Instruments are always expected to keep pace with—and preferably be one step ahead of—newly implemented technology. This is especially true today for RF/microwave signal sources, which must generate the variety of rather unconventional signals for evolving wireless communications systems.
“In recent years the demand for wireless products providing a diversity of capabilities has exploded,” said Chris Rix at Marconi. “Wireless technology is being applied to applications entailing remote utility readings, telephone service for entire neighborhoods, paging, cellular, PCS and satellite global positioning systems.”
Newly efficient digital modulation schemes are being used to achieve high data transmission rates, optimization of the RF spectrum and low power consumption for mobile applications. Since the purposes served by these new wireless communications systems—as well as the regulations they are subject to—differ, so do the signal formats and modulation schemes.
Required Communication-Specific Test Signals
Most RF/microwave signal generators used for communication test applications include AM, FM and f M capabilities. A combination of these facilities may be used to generate a modulated carrier output which conveys digitally encoded information in accordance with a variety of application-specific standards. Alternatively, an I/Q modulator circuit may be used alone or in combination with the other modulation facilities to generate digitally encoded RF/microwave signals.1
The nature of the baseband signal applied to the modulator depends on the particular communications signal to be emulated. It may be generated internally or supplied from an external source.
The Hewlett-Packard HP ESG-D series of digital signal generators, for instance, includes conventional modulation capabilities plus an I/Q modulator. “An RF engineer may connect his own baseband generator to the HP ESG-D broadband analog I/Q inputs to generate complex modulation formats, such as 64 QAM or QPSK,” said Helen Wright, RF sources product manager at Hewlett-Packard.
Alternatively, optional built-in personalities matching specific common digital communications standards may be used. “These internal facilities generate the p /4 DQPSK and GMSK signals necessary to meet the test requirements of NADC PDC, PHS and GSM receivers,” Ms. Wright continued. “The signal generator sets up the correct modulation, bit rate, filter and number of bits per symbol that are defined in these standards.”
Similarly, the Rohde & Schwarz SME line of signal generators provides more than 25 different standard signals using its internal digital coder. Data lists defining sequences of up to 8 MB can be written, internally stored and applied or provided by the user through an external input. A new software package, the SME-Kz, provides a Windows interface. This makes it easy to select a standard, choose a time slot, and insert a variety of standard or user-defined data sequences.
The Marconi 2050 Series of signal generators also includes the digital modulation facilities needed to generate signals required for testing components and subsystems used in digital wireless systems. “The 2050 series supports modulation formats such as p /4 DQPSK (Root Nyquist or Nyquist), 16 or 256QAM, GMSK, GFSK, and 4FSK,” said Mr. Rix. “Communication systems using these formats include APCO, NADC, GSM, PDC and Tetra.”
Testing of CDMA-based cellular and PCS systems and subsystems places special demands on signal-generation capabilities. “For instance, simple single-channel signals are not adequate to fully test linear amplifiers used in CDMA systems,” said Steve Stanton, product marketing manager at Tektronix. “The IS-98 standard requires that test signals include pilot, sync, paging and six traffic signals on separate Walsh codes.”
The baseband signal containing all this data can be generated by dedicated internal circuitry or appropriately programmed arbitrary waveform generators. However, when combining multiple traffic channels, such as sync and paging data streams, very large crest factors can result. The modulator and RF/microwave output stage of the signal source must handle these without discernible distortion.
Wireless Communications Test-Application Examples
Common characteristics of the signal sources used for wireless communications testing include inherently pure signals, multimodulation facilities and internal or external programming capabilities. Here are some typical examples of how sources possessing these features are used to perform specific types of tests.
Multistandard System and Subsystem Tests
: The Rohde & Schwarz SMHU-58 signal generator contains an I/Q modulator. When used in conjunction with an arbitrary waveform generator and IQSIM-K software, it provides a variety of pure and impaired signals.
“The IQSIM-K is used to generate commercial communications digital modulation standard signals with user-selectable data rates, baseband filtering and custom digital data generation,” said Mr. Stanton. “IQSIM-K also enables signal impairment simulation, including AM and f M distortions, interfering carriers, VCO phase noise, I/Q offset, stationary multipath signals, feedthrough and I/Q gain imbalance.
Amplifier Tests
: Since traditional multitone testing is inadequate for assessing the performance of linear amplifiers used in spread-spectrum communications systems such as CDMA-based PCS, a variety of alternate test techniques has been devised. NoiseCom, for instance, developed the Multi-Channel CDMA Simulator, where filtered white Gaussian noise is used to simulate the real CDMA signal.
Spectrian, an RF amplifier design and manufacturing company, combines a Marconi 2024 Signal Generator with a Spectrian-designed CDMA power mask generation system, filters and a noise source to test linear power amplifiers. These amplifiers must provide particularly low levels of interference and minimal intermodulation distortion. “The 2024 was chosen primarily for its low noise characteristics and its capability to provide a high output power of +25 dBm,” explained Mr. Rix.
“Spectrian uses the Marconi 2051 digital modulation signal source for testing amplifiers destined for NADC applications,” Mr. Rix added. “The 2051 provides the required modulation format and accuracy and has a large stand-off performance margin for amplifier measurements, such as adjacent channel power (ACP), harmonics and detection levels. The phase-noise specification allows at least a 10-dB margin for testing ACP performance.”
RF Pager Tests
: The variety of VXIbus-based RF/microwave sources and arbitrary waveform generators available, as well as their programmability, makes them well suited for many wireless communications test applications. A typical VXI-based test setup configured for assessing the sensitivity of RF pager receivers consists of a Racal 3271 RF/microwave signal generator, a 3151 waveform generator and a 6065P multichannel serial interface housed in a 1269 nine-slot VXIbus mainframe.
“The 3151 waveform generator is used in a sequenced mode to generate coded information for the pager, typically POCSAG, GSC or FLEX protocols at a 600 to 1,200 baud rate,” explained Charles Greenberg, product marketing engineer at Racal Instruments. “The output of the 3151 is fed into the 2-level FSK (or 4-level FSK for FLEX applications) input of the 3271 signal generator. The 3271 is set to the pager frequency (typically 280 MHz or 930 MHz) and transmits the FSK modulated carrier frequency to the pager. The pager itself is set to the print mode, so that it forwards all received pages to its output serial port. This port is then connected to the 6065P which can simultaneously receive readback data from up to eight pagers.”
Application-Oriented Sources
When signal sources for wireless equipment tests are considered, the focus ordinarily falls on signal simulation. However, RF/microwave signal sources are used for additional communications test-related purposes. Some of these, as well as the required signal source characteristics, were enumerated by Ray Beers, product marketing manager at Anritsu Wiltron:
Local Oscillator (L.O.) Applications: Many tests can be performed more efficiently at RF than at microwave frequencies. Consequently, the first precept of microwaves is to down-convert to lower frequencies with the aid of an L.O. Required characteristics of a L.O. source are good spectral purity (to preserve the purity of the signal being down-converted) and high output power (to adequately drive mixers).
Network Analysis: Components, cables and antennas and frequency converters are tested with network analyzers which require sources. A source for network analysis must provide fast sweep speeds and broad frequency coverage.
Signal Simulation: Receiver testing is best performed with the same signals the receiver will see in the real world. The signal source, therefore, must be able to create those signals. A source for signal simulation must provide comprehensive modulation and the capability to produce accurately controllable power levels.
“The one-size-fits-all approach is simply not cost-effective for all these varied needs,” said Mr. Beers. “As a result, we have chosen to produce ‘families’ of products to meet specific requirements. “For example, the recently introduced 69000A, aimed at L.O. duty, features high output power and outstanding phase noise performance. The 69100A features fast analog sweep and is aimed at network analysis applications. The 69200A is configurable to provide the needed new complex modulation capabilities and is aimed at signal simulation,” Mr. Beers concluded.
Programmed Test Sources (PTS) addresses the issue of targeted performance by providing a series of CW sources covering limited frequency bands in addition to their wideband products. “For example, to test equipment in the 800 to 960 MHz band, the PTS W800 Frequency Synthesizer offers frequency coverage targeted specifically for this band, without the additional costs inherent in wideband CW sources,” said Michael Lohrer, vice president of operations at PTS. “A vector modulator option accepts I/Q waveforms for up-conversion to wireless frequencies where the signal can act as a stimulus for demodulator, component or subsystem tests.”
Conclusion
All signal sources used for wireless communications tests are, to some extent, already application specific, not only by being targeted at a particular industry segment, but also in the frequency range and modulation capabilities provided. Most RF/microwave sources are available in a range of identical models except for frequency limits, because instrument costs escalate rapidly for the higher frequencies.
As to modulation capabilities, an I/Q modulator is essential for generating CDMA signals. This does not necessarily target the generator solely toward CDMA applications since an I/Q modulator can generate any type of modulated signal, provided that suitable baseband signals are applied.
Generating baseband signals containing the proper structure and protocol to emulate any digitally modulated real-world signal may not be a trivial task. Of course, this is not a problem when buying a signal source that has built-in personalities to generate signals matching the communications systems you plan to test. Otherwise, they may be generated with an arbitrary waveform generator which has been programmed via dedicated or general-purpose software.
In either case, the ease with which the signal source can be used and the range of applications it is intended for should be prime considerations in the signal-source selection process. “Ease of use and versatility are the key factors in maximizing cost-effectiveness and minimizing the cost of owning a signal generator,” said Ms. Wright. “Ease of use cuts training time and versatility ensures that the investment you make today will meet your needs tomorrow.”
Reference
1. Jacob, G., “Why Special Signals Are Needed for Digital Communications Testing,” EE-Evaluation Engineering, November 1995, pp. 62-66.
Note: The article can be accessed on EE’s Test Site at www.nelsonpub.com/ee/. Select EE Archives and use key word search.
Signal Sources
2.4-GHz Signal Generator Meets
Field, Bench, System Needs
The PSG2400A RF Signal Generator is portable, operating from external DC or an optional rechargeable battery pack. The frequency range extends from 100 kHz to 2.4 GHz and the output level is -143 dBm to +16 dBm. Simultaneous AM, FM and f M with AC/DC wideband and high-rate capabilities are featured. Pulse modulation is optional. Two internal modulation sources generate signals of 0.1 Hz to 500 kHz. Sixteen frequencies and durations may be programmed. Supported modulation systems include those used by CTCSS1 DTMF and SELCALL systems. $9,950. Wayne Kerr, (617) 938-8390.
Microwave Synthesizers Feature
Ultra-Low SSB Phase Noise
Consisting of 60 models, the 69A family of Microwave Synthesizers covers frequency ranges up to 65 GHz. The 69X97A, for example, provides 10-MHz to 65-GHz frequency coverage from a single coaxial output. The family is comprised of four subfamilies: 69000A Synthesized CW Generators, 69100A Synthesized Sweep Generators, 69200A Synthesized Signal Generators which offer internal AM, FM, f M, and pulse modulation, and 69300A Synthesized Sweep/Signal Generators which combine these features. Anritsu Wiltron, (408) 778-2000.
Stimulus Instruments Target
Emerging Communications Bands
The SMT06 and SME06 Signal Generators cover a frequency range from 5 kHz to 6 GHz. The SMT06, a general-purpose instrument, features AM, broadband FM and f M plus options that include pulse modulation, a function generator and a low-frequency generator. The SME06 supports standard analog modulation schemes and produces a variety of digital modulation signals. It supports 25 of today’s digital modulation standards including GSM, DCS 1800, PCS 1900, DECT, PDC and FLEXTM. SME06: $33,450; DM Coder option: $1,960; SMT06: $22,450. Tektronix, (800) 426-2200 (press 3, code 512).
RF Digital Signal Generators
Offer Flexibility, Performance
The HP ESG family of digital and analog RF signal generators provides frequency and level control, versatile digital-modulation capabilities and a choice of frequency coverage from 250 kHz to 1, 2, 3 or 4 GHz. It offers a cardcage architecture, flash-ROM firmware, 100 internal storage registers and multiple data generation options. Broadband analog I/Q capabilities enable generation of complex modulation formats. The digital signal generators of the series include four built-in communications standards and optional personalities for more digital communications standards. $7,000 to $19,000. Hewlett-Packard, (800) 452-4844, ext. 2032.
Multiple Source Generator
Offers User-Defined Tracking
The 2026 Multi-Source Generator features two RF signal generators in one enclosure, with a third source as an option. Each source is a fully functional generator covering 10 kHz to 2.4 GHz with AM, FM, f M and pulse modulation capabilities. The 2026 is suited for applications requiring two or three combined sources for testing components and receiver assemblies. Application-specific operating modes, such as amplifier two-tone/three-tone intermodulation and receiver intermodulation/selectivity tests, make the instrument easy to use. Under $20,000. Marconi Instruments, (817) 224-9200.
C-Size Signal Generator
Covers 10 kHz to 2.4 GHz
The C-size, dual-slot 3271 is a VXI plug-and-play-compliant full-function RF signal generator with frequency coverage from 10 kHz to 2.4 GHz and 1-Hz resolution. It provides RF output power from -137 dBm to +25 dBm with a resolution of 0.1 dBm. The 3271 has <4.5-Hz residual FM and typical sideband noise of -126 dBc/Hz. Features include an internal modulation source, a built-in attenuator, a sweep mode and VXIplug-and-play drivers. Frequency, amplitude, phase, pulse and 2- and 4-level FSK modulation are supported. From $9,750. Racal Instruments, (800) RACAL-ATE.
RF/Microwave Synthesizers Test
Cellular and WLAN Applications
The Series 1055A and 1056A Narrowband VXIbus Synthesizers are designed for cellular and WLAN applications. They operate at frequencies from 820 MHz to 960 MHz and 2.4 GHz to 2.5 GHz, respectively. Typical phase noise for a 900-MHz unit with 10-kHz resolution is -115 dBc at 1 kHz and -120 dBc at 100 kHz from the carrier. Output power is +12 dBm. Spurious levels are less than -80 dBc. From $9,000. Communications Techniques, (201) 884-2580.
VXIbus Microwave Synthesizer
Covers 10 MHz to 20 GHz
The Series 50000A VXIbus Microwave Synthesizers generate and modulate leveled RF output signals from 10 MHz to 20 GHz in a two-slot VXIbus module in conjunction with the one-slot Model 52000A Synthesizer Control Module. The 52000A offers the digital programming and analog time-base signals required to operate up to eight synthesizer modules. AM, FM and f M can be applied individually, alternately or simultaneously. The 50000A controls the power level from -100 to +10 dBm with 0.1-dB resolution. Starts at $20,750. Giga-tronics, (800) 726-4442.
Synthesizer Supports
Wireless Communications
The PTS W800 is a frequency synthesizer for wireless communications testing in the 800-MHz to 960-MHz band. It features a frequency switching rate of 5 m s to 20 m s, a noise floor of -135 dBc/Hz, 0.1-Hz resolution, and -65-dBc spurious outputs. The synthesizer is available with front-panel control or remote-control capability. $4,800: remote control); $5,100: front-panel manual control. Programmed Test Sources, (508) 486-3008.
PCS CDMA Amplifier Test Set
Provides White Gaussian Noise
The CDMA-1900 Series Test Sets simulate CDMA signals needed for PCS amplifier linearity evaluations. The instrument generates clean, band-limited white Gaussian noise with a sharp spectrum roll-off, achieving intermodulation distortion of
-60 dBc at 750 kHz from center frequency. Up to four channels are available, each with an output power level selectable from 0 to -70 dBm in 0.1-dB increments. Frequency is tunable in 100 kHz steps. NoiseCom, (201) 261-8797.
Sidebar
Glossary of Terms
APCO Association of Public Safety Communications Officers
CDMA Code Division Multiple Access
DCS Digital Cellular System
DECT Digital European Cordless Telecommunications
FLEX Flexible High-Speed Paging System
4FSK Four Levels of Frequency Shift Keying
GFSK Gaussian Filtered FSK
GMSK Gaussian Minimum Shift Keying
GSM Global System for Mobile Communication (ETSI, Europe)
I/Q In-phase/quadrature-phase
NADC North American Digital Cellular
PCS Personal Communications Services (U.S.)
PDC Personal Digital Cellular (Japan)
PHS Personal Handyphone System (Japan, formerly PHP)
p /4DQPSK Differential Quadrature Phase-Shift Keying with a p /4 radian phase shift between successive symbols
POCSAG A communications protocol used between paging towers and mobile pagers
QAM Quadrature Amplitude Modulation
QPSK Quaternary Phase Shift Keying
TDMA Time Division Multiple Access
TETRA Trans European Trunked Radio System
Copyright 1997 Nelson Publishing Inc.
February 1997
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