements. Te resulting link budget net loss is combined with the measurements made by the mmWave tester instrument to determine the actual radiated power and phase being generated by each DUT antenna array elements, or, likewise, when generating mmWave signals from the test horn antenna into the DUT. Once the choices have been made for a DUT antenna array size with aperture D1, a chamber test horn antenna with aperture D2 and a chamber with a far-field distance R, the Friis transmission equation can be used to calculate the overall link budget of the OTA test setup, as per:
PR = GR GT PT [λ2 / (4πR)2 ]
where PR = Power at the receiving antenna; GR GT
= transmitting antenna; PT = receiving antenna; = Power at the transmitting antenna; and
R = FF distance between the two antennas. With the advent of 5G mmWave wireless devices of various sizes and
applications, each requiring different architectures and sizes of mmWave antennas, it’s critical for test engineers to understand the range of OTA test chambers and test techniques. Direct far field (DFF) and compact antenna test range (CATR) are two types of test methods supported by the 3GPP technical report 38.810, Study on Test Methods for 5G Frequency Range 2 (FR2) in mmWave bands devices. Since CATR OTA test chambers can cost up to ten times more than DFF 1 chambers, a test engineer must decide which one is best suited for the intended application and test requirements.
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