there is no single test system that can economically test all the RF ICs designed into today’s wireless applications. For testing low-complexity devices (such as power amplifiers or low-noise amplifiers), available ATE generally exceeds requirements. Consequently, the lowest-cost test approach is to use a custom rack of instruments. Note that testing these simple RF ICs requires little or no digital test capability, so the low-cost RF test system does not need a dedicated digital test subsystem or a traditional test head.
Instruments are readily available from Agilent, Anritsu, Keithley and Rhode & Schwarz. However, integrating them to achieve low test times requires specially designed and optimized software that fully utilizes the instrument’s capability. Test integrators like Amkor Technology develop test systems and user-optimized software to effectively integrate multiple instruments into one RF IC test stand.
Midcost testers originally were required to test a single tone stimulus and measure the distortion of the receiver so the digital pins did not require speeds greater than 10 MHz. However, digitally based modulation schemes such as those used by WLANs have generated new test requirements. ATE must now test parameters such as EVM and BER.
With signal bandwidths around 20 MHz and nominal and dynamic range requirements in excess of 80 dB, the test for 3G wireless devices requires a difficult combination of high sample rates, fine resolution and high dynamic range.
The tester must be able to digitize data with at least 14 bits’ resolution and a sample rate greater than 65 Msamples/s. To increase dynamic range, the testers are typically designed with a two-stage down-conversion architecture. For GSM and W-CDMA, RF testers must go up to 6 GHz, with baseband and digital capability up to 50 MHz.
RF-IC Transmitter tests
The RF-IC transmitter is characterized by developing a digital IQ representation of the RF signal using signal studio and loading it into the System Controller. The exercising module packetizes the IQ data, inserts the specified control packets and drives the bit stream into the RF-IC digital baseband input over the DigRFSM V4 interface. The RF-IC converts the digital signals into analog, processes the serial bit stream and generates an RF signal that is captured by the Agilent MXA signal analyzer and can be analyzed using the Agilent 89600 VSA software. Designers can compare the output RF waveform produced by the RF IC to the intended baseband waveform as produced by the BB-IC to characterize transmitter behavior, including the effects of conversion of the data from Digital to Analog, and impact of frequency upconversion for example.
RF-IC Receiver Tests
The receiver is characterized by driving a known RF signal into the RF-IC Analog RF Rx port using an E4438C or MXG Signal Source. The RF-IC processes the RF signal and renders the digital IQ, which is packetized and provided out in the DigRFSM V4 digital serial format. Again, the 89600 VSA software in conjunction with the MXA signal analyzer, and the DigRFSM exerciser and analysis modules (captures the serial bit stream and extracts the relevant IQ data for analysis.
The Agilent solutions enable BB-IC development teams to evaluate and tune the behavior of their components independent of the RF-IC:
System Integration and Validation
Using the acquisition probe, validation teams can monitor transactions on the DigRFSM V4 interface generated by the BB-IC. Control packets are displayed on the system controller and IQ from the data packets can be evaluated using the vector signal analysis package. The receive path on the BB-IC can be characterized by driving the DigRFSM V4 interface with IQ data packets from the stimulus probe. The IQ data packets are creating on the logic analyzer using the digital IQ representation of the RF signal generated in signal studio.