Cellphones connect through Ethernet
To test its base stations, RadioFrames modified a commercial tester to create a system that makes RF measurements, frequency measurements, and power measurements under digital control.
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 4/1/2008 2:00:00 AM
PROJECT DESCRIPTION
Cellphones may not work indoors because of weak signals. To help people who need to use their cellphones in home offices and in small businesses, RadioFrame Networks (Redmond, WA, www.radioframenetworks.com) developed base stations that let users connect to a cell network through a DSL or cable modem.
| More about the device under test, the challenge, and the tools. |
To test the RadioFrame base stations, test development manager Paul Knight needed a system to measure and calibrate the transmitters and receivers. He based his system on a commercially available RF test system and added components that let it perform a complete functional test. The system tests production base stations by making RF measurements, frequency measurements, and power measurements under digital control.
The basic RF test system consisted of an eight-slot PXI chassis, an embedded controller, a signal generator, a vector signal analyzer (VSA), and a software library of functional tests. RadioFrame engineers substituted an 18-slot PXI chassis for the eight-slot chassis, then added PXI modules, GPIB instruments, and the code needed to control them (figure).
 A PXI-based system tests cellular-to-LAN base stations. |
After a test operator connects a unit under test (UUT) board to the test fixture, the system programs an address into the board through a serial port. Once programmed, the UUT can operate over Ethernet with a Telnet interface.
A test starts with basic functional measurements such as current consumption and oscillator frequency. The power supply measures current, and the counter/timer card measures oscillator frequencies on the board’s processor clock, field-programmable gate array (FPGA) clock, and RF reference clock. The DMM measures all voltage levels on the board: 2.5 V, 3.0 V, 3.3 V, 3.6 V, and 5.0 V.
| Cellular band |
Transmit frequency range (MHz) |
||||
| GSM 850 | 869.2 to 893.8 | ||||
| GSM 900 | 925.2 to 959.8 | ||||
| DCS | 1805.2 to 1879.8 | ||||
| PCS | 1930.2 to 1989.8 | ||||
Calibration takes most of the 15-min test time. In a transmitter calibration, the system commands the UUT to produce a specified power (19 dBm, 42 dBm, or 67 dBm). The VSA measures RF power, and the system adjusts the power until it’s within tolerance. It then repeats the measurements across four frequency bands. After measuring each power level, the system calculates correction factors and stores them in EEPROM.
With a transmitter calibrated, the VSA measures transmit power, frequency error, phase error, adjacent-channel power, and power-versus time (for a transmit burst).
The system calibrates a UUT’s receiver when the signal generator produces a modulated carrier of known frequency and power. The UUT contains diagnostics that report received power levels. Using those reports, the test system calculates correction factors. Following calibration, the system performs BER tests on the receiver.
In addition to making measurements, the test system automates parts of the test setup by moving Ethernet cables into place. “We broke off the clips on the Ethernet cables and use solenoids to connect them to the UUT,” said Knight.
LESSONS LEARNED
“When we first began testing the boards, we had set the RF measurement limits too tight,” said Knight. “We were failing good boards. Some tests limits were flexible, and we needed a statistically large sample of tests before we could properly set test limits.”
DEVICE UNDER TEST
Base stations that let you use your cellphone indoors through a LAN. The base station connects to a DSL or cable modem and to a cellphone through GSM, digital communications system (DCS), or personal communications services (PCS) networks.
THE CHALLENGE
Measure and calibrate transmitter output power and receiver sensitivity. Test RF transmit and receive functions for power, frequency, spectrum, and bit-error rate (BER). Measure voltages and power consumption. Control electromechanical devices that automate the connecting of test cables.
THE TOOLS
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Agilent Technologies: rack-mount power supply, digital multimeter (DMM) with internal switching card. www.agilent.com.
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Amfax: PXI-based RF test system consisting of a chassis, controller, signal generator, and vector signal analyzer. www.amfax.co.uk.
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National Instruments: PXI data-acquisition card, PXI digital I/O card, PXI counter-timer card, PXI RS-232 interface card, PXI RS-485 interface card, graphical programming language. www.ni.com.
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