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An IS-95 16-channel CDMA Source for Power Amplifier Testing
Berkeley Varitronics Systems Inc.
Communications systems that use CDMA techniques are placing new demands on component manufacturers, test equipment and measuring techniques. Digital TDMA and CDMA standards use quadrature phase-shift keying (QPSK) modulation formats as opposed to the constant modulation envelope formats of the older analog systems. These QPSK formats feature greater spectral efficiency, but they are subject to spectrum regeneration from nonlinearities in the transmit systems. CDMA, as implemented per the IS-95 standard, provides increased channel capacity as a result of its multiplexing re-use. However, in order to reduce capacity degradation, high demands are placed on the power amplifier to minimize nonlinear effects. Performance testing of these power amplifiers has become much more challenging and, in an operating network, it is necessary to provide the device under test with signals that have similar amplitude distributions to the system signal.
A CDMA source that generates real-time stimuli for testing power amplifier performance for use in cellular and PCS applications is now available. The Zebra™ IS-95 16-channel CDMA source provides a true baseband CDMA signal for testing power amplifiers per the IS-97 specification. The I and Q baseband outputs can be fed to any signal generator to create an RF CDMA signal.
The CDMA Source
The Zebra generator is a simple multiple-CDMA source with a built-in IS-95 filter. A block diagram of the Zebra 16-channel CDMA source is shown in Figure 1 . The CDMA channels are combined using a digital combiner. The generator produces up to 16 code channels and provides independent control of the gain and Walsh code spreading for each channel. Because the channels are a digitally combined stimulus, well-defined RF envelopes per the IS-97 specification are generated, as shown in Figure 2 .
The unit produces baseband I and Q signals into a 50 W load. The output amplitude is selectable and a modulated 70 MHz IF I and Q output is available as an option. In addition, the generator’s output also may be combined with an external additive white Gaussian noise signal for simulated noise loading of the test signal. An option is also available to modulate actual data packets.
Specifications for the Zebra source include a 1.2288 Mbps chip rate, an IS-95 forward link pilot channel and 16 orthogonal channels with 0 to 63 selectable Walsh codes. An internal temperature-compensated crystal oscillator (TCXO) is incorporated with a stability of 2.5 ppm from 10° to 50°C. The unit is portable and operates from 110/220 V AC at 1 A.
Zebra options include a selectable internal or external 10 or 19.6608 MHz clock source and an external even-second pulse input. Also available is a rubidium clock source with a stability of 1 x 10–10 ppm that can be phase locked to the Global Positioning System signal.
The PC interface supplied is the ZebraEye™ application software, which enables the user to control and configure the Zebra CDMA source for the required performance. The software allows either predefined or custom waveforms to be loaded into the generator for transmission at the appropriate times. The software also permits the user to mark a specific waveform as the default, allowing the source to operate individually without PC control by transmitting the default waveform. Waveforms may be created in ZebraEye and saved to disk for later use. The software also allows the user to select a clock source (the internal clock or either of two external clock sources).
The ZebraEye software is designed to allow a waveform to be created quickly and sent to the Zebra source for transmission. The ZebraEye main screen, shown in Figure 3 , contains a main menu bar, progress indicator, bank selection control, 16-channel configuration area, system control buttons and system status bar. The four submenus are FILE (for loading a predefined waveform or one that was configured and saved previously), OPTIONS (for changing the settings for the clock source), COMMUNICATION (for selecting the connecting ports) and HELP (for displaying the operating manual).
The bank selection control enables the user to switch between eight selectable and configurable waveforms. (Up to eight waveforms may be stored in the unit.) The control buttons allow the user to read and load bank information to and from the unit, queue data, and start and stop transmission. The status bar displays connection and Zebra version information as well as any pertinent status and system clock information.
In creating a CDMA waveform, a desired combination of pilot, sync, paging and traffic channels must be selected. The pilot channel selected always is Walsh code 0. The default waveform has all of the channels de-selected except the pilot channel. Any channel can be activated by placing a check mark in the appropriate channel box. The data for each channel are selected by choosing the pattern from the combination box for that channel. Power for the pilot, sync and paging channels may be selected manually. Any remaining power is distributed among the active traffic channels.
A simple signal source now is available to generate the ideal stimuli for testing power amplifiers designed for IS-95 CDMA operation in today’s cellular and PCS applications. The source provides a 16-channel output for evaluating adjacent-channel power and error vector magnitude performance realistically per IS-97 specifications in an easy-to-use, compact, portable generator.
Berkeley Varitronics Systems Inc.,
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