The RFIC evaluator
An exclusive interview with a test engineer
By Martin Rowe, Senior Technical Editor -- Test & Measurement World, 3/1/2009 2:00:00 AM
Joe Flynn is a staff engineer at Sequoia Communications, a start-up fabless semiconductor company that develops and produces single-chip RF transceivers for multimode cellular handsets. The transceivers enable handset manufacturers to use a single device to communicate over GSM, EDGE, and WCDMA networks. Martin Rowe spoke to Flynn by telephone at his office in San Diego.
Q: What is your role in test?
A: At Sequoia, four test engineers develop evaluation boards and automated test stations for evaluating designs. We also perform the evaluations and analyze test data.
Q: What’s in an evaluation board?
A: In-house evaluation boards provide us with access to every signal on the device. Our device supports multiple bands and modes, which means we need to test five transmit paths and seven receive paths. We use a socketed board for functional testing and a soldered-down version for characterization. For customers, we provide a reference design board with a single antenna port that includes all of the components required to simulate the RF section of a cellphone.
Q: What are some of the tests you perform?
A: We test transmitters by feeding them a baseband analog signal and measuring the output. We use predefined waveforms built into a signal source along with proprietary waveforms and store them in the instrument. We also test receivers using modulated signals and measure any distortion that the receiver produces during downconversion. For receiver EDGE testing, we use 8PSK (8 phase-shift keying) modulated waveforms. We have many waveforms for WCDMA testing, starting with QPSK (quadrature phase-shift keying) waveforms and then moving on to more complex waveforms.
Q: How many devices do you test in an evaluation?
A: We typically test 50 to 100 pieces. That gives us enough data to characterize how the part will perform in production. We also evaluate parts from multiple wafers within the same lot. In addition, we have our silicon supplier fabricate parts using process parameters at the corners of their specifications. This provides us with data on how well the parts behave over the full range of production parameters. We also test the parts over a wide range of power-supply voltages.
Q: How do you test for environmental effects?
A: We test our receivers in an environmental chamber that’s installed in a screenroom. It minimizes interference from commercial radio transmitters and nearby cellphones and base stations. Transmitter test stations don’t need screenrooms.
Q: What are the functions of your test stations?
A: We have several test stations. One tester performs functional test on new parts so we can verify that they work before we characterize them. Another station calibrates transmitter and receiver gain stages. To minimize cost and complexity, we have independent receive and transmit test stations. The receiver test station includes a vector network analyzer for measuring S-parameters and a modulation analyzer to test the baseband output of the receiver. Measurements include gain, noise figure, intermodulation distortion, cross modulation, signal-to-noise ratio, and error-vector magnitude. A digital multimeter measures power-supply current on every power pin through a switching matrix.
Q: How do you analyze test data?
A: The test data is stored and is manipulated using a custom Web-based charting tool that allows us to compare measurements on the devices over environmental, fabrication process, and power-supply conditions. Due to the complexity of the device, we produce as many as 1200 plots.
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