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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