Reproducible tests need a test system, not a box

Hundreds of mobile phones are returned to shops every day because the customer thinks the phone is broken, especially during the warranty period. But how does the shop or a service centre find out if it is really defective or just not operated correctly? Setting up a phone call in a real network for testing is like a finger in the wind to determine the wind direction and speed. This article shows you how these phones are tested effectively and efficiently.

It is rather unlikely that trying out a partially defective phone at a real network reveals the problem and its source. A simple phone call may work at one specific reception level, but may not under different conditions or in a different mode; e.g. data transmission in EDGE or WCDMA mode uses different circuits of the phone. Customer satisfaction is only achieved if the problem has been solved or if the shop proves the quality with a test protocol. There are test instruments available to take the necessary measurements, ideally also providing a pass/fail verdict along with a test protocol. Modes for fault finding and calibration after repair are essential as well. The instrument measures the transmitter and receiver quality at different input and output levels; it can be switched into different modes such as GPRS, EDGE, WCDMA or HSPA for full evaluation of the unit under test.

This actually sounds easier than it is. If quality and efficiency are of concern, the repair centre will need much more to conduct reliable testing than just the right instrument. Additional obstacles can be taken care of by selecting individual components to establish a complete test system – or a solution is chosen that is working right away. The next sections will present a few challenges that you will come across when trying it on your own.

Aeroflex-Image1Fig. 1: A complete test system need not be very large or expensive

The right connection
Only few mobile phones are equipped with an RF connector today. Connecting them to the instrument requires an antenna coupler that picks up the phone’s signal and sends signals from the instrument to the phone. The coupler should be connected to the instrument with a double-shielded RF cable to avoid distortion of the signal by radio waves in the air. Most of the interference, however, can be caused at the coupler, and the signal from the phone can even interfere with the real network, blocking a complete WCDMA radio cell. So the combination of the unit under test with the antenna coupler should be placed in a shielded chamber with a shielding factor of 80 dB. Relatively small shielding boxes with suitable antenna couplers are available as a special solution for mobile phone testing.

The obvious problem with the coupler is that only a fraction of the transmitted signals is forwarded to the receiving side while a lot of the signal power is radiated elsewhere. This power loss must be compensated or taken into account in the measurements. The coupling factor depends on the coupler, the environment (e.g. the shielding chamber), the exact type of phone and even the frequency. So the minimum requirement is a list of coupling factors for each type of phone with the shielding box and coupler at hand. Such a list should be amended frequently to account for all the new phones coming to market every other month. 
Aeroflex-Image2Fig. 2: Small options extend the capabilities of the test system, e.g. for Wi-Fi or Bluetooth

Avoiding errors
A underestimated source of errors is the operator of the instrument. Entering the wrong coupling factors in the instrument renders the test results invalid; trying tests in frequency bands not supported by the phone is costly, and forgetting to test an important frequency band may prevent a problem from being uncovered. Test automation is the way out. It is the only way to ensure that all the different parameters at different levels, in different frequency bands and modes are tested. Manual operation is not only error-prone but also very time consuming and hence cost intensive as it requires qualified personnel to set up the tests and interpret results. Writing the automation software, however, is an expensive undertaking as well, and commercially available support software is typically – but not always – written for a single test platform only or does not take all the variables into account, such as the device-dependent coupling factors.

Each phone vendor requires slightly different test sequences and limits which also depend on the capabilities of the phone. A concise test protocol should document the measurements and results with an overall verdict as a means of proof to either the phone vendor (in cases of warranty claims) or the customer (in the no-fault-found case). These individual test sequences and limits must be taken into account in the testing or in the test automation software.

In addition to the cellular standards, smartphones often support other technologies such as Bluetooth and Wi-Fi. This may require more tests to be integrated in the complete hardware and software system – an obstacle often overlooked by service shops.

Ideally, the test system includes a database to identify the type of phone from its serial number and subsequently set the required tests and parameters. The serial number can be found out either when the mobile registers with the simulated network, or with a barcode reader.

Once a mobile device has been recognised as defective, it is forwarded to the repair loop. The problem here is that the test protocol from an in-depth tester usually differs widely from that produced in the incoming inspection because the test equipment is different. Test technicians then need additional time to interpret the protocol or repeat the test. However, with a standardised test protocol, the type of instrument could be exchanged and still result in the same test protocol format.

Aeroflex-Image3Fig. 3: Small repairs can be done locally and verified with Aeroflex’s 3G Test System for Service 

Complete, off-the-shelf test solution
The test industry is offering a 3G test system for service exactly matching the needs of service shops and repair centres for mobile phones and other wireless devices. Available at a reasonable price, the system is scalable with software and hardware options to meet requirements as they evolve.

A small and handy instrument for GSM and WCDMA, an antenna coupler and a shielding device, cables and Windows-based software form the basis of the system. The system is easily set up and started; when connected to the Internet, the PC software can download the latest coupling factors, test scripts and individual limits within seconds.

The test system automatically detects the mobile phone type, sets up the test parameters and performs a comprehensive test. Additional options for different technologies and speed improvements extend the capabilities and the throughput of the system. If, for example, a Bluetooth or a Wi-Fi option is available in the test system, a short Go/NoGo test for these interfaces can easily be included as well.

Leading manufacturers such as Samsung accept, support and require the test system from their service partners. Frequent updates with the latest phones and test specification amendments are available and can be loaded automatically from an FTP server on the Internet. Alternatively, the updates can be made available on a LAN server or on a USB stick.

Large service centres use different types of testers for different tasks, such as the small Go/NoGo tester for incoming and outgoing inspection and a more versatile RF tester for in-depth testing and electronic correction.

Aeroflex’s 2201 ProLock forms the basis of this affordable 3G test system. The company cooperates with leading mobile phone vendors, ensuring that the future-proof system is always up to date. Level-1 service centres simply replace the 2201 by the 4403 Mobile Phone Tester to add alignment and calibration capabilities in the repair loop. Phone vendors acknowledge that this system is a unique test solution for 2G and 3G service.

Posted by Janine E. Mooney, Editor

March 5, 2012