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A Vector Modulator to Convert Analog Signal Generators to Digital Formats
IFR Systems Inc.
There is much capital money invested in analog signal generators within today's research and development laboratories and manufacturing floors. Yet the trend in wireless technology is to use complex digital modulation schemes that require sophisticated digitally modulated signals. Replacing all those analog generators can represent a staggering cost. However, the newly introduced model 2029 vector modulator has the ability to convert any of today's analog signal generators to a digital signal format at a fraction of the cost of a new digital generator. This saves major capital funds and permits fast conversion of manufacturing test lines from the older wireless formats to the latest 2G, 2.5G and 3G digital formats.
The new vector modulator operates from 800 MHz to 2.51 GHz, and when combined with any analog RF signal source, can output digital signals in wideband CDMA, cdma2000, IS-95, GSM, IS-136 and Enhanced Data Rate for GSM Evolution (EDGE) formats. The new modulator eliminates the need to purchase more expensive signal generators and allows a reduction in the dependence on expensive protocol and air interface specific radio test sets as the world migrates towards software radio.
As the name implies, the 2029 vector modulator provides a vector-modulated signal from an external local oscillator. Any analog signal generator can provide the LO signal. Suitable signal generators include the IFR 2023/5 family and the 2030 and 2040 series. Figure 1 shows the vector modulator's simplified architecture comprised of an arbitrary waveform generator (ARB), an IQ modulator and an RF output control system.
The ARB, shown in Figure 2 , operates with an interpolator. The interpolation schemes used ensure that the size of the source file of the ARB is kept to a minimum, while at the same time preserving the quality of the ARB output signal. Therefore, the ARB files require only four times oversampling compared to most other systems which rely on higher oversampling rates, requiring increased file sizes and switched banks of anti-alias filters. Figure 3 shows the ARB file sampling IS-95.
The 2029 ARB's memory is segmented into 15 equal blocks, so it can store 15 different waveforms (for example, 15 IS-95 waveforms, each with a duration of 80 ms or four frames). Because the waveforms can occupy more than one block, the ARB can also store a smaller number of larger waveforms, or even a much larger single one.
The large memory provides enormous test flexibility, thus eliminating down time to load test routines. The waveforms are not required to be from the same transmission standard. This capability allows handsets from two different manufacturing lines to be fed into a single test station. The 2029 can instantly switch back and forth between the two formats without waiting to download the new waveforms. In addition, the large internal memory eliminates the need to read operations traditionally stored in nonvolatile memory, hence, there is no significant lag time to change waveform files.
The vector modulator is supplied with an ARB packager and loader program that is part of the soft front panel. The packager configures the source files into a format that is readable by the ARB over the GPIB. This capability permits flexible file management, maximizes storage and enables fast changeover between files.
THE IQ MODULATOR AND RF OUTPUT SYSTEM
The IQ modulator, shown in Figure 4 , allows accurate simulation of a variety of digital communications signals. The modulator features excellent IQ accuracy with a wide bandwidth and good linearity, thus providing, along with the RF output system, an RF output that has low adjacent channel power (ACP) for testing high performance amplifiers for linearity and receivers for selectivity.
The IQ modulator includes an RF level control system to allow the output to be varied over a wide range, permitting measurements of both receiver sensitivity and overload. Good RF level accuracy in the output control system and attenuator minimizes uncertainty and maximizes repeatability. An RF combiner is available as an option so that other signals may be combined with the digitally modulated carrier (for example, to create an interfering signal), or for connecting measuring devices such as power meters or spectrum analyzers. The 2029 modulator has no user front panel interface and is designed for operation from the GPIB. An RS-232 port is used for firmware and software upgrades.
The 2029 vector modulator operates from 800 MHz to 2.51 GHz and requires an external signal source level of +7 dBm. The RF output level is 0 to 138 dBm for signals with a 14 dB crest factor. RF output resolution is 0.01 dB and the level accuracy is ±0.5 dB for CW signal levels greater than 110 dBm. The modulated RF level accuracy changes by < 0.15 dB, compared to a CW signal, when an IS-95 multichannel signal with a 14.5 dB crest is applied.
The unit's harmonics are < 30 dB and spurious are < 70 dB excluding the contribution from the external LO. The noise floor is typically 138 dBc/Hz when modulated with an ARB file at 0.2 RMS of full scale. The ACP is typically 70 dBc for an IS-95 pilot channel file, optimized for ACP, with a crest factor of 5.5 dB.
A soft front panel is supplied that requires Windows 95/98.NT and a GPIB card. The modulator contains a selection of waveforms for testing 2G, 2.5G and 3G radio systems. Other waveforms can be supplied as well.
The 2029 vector modulator can be used with any analog signal generator to test systems that conform to today's wireless communications formats, such as CDMA, cdma2000, wideband CDMA, EDGE, GSM, Bluetooth, DECT and IS-95. The unit was designed specifically to simplify the testing of cellular telephones in manufacturing. The included ARB provides an ideal solution for manufacturing tests that require good repeatability, high precision and inherent accuracy.
The compact 3.5" (2U) * 16.5" * 20.5" unit is ideal for retrofitting into existing test station racks, and the GPIB interface permits complete operation to be controlled from a central location. The 2029 vector modulator coupled with a suitable analog generator is capable of testing any digital wireless RF component or system. Applications include RF amplifiers and base station equipment. Additional information may be obtained from the company's Web site at www.ifrsys.com.
IFR Systems Inc.,
Wichita, KS (316) 522-4981 or (800) 835-2352.
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