Electronic Design

Signal Generators Step Up And Deliver Come Test Time

With the variety of instruments available today, designers can put their projects through the paces with just about any kind of imaginable signal.

All electronic circuits and equipment receive input signals and process them into new and different output signals. When you’re designing and testing circuits and equipment, where do you get those input signals? You could build your own signal source for a specific application, but that isn’t necessary.

That’s because there’s a signal generator available for any type of signal, no matter what type of equipment is being designed or under test. The signal generator is like the scope, multimeter, and power supplies on your bench. With analog and digital models alike, it’s an essential instrument that saves time and ensures that your product works properly (Fig. 1).

FUNCTION GENERATORS
A basic function generator produces sine, square, and triangular waves from about 0.2 Hz up to 20 MHz or so. Some units offer linear ramps and positive and negative pulses. They’re used for basic audio, ultrasonic frequencies, and low RF testing. Pulse outputs are TTL/CMOS levels, while linear outputs are variable up to about ±20 V p-p.

Low-cost generators are implemented with analog circuits that feature continuously variable frequency and output voltage. While some low-cost analog function generators are still available, most modern function generators today use digital signalgeneration methods and frequencysynthesis techniques.

In fact, a majority of engineers prefer one of the digital models. These are more commonly known as arbitrary function generators (AFGs) or arbitrary waveform generators (AWGs), both generically referred to as ARBs (Fig. 2).

The AFG, the simpler of the two, is set up to produce only the most commonly used signals, such as sine, triangle, sawtooth, or square waves. Meanwhile, the AWG can be set up to produce virtually any type of signal. Most AFGs employ direct digital synthesis (DDS) along with a waveform storage memory containing standard waveforms and DAC output (see “DDS Basics” at www.electronicdesign.com, Drill Deeper 19147).

The output signal is stored in a RAM or ROM as a sequence of binary samples of the desired waveform. This data is output to the digital-to-analog converter (DAC) that generates a stepped approximation of the desired output signal. Some AFGs can produce sine and other waves up to 300 MHz.

An AFG has all of its standard waveforms, which are selected via the front-panel control, pre-stored in the memory. Standard waveforms are also available with an AWG, but users can enter any desired waveform into RAM. External software is used to create the binary file that defines the desired waveform.

A frequency synthesizer provides an incremental address to the RAM, which delivers the waveform samples to the DAC. Also, an analog low-pass filter eliminates residual digital artifacts. An output level control sets the desired amplitude.

Some function generators can also supply basic modulation. These include amplitude modulation (AM), amplitude shift keying (ASK), on-off keying (OOK), frequency modulation (FM), frequency shift keying (FSK), phase modulation (PM), phase-shift keying (PSK), and some digital modulation types.

Examples include Tektronix’s AWG 5000, which uses a standard fractional-N phase-locked-loop (PLL) synthesizer (Fig. 3). It also features two channels of output that can be single-ended or differential. With a DAC sampling rate to 1.2 Gsamples/s, it can produce output waveforms with a maximum frequency of 600 MHz. Because of its high frequency capability, it can be used for RF testing in some applications.

Its key specification is dynamic range, which is determined by the 14-bit DAC resolution. Maximum waveform storage capability is 32 Msamples. The two outputs are set up so that the I and Q signals are available simultaneously for digital modulation tests.

The AWG 5000 brings flexibility due to its wide frequency range and waveform programmability. It can perform virtually any form of digital modulation, and it’s well-suited for testing DACs and analogto- digital converters (ADCs) thanks to excellent bit resolution. For DAC testing, the 14-bit parallel digital words that are output from the waveform memory to the internal DACs are available as outputs.

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Tektronix also offers the AWG 7000, featuring a basic DAC sampling rate of 10 Gsamples/s. With an interleaving technique on the two channels, 20 Gsamples/s are possible, allowing up to 10-GHz waveforms. Output resolution is 10 bits.

This instrument is targeted at testing high-speed serial devices with interfaces like PCI Express (PCIe), SATA, Rapid- IO, and Ethernet. The programmability lets users create waveforms with noise and other impairments for more robust testing.

Signal-generation software is available for the AWG 5000 and AWG 7000. RF Express handles digital modulation wave creation, and SerialExpress is used to set up waveforms to test high-speed serial interfaces and devices. Software like Matlab or LabVIEW can also be used.

RF GENERATORS
To test wireless equipment, engineers typically turn to radio-frequency (RF) generators. They generate signals from 10 MHz to more than 30 GHz. The two basic types of RF generators—continuous wave (CW) and vector signal generator (VSG)—both provide some form of modulation capability (Fig. 4). Digital signal generation is the most common, but some analog types still make the rounds.

RF generators are commonly used for local-oscillator (LO) substitution. A highly stable and accurate reference crystal oscillator drives a PLL synthesizer. The fractional-N divider provides frequency selection via front-panel control. The PLL output goes to an automatic-level-control (ALC) circuit that maintains a constant output signal. A power amplifier and variable attenuator make up the output circuit. Extra circuits provide modulation.

The device performs FM and PM by having the modulating signal vary the voltagecontrolled oscillator (VCO) frequency or phase via some associated circuitry. AM is implemented with an extra variable attenuator in the output.

On occasion, engineeers will add an extra upconversion stage that consists of a mixer and high-frequency LO to increase the output to a desired range. For example, an yttrium-irongarnet (YIG) VCO is a commonly used LO to translate signals into the uppergigahertz range.

National Instruments’ PXI-5652 modular RF generator plugs into a PXI chassis to work in a virtual instrument environment with software such as LabVIEW (Fig. 5). It has a frequency range of 500 kHz to 6.6 GHz. Other models have upper frequency limits of 1.3 or 3.3 GHz. Step increment size is 4 Hz at 6.6 GHz maximum or 1- and 2-Hz increments at 1.3- or 3.3-GHz maximum outputs. An internal frequency standard of 10 MHz provides an accuracy of ±3 ppm max. Output impedance is the standard 50 O.

Key specifications include spectral purity, harmonics, output amplitude, and modulation capability. The PXI-5652’s spectral purity is a phase noise of –90 dBc/ Hz, a jitter value of 50 fs at 2.488 GHz with 5-kHz to 15-MHz jitter bandwidth, and residual FM of less than 1.5 Hz rms at 2.4 GHz. Harmonic output measures –20 dBc from 3.3 to 6.6 GHz.

Output power can be varied from about –100 to 10 dBm up to 3.3 GHz or 0 dBm at 6.6 GHz in 0.1-dB steps. This unit also offers internal modulation capability for FM, FSK, and OOK. Maximum FM deviation in the 3.3- to 6.6-GHz range is 8 MHz. The FSK symbol rate is 763 Hz to 100 kHz, and the OOK symbol rate is 153 Hz to 100 kHz.

Newer CW RF generators use a DDS synthesizer to create the basic sinewave signal. Then, that signal is filtered to remove harmonics generated in the stepped-approximation process and sent to a power amplifier and attenuator. Again, a heterodyne upconversion stage may be used to bump the output frequency into the desired higher range.

The most popular RF generator today, the VSG, is used to create the RF signal most commonly used in testing digital wireless products. Virtually all digital modulation schemes use the IQ signal generation scheme (Fig. 6). Most VSGs also have a built-in AWG baseband section to create the digital modulation in software and then output it via DACs to the VSG modulator shown. In some generators, the LO is at the desired frequency, so there’s no need for subsequent upconversion.

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Wireless test sets exist for the formal testing of cell phones and wireless gear to specific standards. Among them is an RF generator that can create all of the modulations, such as Global System for Mobile Communications (GSM), Enhanced Data for GSM Evolution (EDGE), codedivision multiple access (CDMA), High- Speed Downlink Packet Access (HSDPA), or orthogonal frequency-division multiplexing (OFDM) for Long-Term Evolution (LTE) and WiMAX.

One modern VSG, Keithley’s 2910, generates signals from 10 MHz to 2.5 GHz (Fig. 7). The model 2920 has an upper frequency limit of 6 GHz and can accommodate 25, 40, and 80 MHz. A built-in AWG generates the modulation waveforms. Also, its software-defined radio architecture creates lots of flexibility, and it can be programmed for almost any digital modulation or wireless standard.

The desired modulation/standard, which is created with Keithley’s SignalMeister software or software like Matlab or Lab- VIEW, is stored in a huge 100-Msample RAM. An internal 500-MHz DSP processes the waveform data. An FPGA and digital up/downconverters process the resulting signal further before it’s sent to the DACs and vector modulator with a DDS synthesizer. In addition, an output amplifier and attenuator provide a variable output power range of –130 to +13 dBm, depending on the frequency.

As for spectral purity, single-sideband (SSB) phase noise is –101 dBc for a 20-kHz offset at 6 GHz. Harmonics are typically –40 dBc at 6 GHz. Phase noise for a 100-kHz offset is –104 dBc at 6 GHz. Modulation available includes AM, GSMGPRS- EDGE, cdmaOne and cdma2000, wideband CDMA (WCDMA), and GPS. Newer forms like HSDPA, LTE, and WiMAX are able to be produced in software. Multiple 2920s can be used together with the 2895 synchronization unit to produce multiple-input/multiple-output (MIMO) signals for testing.

Agilent has two notable RF generators in its MXG series. The N5183A microwave analog signal generator is essentially a high-end RF CW generator. It comes in models with output frequency ranges of 100 kHz up to 20, 31.8, or 40 GHz and an output power of up to +18 dBm at 20 GHz. Its fast frequency switching speed is less than 900 µs and typically less than 600 µs. Phase noise usually measures less than –98 dBc/Hz with a 20-kHz offset.

While it’s most useful in manufacturing test of antennas and microwave devices, it also offers modulation capability with AM, FM and PM, and pulse. Its pulse capability is less than 10-ns rise/fall with a minimum of 20-ns pulse width. Digital-step and continuous-sweep frequency modes are available.

There are two versions of Agilent’s N5182A MXG VSG—one has a frequency range of 250 kHz up to 3 GHz, the other up to 6 GHz. Output power runs up to +13 dBm at 1 GHz, while phase noise at 1 GHz is less than –121 dBc/Hz with a 20-kHz offset. With the same modulation modes as the N5183A, a key feature is its internal baseband generation and modulation signal creation capability. The internal AWG baseband section can deliver up to 125 Msamples/s to the DACs with a bandwidth to 100 MHz. With 16-bit DACs, dynamic range is excellent for testing almost anything.

The waveform playback memory can hold up to 64 Msamples, while the storage memory holds up to 100 Msamples. With this capability and software like Agilent’s Signal Studio software, users can produce standard wireless signals like Wi- Fi wireless local-area network (WLAN), WiMAX, WCDMA, cdma2000, GSM, time division-synchronous code multiple access (TD-SCDMA), and even the more advanced 3G and 4G wireless standards like HSDPA and LTE. Both MXG generators have general-purpose interface bus (GPIB), USB 2.0, and Ethernet 100Base- T interfaces and comply with LXI class C.

The updated N5161A analog and N5162A VSG MXG ATE versions of these generators target cost-sensitive automated test equipment (ATE) applications. Improved features include higher output power to +23 dBm, improved distortion specifications, and phase coherency support for MIMO and beamforming antenna applications. Also, these models lack a standard front panel and move all connections to the rear (Fig. 8).

DIGITAL GENERATORS
A digital generator produces binary signals or pulses. A basic pulse generator generates pulses over a wide frequency range and can control pulse attributes such as rise/ fall time, duty cycle, and jitter. Pulse formats may be standard NRZ, RZ, or other formats, as well as positive and/or negative pulses. Pulse amplitude can also be varied.

A data or pattern generator has RAM and/or ROM that stores digital data. Data may be user-defined to exercise the device under test (DUT) or standard test patterns such as pseudorandom bit sequences (PRBS) like PN9 (109-1 points) or PN 15 (1015-1 points). Some generators can produce multiple output streams. Pulse input triggers are usually provided. And with the delay feature, users can generate delayed sequences to set specific devices.

The Berkeley Nucleonics Model 575 digital delay/pulse generator is two generators in one (Fig. 9). A pulse generator section provides independent control of pulse rate, delay, and width and includes an external trigger input. It offers two, four, or eight output channels, each individually settable to different pulse conditions.

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Frequency range is 0.0002 Hz to 20 MHz with a 40-MHz option available. Resolution measures 5 ns, and jitter less than 200 ps. The standard output is transistor- transistor logic (TTL) or adjustable from 0 to 4 V with a 3-ns rise time typical. An adjustable high-voltage output option is also available. Each channel may have its own input trigger, or all channels can be triggered simultaneously.

The 575 has a separate delay generator. It offers fine resolution and accuracy of delays and widths. Basic resolution is 250 ps with an accuracy to 1 ppm. The rate, selected by period, is set to a resolution of 10 ns. Separate delay channel triggering inputs are available, or triggering may be done on all channels at the same time.

Also, the 575 includes optical outputs with ST connectors. The output LEDs operate at 820 and 1310 nm with a rate to 5 Mbaud. Resolution is 500 ps. Maximum optical link distance runs 1.5 km. Up to two optical inputs are also available. It offers external programmability through standard RS-232 or USB ports. An option provides for GPIB or Ethernet programming interfaces. In addition, onboard storage helps save setting profiles. National Instruments LabVIEW drivers are available as well.

The Tektronix DTG5000 pulse generator is another data generator (Fig. 10). Depending on which module is selected and which modules are used, the generator can produce pulses and data streams at speeds to 3.35 Gbits/s on one, eight, 16, 32, 64, or 96 channels. Users also get full control over all pulse characteristics. Pulse delay capability is available on each channel with a 0.2-ps resolution.

The different models deliver pulsegeneration frequencies from 50 kHz to 750 Mbits/s, 2.7 Gbits/s, or 3.35 Gbits/s. Typical pulse formats available are NRZ, RZ, and R1. Pulse width, duty cycle, and delay are fully variable. Pulse-width resolution is 5 ps. Random jitter can be added at less than 3 ps rms. A PRBS PN15 data pattern capability up to PN23 is available. Pattern length per channel is 8, 32, or 64 Mbits, depending on the model.

A range of pre-stored patterns is available as well, such as binary, Gray, Johnson, or checkerboard codes. For instrument control and data transfer, the DTG5000 has GPIB and Ethernet interfaces. I/O ports available include USB, RS-232, RJ45 for 10/100 Ethernet, and VGA out.

Many other types of generators exist for special testing. For example, video generators produce the signals for TV and video testing. Also, noise generators create white or pink random noise that can be added to the output of another signal generator to test noise immunity or performance of amplifiers or other circuits. Other special generators can produce the signal to create jitter in pulse generators or AWGs.

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