Reducing the cost of test
07 February 2008
Wireless mobile phone manufacturing is an extremely competitive industry. With the explosive growth of worldwide mobile phone subscribers in emerging markets such as China and India, manufacturers feel pressure to reduce costs and improve margins to remain competitive.
Consequently, they are taking a close look at processes and particularly at the cost of test, which has grown to be a large percentage of the total manufacturing cost of a mobile phone.
As mobile phones increase in complexity, requiring more tests, manufacturers must find strategies that will reduce rising test costs. One approach is to design products that can be tested more efficiently. Another is to adopt test equipment with new architectures, faster measurements and calibration techniques that can increase throughput and lower ownership costs.
What is driving the cost of test?
The latest mobile phones integrate advanced features such as Bluetooth, WiFi, MP3, FM stereo, GPS, cameras, and video. In addition, 3G phones must support multiple access technologies such as GSM/GPRS/EGPRS and W-CDMA. This convergence of technology requires considerably more testing during manufacturing to ensure product quality. One of the most time-consuming measurement stages on a production line is the calibration stage. RF transmitter and receiver calibration tests are performed to verify that all RF functions are operating properly and meet technology and regulatory standards.
Calibration of the transmitter output power affects maximum power, minimum power, power control, modulation quality, adjacent channels, spectrum emissions, and battery life. Accurate calibration of the receiver input level Receive Strength Signal Indicator (RSSI) is necessary to ensure proper power control and handover operation on the subscriber network. With the latest multi-mode 3G phones, these calibrations must be performed over more frequency bands with different modulation types and standards requirements. Calibration has thus become the dominant stage of manufacturing test.
Traditionally, transmitter calibration tests have used mobile phone test modes to set the transmitter to a specific channel and power level, which is then measured by a wireless communications test set. A correction factor is then calculated and loaded into the mobile phone calibration table. This process is repeated for each power level across multiple frequencies per frequency band. The mobile phone RSSI calibration has traditionally followed a process similar to the transmitter calibration where the test set provides a calibrated modulated signal to stimulate the mobile receiver. The mobile phone then makes an RSSI measurement on the calibrated signal and a correction factor is generated for the RSSI. This process is repeated across frequency bands at multiple input levels per frequency.
The mobile phone transmitter and receiver calibration processes require a large amount of communication time for controlling the mobile via a serial interface, reading the measurement data from the test set, and writing the calibration data into the mobile phone. With multi-band, multi-mode 3G phones, this communication time has become a significant part of the overall calibration time.
Manufacturers face the dilemma of how to deal with the increased functionality and complexity of their products without substantially increasing test costs. One part of the solution is to reduce manufacturing test time by building test modes into the next-generation wireless products. These test modes execute complex sequences and control mobile phone functions to speed measurements and significantly reduce the communication time required for calibration. Another part of the solution is to challenge test equipment suppliers to provide innovative, efficient design and measurement techniques.
Instrument features needed to calibrate a mobile phone
To calibrate a mobile phone, the test set must have a calibrated source and measurement receiver. In the role of calibrated source, the test set provides an expected signal level, which is then measured by the mobile phone receiver.
Power measurements are made to verify that the mobile is transmitting at a set level. However, other modulation and spectral measurements are often used to verify performance parameters of the device, such as phase, frequency, or out-of-channel emissions. A spectrum monitor can be used for filter bandwidth checking and carrier suppression. These capabilities provided by the test set must be able to support the mobile phone chipset calibration requirements being used by the manufacturer.
Figure 1 shows a Dedicated Physical Channel (DPCH) measurement screen for a W-CDMA calibration application running on the Agilent E6601A Wireless Communications Test Set. Using the test set, it is possible to control the DPCH downlink channel number setting, output power, and modulation type. Common power measurements are mean power (mean power in the 5 MHz band), dynamic power (fast power steps over a set frequency), and RRC Filtered Power, which is measured in the 3.84 MHz band according to the standards. Spectral measurements include a spectrum emission mask, occupied bandwidth and adjacent channel leakage ratio, which shows the out-of-channel emissions for the mobile phone. Tests have masks or limits defined by the Standards. Importantly, these measurements do not require call processing.
Increasing measurement speed
Accurate calibration takes many channels (frequencies) and power levels to ensure that a mobile phone operates correctly. For example, a typical W-CDMA device calibration profile measures power across both the 800-1000MHz and the 1700-1990MHz bands with 15 channels at each band and 20 power levels at each channel. This adds up to 600 power measurements. Another 300 receiver measurements are made at each band, for a total of 1,200 measurements needed to calibrate the device. Using typical power measurement methods and sequential receiver measurements, a W-CDMA device calibration would take over 150 seconds. Reducing the number of measurements is not always an option. Test equipment suppliers have therefore developed techniques to improve the measurement process by cutting the set-up, communications and measurement time.
One such technique is Fast Device Tune, a measurement available on the Agilent test set. This technique allows the simultaneous calibration of the mobile phone transmitter output power and RSSI across level and frequency in a single sweep. The measurement is performed with the phone using a complex test mode, which requires chipset support and software. This causes the mobile phone to transmit at frequencies and levels, and synchronize the phone with the test set for receiver measurements.
In the case of Fast Device Tune, the mobile transmits a pre-defined series of power steps at various uplink frequencies and simultaneously tunes its receiver to perform measurements of the test set’s signal at various downlink frequencies and power levels (see figure 2). Calibration of a W-CDMA device using Fast Device Tune can now be completed in 15 seconds, which is in contrast to 150 seconds using standard techniques.
Through techniques such as Fast Device Tune, calibration measurements are hitting the technical limits of speed such as the frame rate of GSM. When this point is reached, test speed is then determined and limited by the mobile phone complex test modes. However, there are other features that test equipment suppliers can provide to further offset the cost of test.
A test set designed on a next-generation platform increases overall operational efficiency and makes allowance for technology convergence. An example is the embedded open PC in the Agilent E6601A that runs Microsoft Windows XP. An advantage of this is that Windows provides a familiar user interface. Furthermore, Windows remote desktop capability allows a manufacturer to access any test set from any other Windows computer.
By turning a test set into a ‘smart instrument’, test equipment suppliers increase the opportunities for mobile phone manufacturers to streamline automated production lines. The test set has the control and interfacing capability that allow it to become the centrepiece of an efficient, automated test system. Using automated test programmes running on the internal PC controller, system rack space can be reduced and the overall system design simplified.
In a next-generation platform, test equipment suppliers can make other improvements that reduce the cost of ownership and the cost of test. Reliability design, support, and calibration and repair tools are all areas in which the supplier can apply technology and a creative approach to maximise the uptime of the equipment.
Test suppliers can even implement flexible licensing strategies such as transportable licenses that allow manufacturers to balance test capacity across their global manufacturing operations to take advantage of under-utilised test assets without having to invest in additional test capability.
Whether focused on improving specific measurements or addressing the broader issues of manufacturing test processes, test equipment suppliers are important allies of mobile phone manufacturers in meeting their goals of high throughput and reduced costs.
DAN AUBERTIN, product manager for the wireless division, Agilent Technologies
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