5G promises substantial improvements in wireless communications, including higher throughput, lower latency, improved reliability, and better efficiency. Achieving these goals requires a variety of new technologies and techniques: higher frequencies, wider bandwidths, new modulation schemes, massive MIMO, phased-array antennas, and more.
These bring new challenges in the validation of device performance. One of the key measurements is error vector magnitude (EVM), which is an indicator of quality for modulated signal as they pass through a device under test (DUT). In many cases, the EVM value must remain below a specific threshold—and getting an accurate measurement requires that the test system itself be very clean (i.e., have a low EVM itself). This includes all fixtures, cables, adaptors, couplers, filters, pre-amplifiers, splitters, and switches, between the DUT and the measurement system.
At 5G bandwidths and frequencies, the test fixture can impose a significant channel frequency response on the test system and adversely affect EVM results. Hence, the measurement now includes the characteristics of the test fixture and the DUT—and this makes it difficult, if not impossible, to determine the true performance of the DUT.
Calibration can move the test plane from the test instrument connector to the input connector of the DUT (Figures 1, 2). Keysight has created a solution that uses a NIST-traceable reference comb generator to enable complete channel characterization of the test fixture on both sides of a transceiver (or any other component or device).
Figure 1. This uncalibrated test system has unknown signal quality at the input to the DUT (A1’). A common mistake is to simply use equalization in the analyzer (A2), but this occurs after the DUT and it also removes some of the imperfect device performance we’re trying to characterize.
Figure 2. In this calibrated test system, the system and fixturing responses have been removed, enabling a known-quality signal to be incident to the DUT (B1). The analyzer errors can also be removed (B2).
Figure 3 shows the uncalibrated test fixture equalizer response for a 900 MHz BW signal at 28 GHz. The upper trace shows the amplitude response with a significant roll off at the upper end of the bandwidth. The lower trace shows the phase response, which also has considerable variation over the bandwidth. These imperfections would limit EVM to being no better than about 5 percent.
Figure 3. These OFDM frequency response corrections for an uncalibrated system show variations of nearly 7 dB in raw amplitude and 45 degrees of phase across a 900 MHz bandwidth at 28 GHz
Figure 4. Here is the same OFDM response for a calibrated system, showing variations of only 0.2 dB and 2 degrees. The resulting signal EVM dropped to less than 1 percent from more than 5 percent.
Figure 5 shows the demodulation results after calibration for single-carrier 16QAM signal nearly 1 GHz wide. The upper-left trace shows a very clean constellation diagram. The lower-left trace shows the spectrum with a bandwidth of approximately 1 GHz. The upper-left trace shows the equalizer response in both magnitude and phase: both of these are nearly flat, indicating the equalizer is not compensating for any residual channel response in the test fixture. The middle lower trace shows the error summary: EVM is approximately 0.7 percent, which is a very good result. This system would be ideal for determining a device’s characteristics.
Figure 5. Calibration enabled the signal generation of a 1 GHz wide signal with an EVM of less than 0.7 percent at 28 GHz. This EVM occurs at the input plane of the DUT.
In pursuit of tremendous improvements in cellular network capability, 5G is using new technologies that pose many challenges to testing. Fortunately, calibration will help ensure that we’re measuring the true performance of the DUT without the effects of the test fixture.
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