When it comes to over-the-air (OTA) testing, more test points equals more test time, says NI Chief Systems R&D Engineer
When it comes to over-the-air (OTA) testing, more test points equals more test time, and this has never been more true than in the case of mmWave. At the Test & Measurement Forum, NI’s Chief Systems R&D Engineer Gerardo Orozco shared with attendees the top three OTA testing challenges and how the company has successfully cut down test time through integrated motion control and dedicated hardware and software tools.
External switching
Orozco stated that the first challenge identified by NI is that most testing instruments on the market have a signal generation box and a signal analyzer box. “Therefore, whenever they try to do a transmit and receive test, they need to have some sort of external switch to do this combination,” he explained, adding that at frequencies in questions — roughly 24 GHz to 54 GHz — this leads to a lot of path loss. “You want to be as close as possible to your … antennas … The moment that you need to put external switches … the cable starts to be a little too long and there is extra complexity,” he said.
To solve this, NI designed mmWave test heads located outside of the main portion of instrumentation. These can be placed a lot closer to the antenna and they are transmit and receive, which allows for testing “bi-directionality” without connecting and disconnecting.
Waveform switching
Another time-consuming challenge in OTA testing is that switching between CW (continuous waveform) and modulated wave form requires different equipment. “We want the same equipment to be able to do a CW type of test, which sometimes is do[ne] to get very, very accurate way of the antenna pattern. But you also need to do modulated measuring … to see what the signal looks like when a wide band signal is going through,” said Orozco.
This is why NI designed equipment, said Orozco, that does both CW and modulated measuring. “That is what the instrumentation is expected to do,” he said.
Movement
With over the air testing, the fewer points you test, the more systematic errors you get; however, more test points means more test time. “If you want a very small systemic error, you’re talking about thousands of points and … that’s minutes per test case,” said Orozco. He explained that this challenge arises because as you’re testing, there is movement; it takes time to accelerate and decelerate to get to the next point. “You stop, then you measure. That takes hundreds of milliseconds and when you multiply that by all of these points, you start getting into the minutes just to get one test case completed,” he continued.
NI achieved five times improvement in test time using what it calls autonomous 3D sweeping, which Orozco said essentially means “do not stop.”
“Instead of move, stop, move, stop. What we do is as we are moving, we have hardware triggers and we start capturing little buckets of RF signal, and then we start mapping them where they were in the location and the position is constantly moving,” he said, noting that this “substantially” reduces test time with the same grid size.
