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Address ATE challenges for mission-mode test of wireless transceivers

31 Jul 2012  | Steven Fields, Scott Therrien, Steve Lyons

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ATE typically only tests a subsection of the end application at a time. This means that the feedback path and correction mechanism may not be present during test. Therefore, in order to understand how the device under test (DUT) will perform in the entire system, the ATE needs to emulate the feedback path along with the correction methodology to simulate the end use of the system. We call testing of the DUT as would be used in its end-use application vs. a simplified alternative test mode a 'mission mode' test strategy. Mission mode testing is a more robust test strategy because it tests the device in the same mode of operation that the device will actually be used. To implement mission mode testing there are numerous challenges that must be overcome when using ATE instruments and fixtures. One challenge is overcoming additional sources of IQ impairment that could be introduced by the analog baseband instrument as well as the quality of the test fixture (figure 2).

Figure 2: Test configuration mimicking the end application providing for a mission mode test strategy.

These impairments, whether from the DUT, ATE or test fixture, can be grouped into two categories; frequency dependent and frequency independent error. Frequency dependent errors, such as filter group delay, are not uniform across bandwidth and are beyond the scope of this article. Frequency independent errors have the same effect regardless of bandwidth. Quadrature error (quad error), IQ amplitude imbalance and IQ timing skew are all considered frequency independent errors. Quad error is the orthogonal error between the I and Q signals. Ideally, I and Q should be orthogonal (90 degrees apart) at the output of the mixers. However due to quad err they are not 90 degrees apart. This error typically can only be introduced by the quadrature modulator / demodulator of the DUT.

Another type of error is I/Q gain imbalance, this is the ratio of the gain of the I signal path vs. Q signal path either before or after the mixers. The ideal gain ratio should be one. Gain imbalance can be introduced by the quadrature modulator / demodulator as well as the ATE and test fixturing. The third type of error is baseband IQ timing skew error. This is the amount of time based skew is between the I and Q data streams of the baseband signal. In ATE systems, time based skew is often due to an unequal start of I relative to Q or differences in path lengths of the test fixture [2].

Often the automated test system's load board design can be a critical element which introduces undesired I/Q amplitude and timing skew impairments. In an end application, such as a cellular handset, the baseband IC and wireless transceiver IC may be physically located mere centimeters or less from one another. In the test environment however, the baseband AC instrumentation acting as proxy for the baseband IC is much farther away than a couple of centimeters. It may be a foot or more away from the wireless transceiver DUT. This opens the door for a multitude of error sources to be introduced that would not normally exist within the handset. Trace length mismatches and parasitic errors from non uniform trace routing, potentially on multiple layers can lead to unwanted skew impairments. Long trace lengths can lead to the addition of active components in the path such as amplifiers which can add further impairments.

Figure 3: Geometric Distortions due to IQ impairments: Plots show geometric distortion effects of IQ impairments on the constellation diagram.

Figure 3 shows the effects of these impairments on the constellation. The effects vary depending on the type of modulation used. For example, single carrier modulations, such as 802.11B (CCK), are demodulated in the time domain and the amplitude of I and Q components at points in time determines the constellation location. Multi-carrier modulations on the other hand, such as LTE and 802.11ac (OFDM), are demodulated in the frequency domain. A complex fast Fourier transform is used to determine each constellation point for each carrier frequency individually. For this reason the effect of quad err, gain imbalance and time skew look different for multicarrier modulated signals. Notice that gain imbalance in single carrier modulation takes on a rectangular shape whereas a multicarrier modulation the constellation produces a miniature constellation diagram at each constellation point. Quad err also takes on different shapes depending on the modulation type, trapezoidal for single carrier, and mini-constellation for multicarrier.

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