Signal Sources Meet Evolving Communications Test Needs

Gone are the days when an instrument was purchased just to meet the needs of a particular project. Multifunctionality–to serve many needs, and upgradability–to avoid early obsolescence, are the key requirements today. The reasons: cost savings achievable by amortizing test equipment over several projects, and the increased pace of technological changes, especially in the communications industry.

The demand for new communications system implementations and corresponding standards is fueled by the need to supply ever-greater data transfer capacity, more secure communications and additional services, noted Renee Cherolis, Market Development Engineer at Hewlett-Packard. Since this is an evolving process, signal sources required to test these new equipments and systems must be readily upgradable to remain useful.

Many recently introduced signal sources have been designed to satisfy a wide range of data-format generation and modulation requirements, and to be easily upgradable to accommodate new standards. Digital implementations of these functions, mandated by the data-oriented nature of most new communications systems, innately lend themselves to such flexibility.

Since digital circuitry also achieves cost and space savings, it is now used for as many signal source functions as possible; for example, user interface, calibration and, within limits, basic signal generation. For the higher frequency ranges, we must still rely on analog techniques, although complementary digital-analog implementations are gaining ground.

New Sources, New Features

Extensive internal modulation features have been introduced in the newest microwave synthesizers from Anritsu Wiltron, reported Donn Mulder, Director of Marketing at Anritsu Wiltron. “The internal modulation generators derive their output signals via digital techniques. Samples in a waveform memory are clocked out through a DAC and filtered to provide a variety of waveforms: pulse trains, sinusoids, triangles, ramps and random noise,” he said.

“Independent channels are provided so that AM, FM and pulse- modulation parameters can be set individually and operate simultaneously on the microwave carrier signal. This is useful for simulating real-life signals to test receivers,” Mr. Mulden continued.

“For example, a microwave signal could be frequency-modulated to simulate communications traffic and then amplitude-modulated to simulate atmospheric fading effects. This real-life signal can be fed to a receiver to determine its capability for receiving and demodulating this signal,” he said.

Digital techniques, primarily Direct Digital Synthesis (DDS), are also used in the SME Series signal sources from Rohde & Schwarz. “DDS provides high vector accuracy at a much lower cost than available with earlier analog implementations upgraded with digital options,” said Gerhard Sonnde, Marketing Manager at Rohde & Schwarz.

“For instance, DDS is one of the factors that enables the SME to cover 27 different digital communications standards, including the new Cellular Digital Packet Data System standard for radio models of portable computers; APCO 25, a new speech and data service; and FLEX, the new Motorola paging standard,” said Mr. Sonnde. “The SME also is upgradable to nearly any upcoming modulation standard.

“The generator also has the capability to generate a bit-synchronous internal power-ramping signal over an 80-dB dynamic range. This provides an important test feature for all communications networks using the Time Division Multiple Access principle. With internal data source, internal power ramping and extensive modulation capabilities, there is no need for more external instruments to generate the required test signals,” Mr. Sonnde concluded.

“Hewlett-Packard’s newest additions to the economy line of RF signal generators, the HP 8648A, B and C, offer performance, reliability and ease of use,” said Ms. Cherolis. Designed to increase productivity in manual and semiautomated test environments, they are suited for receiver, RF component and general-purpose testing.

Other companies are adding features to existing products or increasing the operating range. Hameg, for instance, recently extended the HM8133 RF Synthesizer’s high-frequency range for FM modulation to 990 MHz to satisfy new demands of the communications (cellular/pager) industry, reported Fred Katz, Chief Engineer at Hameg.

How Digital Should We Get?

Digital readouts are common and appreciated by most users. Menu-based selection is hailed by some and merely tolerated by others, but at least one analog-feel main control knob is a must-have requirement of almost every user.

“A large, easy-to-adjust knob and the capability to designate the particular parameter to be adjusted by that knob with a digital display readout is an ideal combination,” said Mr. Katz. “Add to that the capability to slide the decimal range of the adjustment so that the fine/coarse sensitivity of the knob can be varied. An optional numeric keypad control for the entry of precision values and remote data bus controllability are also desirable.”

While there are differing opinions on the optimum analog/ digital trade-offs for user interfaces, the flexibility and multifunctionality provided by digital implementations is universally appreciated.

“The availability of fast, reasonably priced 12-bit DACs make it possible to develop multipurpose generator equivalents to a wide range of analog signal generators,” said Jim Wookey, Product Marketing Manager at Wavetek Instruments. “Also, firmware, providing for a convenient user interface, now exists to give the generator the look and feel of a sweep, pulse, function, trigger, arbitrary, noise or modulation generator.”

While most users value the multifunctionality provided, others are hesitant to buy multifunction generators because they don’t want to pay for capabilities they don’t need. “But in fact, most digitally based multifunction generators have a fixed hardware platform,” remarked Mr. Wookey.

“The multifunction capabilities reside in the firmware. So although the purchaser may contribute a small amount to the firmware development cost of those capabilities he doesn’t need, the cost of manufacturing a digitally based multifunction generator is the same, whether all of the capabilities are used or not,” he said.

Regarding the frequency range achievable with pure digital implementations, there are certainly limits. “Currently, digital technology only provides high-quality RF signal generation over a very limited bandwidth,” said Michael Lohrer, Vice President of Operations at Programmed Test Sources. “Manufacturers will continue to rely on traditional analog technology (direct or indirect synthesis) to provide RF output signals of 100 MHz or higher.”

“The main synthesizer can only be implemented digitally in some instruments, since DDS covers a limited frequency range,” concurred Mr. Sonnde. “There is still a need to up-convert the signal to cover the most frequently used wireless communications bands. Of course, frequency and phase modulation capabilities can be provided conveniently by the DDS.”

As for microwave sources, it is unlikely that digitally based techniques will replace the most often used primary frequency-generating mechanism, the YIG-tuned oscillator. “To realize microwave frequencies using digital techniques, a DAC would have to be developed to convert samples at twice the rate of the highest frequency component desired,” said Mr. Mulder.

“This means a DAC would have to run at 20 GS/s to generate a 10-GHz analog frequency. DAC technology presently is limited to 1 GS/s for 12-bit dynamic range,” he said.

Analog architectures are still often more than adequate for conventional applications. “Where flexible performance is not an issue, analog sources may be more cost-effective,” said Ms. Cherolis. “When only minimal modifications are required to meet a new application need, it may be less expensive and more efficient in terms of time-to-market to leverage an existing analog platform.”

Trends

More RF and microwave signal generators will be designed to meet the needs of the communications industry. Digital modulation capabilities are becoming mandatory, noted Mr. Sonnde. Digital technology will also be used to compensate for the shortcomings of analog components and circuits for which there are presently no economical or technical digital substitutes.

“For example, nonlinearity in the tuning response of a YIG-tuned oscillator can be corrected for by a software look-up table that has correction factors stored in it,” said Mr. Mulder. “The same is true for the frequency response of a step attenuator, which can have an associated correction table in software to compensate for the frequency response. This allows accuracy in frequency and power to be delivered by at least an order of magnitude better compared to relying on the analog performance alone.”

Signal source manufacturers all aim to offer price-competitive products that have the needed sets of features and performance optimized for specific applications. As a result, most sources have feature and performance upgrade capabilities to meet application requirements as they emerge. This offers the user the best economy over the total time of ownership.

SIGNAL SOURCES PRODUCTS

Synthesizer Features

FM Modulation to 990 GHz

The HM8133 Programmable RF Synthesizer provides continuous frequency coverage from 1 Hz to 1 GHz with an accuracy of +3 x 10-7. Frequency resolution of 0.1 Hz allows increment settings in narrowband systems. Internal (400 Hz and 1 kHz) and external (10 Hz to 20 kHz) AM and FM modulation are available. FM deviation may be varied up to +100 kHz. Response time is 10 ms for frequency and amplitude changes. The HM8133 is menu-driven and features digital parameter readouts. IEEE 488/RS-232 interfaces are optional. $3,920. Hameg Instruments, (800) 247-1241.

Narrowband Microwave Source

Available in VXI Configuration

The Model 3261 is a low-noise synthesizer covering frequencies from 1 GHz to 18 GHz in bands. It is a message-based VXI-module, packaged in a C-size, single-slot enclosure. Typical phase noise for a 2-GHz model is -115 dBc at 1 kHz offset. Output power is +15 dBm and spurious levels are less than -85 dBc. Standard tuning range is 15% of center frequency. Step size can be programmed in increments as low as 50 kHz. Amplitude modulation and a digital step attenuator are optional. From $9,000. Racal Instruments Inc., (800) RAC-DANA.

Synthesized Function Generator

Features Ultra-Low Distortion

The DS360 uses direct digital synthesis to provide waveforms from 1 mHz to 200 kHz with total harmonic distortion less than -100 dB (<20 kHz). Functions include sine, triangle, two-tone (such as SMTPE, DIM and CCIF) white noise, pink noise and band-limited white noise. Balanced output ranges from 25 uVrms to 28 Vrms, while unbalanced output ranges from 12.5 uVrms to 14 Vrms. Features include linear and logarithmic frequency sweeps, 1-mHz frequency resolution, 25-ppm frequency stability, and GPIB and RS-232 interfaces. $2,395. Stanford Research Systems, (408) 744-9040.

Programmable Synthesizer

Features Two Channels

The PTS D620 broadband dual-channel frequency synthesizer contains two independent, individually programmable sources, each providing coverage from 1 MHz to 620 MHz. The unit offers fast frequency switching (20 us maximum), low phase noise (-110 dBc/Hz at 1-kHz offset) and low spurious output (-65 dBc, 1 to 310 MHz; -60 dBc, 310 to 620 MHz). Two independent RF channels are contained in the space previously required for a single channel, realizing a space and cost saving. $12,725 including an OCXO frequency standard. Programmed Test Sources, Inc., (508) 486-3008.

Source Provides Functionality

Of Seven Instruments

The Model 395 Universal Waveform Generator provides the functionality of seven types of signal generators, including synthesized pulse, function, noise, sweep and AM waveforms. Standard waveforms include sine waves to 40 MHz and square waves to 50 MHz. User-defined waveforms can be generated at 1-mHz to 100-MHz rates with 12 bits of vertical resolution. At clock rates of 50 MHz and below, the Model 395 employs DDS. Seven sweep modes are provided. $3,495; $495 for optional combined GPIB/DSO waveform transfer. Wavetek Corp., (619) 279-2200.

Sweep/Function Generator

Includes Built-In Counter

The dual-output Model 3040 Universal Sweep/Function Generator includes a 6-digit, 30-MHz frequency counter. The main generator features an output frequency adjustable from 0.1 Hz to 13 MHz. The second generator has a frequency range of 0.01 Hz to 10 kHz and may be used independently or as an internal AM/FM modulation source. When used to sweep the frequency of the main generator, a 1,000:1 linear range is provided. Sine, square, triangle, ramp and pulse waveforms are generated. $1,195. B+K Precision, Division of Maxtec International Corp., (312) 889-1448.

Two-Channel Function Generator

Has Phase Shift Capability

The FG100 Series Function Generators consist of a single-channel Model FG110 and a two-channel Model FG120, both featuring a frequency range from 1 uHz to 2 MHz and a 20-Vp-p maximum output. They employ DDS and provide sine, square, triangle, ramp and pulse waveforms with a frequency accuracy of 20 ppm (1 ppm optional) and stability of 10 ppm (0.5 ppm optional). Resolution is 1 uHz or 10 digits; and harmonic distortion is <0.3%, <0.05% optional. Phase differences are settable in 0.01o increments. From $3,595. Yokogawa Corp. of America, (800) 258-2552.

Generators Handle

Complex Receiver Testing

The 68200B and 68300B Synthesized Signal Generators are used for receiver testing, network analysis and ATE applications. The 68200B delivers CW and internally or externally driven AM, FM and pulse-modulated signals for complex receiver testing applications. Output frequencies may be digitally or manually swept. The 68300B adds analog sweep capability for high-speed network analysis. Both synthesizers feature 1-kHz frequency resolution, low SSB phase noise, a wide dynamic range and fast switching times. From $28,000. Anritsu Wiltron, (408) 778-2000.

Generator Series Provides

Signals to 3.2 GHz

The HP 8648A (1 GHz), HP 8648B (2 GHz) and HP 8648C (3.2 GHz) Signal Generators provide preloaded measurement sequences and one-button testing. Frequency coverage starts at 100 kHz. Residual FM specifications for hum and noise measurements are <7 Hz to 1,000 MHz, <14 Hz to 2,000 MHz, and <28 Hz to 3,200 MHz. Level accuracy is +1.0 dB, even at -127 dBm output at 2.5 GHz. A mouse-like keypad facilitates remote control. High-power options are available for HP 8648B and 8648C. 8648B–$7,665, 8648C–$9,165. Hewlett-Packard Co., (800) 452-4844.

Generator Delivers

Pulses with 15-ps Fall Time

The Model 4015C Pulse Generator produces pulses of -9 V amplitude into 50 with a fall time of 15 ps. The pulse is generated in the associated Model 4015RPH Pulse Head, which is attached to the main unit via a coax cable. This eliminates any extraneous rise-time slowing effects which could be caused by any other interconnecting cables. A -3 V, 22-ps wide impulse can also be generated by the 4015C if an optional Model 5208 Impulse Forming Network is attached to the pulse head output. $7,900 in U.S. Picosecond Pulse Labs, Inc., (303) 443-1249.

Synthesized Generator Provides

Many Operating Choices

The Model 3222 Synthesized Signal Generator covers 100 kHz to 2.7 GHz, and offers 10-Hz resolution to 1.35 GHz and 20-Hz resolution to 2.7 GHz. Frequency accuracy is better than 0.05 ppm; a 0.002-ppm time base is optional. The output attenuator range extends from +13 to -133 dBm, with 0.1-dBm settability. Seven modulation modes are provided (14 simultaneous combinations) including pulse (GaAs FET), logic DC FM, and internal or external FM. Up to 100 programmed setups can be stored. GPIB is standard. $13,300. Leader Instruments Corp., (800) 645-5104.

Function Generator

Includes Counter

The Model B-803 0.02-Hz to 2-MHz Function Generator features a 4-digit LED readout for continuous display of generator output, and produces sine, square and triangular waves. It provides a DC offset, 20-dB attenuation and a continuously variable sweep output. Frequency is controlled via seven pushbutton switches and a tunable dial. The unit can also be used as a frequency counter to measure external signals. $320. HC Protek, Inc., (201) 767-7242.

Signal Generators Suited

For Receiver Testing

The SMY01 and SMY02 Signal Generators cover 9 kHz to 1,040 MHz and 9 kHz to 2,080 MHz, respectively, with 1-Hz resolution. Output extends from -140 to +13 dBm with a level accuracy of 1 dB. SSB phase noise is <-114 dBc at 20 kHz from the carrier. Spurious rejection is at least 70 dB. Residual FM is <2 Hz at 100 MHz (<10 Hz at 1 GHz), and RF leakage is <0.1 uV. A built-in 1-Hz to 500-kHz AF synthesizer with 0.1-Hz resolution can be used as a low-distortion modulation source. Fifty front-panel setups can be stored. $6,150--SMY01; and $7,400--SMY02. Rohde & Schwarz, distributed by Tektronix, Inc., (800) 426-2200.

Pulse Generator Plug-Ins

Serve New Applications

The new 9213 and 9214 plug-in modules extend the application range of the Model 9200 Pulse Generator series. The 9213 generates voltage swings from 0 to +15 V, or 0 to -15 V at rates up to 50 MHz. The 9214 generates pulses with up to 300-MHz repetition rate with 300-ps edges and is intended for use by engineers working on fast logic circuits. It complements the 9211 which generates pulses with repetition rates of up to 250 MHz with variable edge times from 1 ns to 95 ms. $1,575–9213; $3,045–9214. LeCroy Corp., (914) 578-6020.

Synthesizer, Sweeper

Target Performance/Cost

A 2-GHz to 20-GHz model has been added to the GT 9000 Microwave Synthesizer line. Phase noise at 2 GHz is -95 dBc/Hz at 10-kHz offset, and harmonics are <-65 dBc at +6-dBm output power. Internal pulse modulation is standard; built-in amplitude, frequency and scan modulation are optional. The GT 9000S Synthesized Microwave Sweeper has the same specifications and modulation capabilities as the GT 9000, plus analog and digital sweep of frequency and power. GT 9000--$22,950, GT9000S--$24,950. Giga-tronics Inc., (800) 726-GIGA.

Copyright 1995 Nelson Publishing Inc.

February 1995

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