Analog and Digital Signal Generators with High Spectral Purity

Analog and Digital Signal Generators with High Spectral Purity

Hewlett-Packard Co.
Santa Rosa, CA

Due to the increasingly stringent requirements for communications channels, today's signal generators must have extremely low phase noise to determine if a device, component or subsystem meets test specifications. A new series of spectrally pure signal generators has been developed to meet these tight demands that also provides flexible digital modulation capabilities for current and future communications requirements.

The HP ESG family of RF signal generators has been expanded with the addition of new high spectral purity models that include the HP ESG-AP series of four analog model RF generators and the HP ESG-DP series of four digital models. The new generators are designed specifically to provide a source of high spectral purity RF signals with built-in modulation functionality for both analog and digital applications that previously required external test equipment or multiple generators. The new instruments provide good phase noise performance for general-purpose bench-top testing of modern communications devices and equipment. Figure 1 shows a comparison of the typical single-sideband (SSB) phase noise performance of the new HP ESG-AP and -DP series vs. HP ESG-D and -A signal generators and other common signal generator types.

Good spectral purity is essential for achieving low phase noise. A spectrally clean signal when used as an LO for up- and downconversion prevents the masking of low level desired signals by phase noise. The high spectral purity of the HP ESG-AP and -DP generators also makes possible receiver performance testing that meets the phase noise and spurious content specifications of today's communications standards for both in-channel and out-of-channel receiver testing. In addition, radar testing traditionally requires a spectrally clean signal source. For example, airborne Doppler radar measures small shifts in frequency that return echoes undergo in order to determine the velocity of a target. Unfortunately, the return signal includes additional frequency-shifted echoes from the ground. This problem is significantly increased if the transmitter oscillator or receiver LO adds phase noise to the desired signal. The HP ESG-AP and -DP generators reduce the instrument-contributed phase noise on the received signal so that the real target signal is more readily distinguished.

Anaaog and Digital Modulation Capabilities

The HP ESG-A and -AP series generators provide comprehensive analog modulation capabilities that include AM, FM, and phase and pulse modulation. The units also have built-in sweep features and a versatile function generator.

The HP ESG-D and -DP series generators provide broadband I/Q modulation with all of the major communications formats, such as  wideband CDMA (W-CDMA), cdma2000, EDGE and GSM. Flexible digital modulation capabilities allow modification of existing standards or creation of new standards. In addition, the digital series generators include analog model features and many digital options, such as a dual arbitrary waveform generator for creating complex digitally modulated signals and a bit error rate (BER) analyzer for measuring sensitivity and selectivity. A real-time I/Q baseband generator permits mixing and matching of modulation types, data rates and filter types to create custom signals.

HP ESG Family Features

In addition to the choice of spectral purity performance levels and analog and digital vs. analog-only models, frequency coverage can be specified from 250 kHz to 1, 2, 3 or 4 GHz, as listed in Table 1 . The generator family also features an output power range of -136 to +17 dBm with superior output level accuracy of ±0.5 dB (> -127 dBm) below 2 GHz and ±0.9 dB above 2 GHz. The typical level accuracy is much better, as shown in Figure 2 . Even with modulation applied, the output level accuracy is better than ±1.2 dB, thus allowing the most sensitive receivers to be measured. In addition, the generators incorporate an electronic attenuator that easily handles continuous output power cycling with highly repeatable results to 4 GHz and offers outstanding reliability. An optional mechanical attenuator provides an increase in output power by as much as 6 dB for overcoming insertion losses from cabling and switch matrices or for components that require high drive levels. Figure 3 shows typical output power vs. frequency.

FM deviations up to 40 MHz at rates to 10 MHz permit a variety of measurements to be made. The stable DC FM precisely reproduces digital signaling for FM receivers with selective squelch control. Wideband phase modulation capabilities offer deviations up to 360 radians and 4 MHz rates for use in satellite communications applications.

Complex modulation formats such as binary phase-shift keying, quadrature phase-shift keying and 64 quadrature amplitude modulation required for testing RF digital communications systems may be accomplished by driving the I/Q inputs with analog signals. A built-in quadrature modulator processes the I/Q input signals to provide superior modulation accuracy and stability over a 30 MHz RF bandwidth (15 MHz baseband I/Q). In addition, internal I/Q modulator calibration routines generate precisely controlled and repeatable signals that minimize measurement errors; the calibration results remain valid for 30 days when the instrument is operated within ±5°C of the calibration temperature.

The generators also provide a coherent carrier output to simplify the detection of digital baseband signals. (The coherent carrier output provides frequency and phase information about the transmitted carrier.) Using this signal as a reference eliminates the need for complex demodulation techniques.

Built-in Functionality

The HP ESG generator family provides built-in multiple functionality that has traditionally required external test equipment or multiple generators. For example, an internal dual arbitrary waveform generator that combines flexible baseband generation and I/Q modulation in a single generator is available. This capability provides a multicarrier CDMA signal to stress active components with multiple carriers or allows the signals to perform intermodulation distortion measurements on amplifiers. An internal BER analyzer allows quick and easy sensitivity and selectivity measurements of communications subsystems and components. The BER analyzer also indicates pass or fail conditions for user-specified test limits. GSM loopback BER base station tests are made possible using an HP VSA series transmitter tester in conjunction with an HP ESG generator.

In general, the HP ESG-D and -DP's built-in flexible baseband generator provides complete control over a transmitted signal. The user can supply data, I/Q modulation waveforms or no data at all and generate digitally modulated signals, I and Q baseband outputs or data streams. This signal may then be used with an arbitrary digital transmitter at any of the major interfaces to test components to full receivers. A proprietary application-specific IC performs symbol building and finite-duration impulse-response filtering, thus giving the user complete flexibility in modulation generation. One-button generation of IS-95 Walsh-coded signals with up to 256 configurable channels for each carrier and up to 12 carriers is available, thus providing a complete CDMA personality. In addition, the instrument can keep pace with the emerging 3G wireless standards by providing W-CDMA, cdma2000 and EDGE personalities.

Finally, the generator is lightweight and portable and may be mounted in a five-inch-high rack space. An expandable, modular architecture makes the instrument a versatile tool for future growth. As digital communications advance, the programmable hardware of the HP ESG-D and -DP models will allow complete modification of the existing standards to create new formats. Updated firmware from the HP ESG Web site will provide support for future measurement personalities and performance options. For more information, consult the company's Web site at www.hp.com/go/esg.

Hewlett-Packard Co.,
Santa Rosa, CA
(800) 452-4844.

Circle No. 301