Signal generators meet modern design challenges
31 March 2011
A versatile signal source is an important part of the electronic engineer’s toolbox. Emily Reed examines how the latest innovations can speed-up testing of modern high-tech equipment
Engineers needing to validate designs, test production units, or repair faults will continue to use signal generators to generate test signals such as analogue waveforms and digital data patterns, to apply modulation, and to inject intentional distortion and noise.
The latest families from manufacturers such as Agilent Technologies, Iso-Tech, LeCroy, Tektronix and Thurlby Thandar Instruments (TTi) provide performance and features supporting today’s design priorities, including high-speed interface standards such as HDMI and USB 3.0, the growth of RF, and widespread use of pulse-width modulation (PWM) in equipment such as audio amplifiers, motor controls and solidstate lighting.
As with any equipment selection process, making the right choice is guided by the task at hand. In addition, when choosing a signal generator for general-purpose use, future needs must also be considered.
Sine, square, triangle Signal generators built using purely analogue circuitry provide an effective low-cost option for engineers seeking a dependable instrument capable of generating basic sine, square and triangle waveforms.
They provide simple and instantaneous control of frequency and amplitude for signals typically ranging from 2 MHz up to 20 MHz or more.
Gating or triggering may also be incorporated. Instruments featuring variable symmetry are able to generate saw-tooth and variable-duty rectangular waveforms from triangle and square signals.
An example is the TG330 from TTi, which has a frequency range from 0.03 Hz to 3 MHz, amplitude from 2 mV to 20 V pk-pk, and variable symmetry.
Frequency is set with four-digit resolution, and a digital display of frequency and amplitude is provided.
A basic digital signal-generator architecture uses a digitally controlled voltage controlled oscillator (VCO) to achieve a highly stable output.
More versatile instruments featuring Direct Digital Synthesis (DDS) or a true variable-clock architecture are able to construct complex waveforms over a range of frequencies.
DDS signal generators such as the Iso-Tech GFG2000 and GFG2110 series are units capable of producing sine, triangle and square waveforms up to 4 MHz, 7 MHz or 20 MHz.
TTi’s TG1000 and TG2000 provide frequencies up to 10 MHz or 20 MHz with six-digit frequency resolution.
As well as being able to produce clean, stable sine waves, DDS technology is also capable of synthesising an arbitrary waveform or signal from values stored in memory.
With this in mind, parameters such as the maximum sample rate and memory depth (maximum number of data points) are important selection criteria for engineers seeking an arbitrary waveform or function generator.
The Tektronix Tekdata AFG3000 series can generate arbitrary waveforms up to 2 Gsample/s, with up to 128 K data points and 14-bits resolution. LeCroy provides extremely long memory with its ArbStudio range, enabling engineers to generate arbitrary waveforms up to 2 million points per channel at up to 1 Gsample/s and 16-bits resolution.
The output waveform can be a sample captured using a digital oscilloscope, or can be composed with the aid of a software application.
Signal sources with arbitrary waveform capability typically have accompanying software such as TTi’s Waveform Manager, Agilent’s Intuilink, LeCroy’s ArbStudio and Tektronix’s ArbExpress packages.
These enable engineers to create and manage waveforms of virtually any technically feasible shape and frequency.
The AFG3000 series introduced the added convenience of an integrated 5.6 inch LCD graphical display at its launch in 2008, which presents a visual conformation of the generated waveform without requiring connection of an oscilloscope.
Waveform editing capabilities also allow setting of the modulation shape, duty cycle and aspects such as rise and fall times for a PWM signal, providing a convenient way for engineers to test many types of modern control schemes.
With the latest equipment, tasks such as modulating waveforms or introducing noise or jitter have also become significantly faster and easier.
Internal AM, FM and FSK modulation, featured in models such as Agilent’s 80 MHz 33250A save engineers from using a separate modulation source.
In addition, the 10 MHz 33210A and 20 MHz 33220A have pulse capabilities including PM and PWM modes. These units provide straightforward, user-friendly features, and are also compliant with LXI, allowing convenient, high-speed connection to PCs or automated test systems.
TTi’s 25/50 MHz TG2511/5011 also support LXI connectivity.
Introducing controlled quantities of noise or jitter is a typical requirement when certifying data-communication equipment to high-speed digital standards such as PCIe, HDMI or USB.
Traditional practice calls for a separate source to generate a noise signal, which is used to modulate the original carrier waveform.
However, a growing number of test committees are now sanctioning new methodologies, allowing developers to take advantage of these more modern signal generators to produce a waveform featuring the required amount of distortion.
In the future, these methods will be preferred for their speed, efficiency and immunity to human error.
EMILY REED is Product Manager, RS Components
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