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Reducing the cost of cellphone testing

Posted: 16 Sep 2006 ?? ?Print Version ?Bookmark and Share

Keywords:Agilent Technologies? cellphone? testing? cellphone testing? test?

By Dan Aubertin
Agilent Technologies

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, all manufacturers feel extreme pressure to reduce costs and improve margins to remain competitive. Consequently, they are taking a close look at processes, particularly at the cost of test, which has grown to be a much larger 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 keep rising test costs down. 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, Wi-Fi, 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 multimode 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 then repeated many times 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 described above require a large amount of communication time for controlling the mobile via a serial interfacereading the measurement data from the test set and writing the calibration data into the mobile phone. With multi-band, multimode 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 special complex 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 in the test instrumentation used to calibrate mobile phones.

Instrument features needed to calibrate a mobile phone
To calibrate a mobile phone, the test set itself must have a calibrated source and a calibrated 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.

As a calibrated measurement receiver, the test set performs four types of measurement needed to calibrate the mobile phone:

  • Power - to verify that the phone is transmitting at the correct level;
  • Out of channel spectral emissions;
  • Spectrum monitor; and
  • Modulation quality

Power measurements are made to verify that the mobile phone is transmitting at a set level. These are the key calibration measurements. But other modulation and spectral measurements are used often 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.


Click for full image
Figure 1: DPCH measurement screen for a W-CDMA calibration application running on a wireless communications test set.

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, you can control the DPCH downlink channel number setting, output power, and modulation type. Common power measurements are mean power (mean power in the 5MHz band), dynamic power (fast power steps over a set frequency) and RRC filtered power, which is measured in the 3.84MHz 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.

Notice that 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 800-1,000MHz and the 1,700-1,990MHz 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 150s. 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 modethere is no call established between the phone and the test set. Test mode requires chipset support and software, which cause the mobile phone to transmit at frequencies and levels, and also synchronize the phone with the test set for receiver measurements.

In the case of Fast Device Tune, the mobile phone transmits a predefined 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 be completed in 15s, in contrast to 150s using standard techniques. This order-of-magnitude improvement eases a costly bottleneck in the manufacturing test process and can significantly improve production throughput.


Figure 2: Fast Device Tune measurement allows simultaneous calibration of mobile phone transmitter output power and receiver input power.

Next-generation test platform
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. The advantages of such an architecture are many: Windows provides a familiar user interface that is easy to navigate. The structure of programming, help and troubleshooting tools is familiar. Windows remote desktop capability allows a manufacturer to access any test set from any other Windows PCacross the desk, across the building, or across the world. Test sets can communicate with each other and run any PC program. In an era of outsourced manufacturing, this could be an especially powerful aid to productivity.

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 centerpiece of an efficient, automated test system. The need for separate system controllers is eliminated. Using automated test programs 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 thus 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 maximize 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 underutilized test assets without having to invest in additional test capabilitythus enabling much higher test asset utilization.

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 high throughput, reduced-cost goals.

About the author

Dan Aubertin is a product manager for the wireless division within Agilent Technologies' Electronic Measurement Group. Dan has spent twenty three years working for Hewlett Packard and Agilent Technologies in a variety of marketing roles, including Applications Engineering, Business Development, Sales Support and Service Engineering. Most recently he was a Market Development Manager and Applications Engineer based in Asia and is currently managing new product introductions for the Wireless Division.




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