Once inside the shielded chamber, the hoses connect to an air rotary joint. It separately supports airflow in both directions (supply and exhaust) through the elevation axis of the combined azimuth-over-elevation positioner, while not limiting its angular movement capabilities. By using well selected seals, it keeps the leakage of air through the moving parts of the air rotary joint to a minimum over the entire air temperature range. The supply and exhaust tubes run along the elevation swing and connect to the lower shell of the thermal enclosure, which is made of robust plastic material and fixed on the elevation swing of the 3D positioner. The azimuth rotation stage of the positioner is guided in an isolated manner through this lower shell into the thermal compartment, enabling full rotation of the DUT in the second axis and achieving full 3D assessment of the DUT across the extreme temperature range.
Figure 2 Temperature and air flow simulations of the 50 l thermal enclosure with direct pipe entry (a) and added diffusor (b).
To close the thermal compartment, the upper dome—made of RF transparent Rohacell® material—interlocks to the lower shell via an air-sealed locking ring. The dome material enables high-quality RF measurements with the dome in place since its permittivity is close to air to minimize any impact on the RF radiation. The shape of the dome, the thickness of its wall and the processing of the material were optimized to close the foam cells as much as possible to increase the air-tightness and robustness of the dome to withstand increases in internal air pressure, while minimizing RF perturbations. Different sizes of the dome provide smaller and bigger volumes, either for a larger DUT or to support faster temperature cycles. The larger dome is compliant with 3GPP and CTIA DUT alignment, as well as the quality of the QZ assessment mandated by the 3GPP RF conformance testing specifications.
Once inside the thermal enclosure, the air is guided using a patented diffusor. By designing mechanical pieces to guide the air flow at the output of the air supply pipe toward the exhaust pipe, the homogeneity of the temperature within the enclosure was increased significantly. This ensures a fast and equalized temperature distribution and eliminates hot or cold spots, increasing performance and accelerating the time for stable temperature convergence. Ideally, the sensor-controlled air flow volume provided by the Thermostream is maximized, however, the supply air temperature range must be adopted to the other materials used in the air flow chain. These parameters also influence the air cycle times, so they had to be chosen carefully.
After the temperature energy of the air is provided into the thermal compartment, the air exhausts through pipes in the same, yet separate, way through the air rotary joint and the exhaust air feedthrough out of the chamber. To reduce noise, the hose ends at a specially designed noise canceller. Since the diameters of the exhaust path affect the pressure increase inside the thermal enclosure, they were selected to keep the internal pressure low enough, with headroom, to avoid damage from excessive pressure.
ETC OTA PERFORMANCE
Multiple optimization rounds were necessary to develop a solution meeting both major test specifications requirements and user needs for high speed testing. These involved electromagnetic, air flow and thermal simulations used to optimize the air distribution within the thermal enclosure (see Figure 2). Many prototypes were required, accompanied by hundreds of hours of testing to validate the numerical findings and optimize the design. The multiple versions of the ETC OTA leading to the final solution yielded a very compact and easy to handle test environment where various size devices can be tested in full 3D and across a wide temperature range (see Figures 3 and 4).
Figure 3 ETC OTA test system (R&S©ATS1800C) with a commercial Thermostream.
Figure 4 Inside of the ETC OTA test system with 35 l Rohacell thermal enclosure closed (a) and open showing a DUT (b).
With this setup, temperature changes well beyond the 3GPP required limits can be achieved in a short time using an air flow rate up to 700 l/min. At that flow rate, a temperature change between +85°C to -40°C is possible in 10 to 14 minutes within a 50 liter ETC compartment. Even without a need for the 125°C wide temperature window, having it is an advantage because the additional temperature range enables fast temperature ramps when testing across the 3GPP specified temperature range. A temperature change between -10°C and +55°C can be achieved in less than 3 minutes in the same thermal enclosure size. Cooling takes longer than heating, as expected; over the full 3GPP temperature window, cooling takes about 40 seconds longer (see Figure 5). Using this ETC OTA setup supports all 3GPP conformance testing and additional stress testing while keeping test time reasonable.
Figure 5 DUT temperature cycling times in 50 l enclosure at 450, 500 and 700 l/min air flow rates: –40°C to +85°C (a) and 3GPP range from –10°C to +55°C (b). Supply air temperature range from –60°C to +125°C.
References
- 3GPP, “NR; User Equipment (UE) conformance specification; Radio transmission and reception; Part 2: Range 2 standalone,” TS 38.521-2, Version 16.12.0, June 2022.
- CTIA, “Test Plan for Wireless Device Over-the-Air Performance,” Vol. 4.0, Feb. 2022.
- B. L. Schoenholz, J. M. Downey and M. T. Piasecki, “Design of a Thermal Testbed for Metrology of Active Antennas,” 2022 Antenna Measurement Techniques Association Symposium (AMTA), 2022.
- B. Derat et al., “Acceleration of Over-The-Air Measurements Under Extreme Temperature Conditions Through Optimization of Air Flow and Thermal Efficiency,” 2022 Antenna Measurement Techniques Association Symposium (AMTA), 2022.
