Digital Electronics: Principles, Devices and Applications

690 Digital Electronicssynthesizer with additional circuitry to produce pulse, ramp, triangle and square functions. Theseinstruments with additional modulation capability are referred to as synthesized signal generators.Direct digital synthesis described in the earlier pages of this chapter is almost invariably usedin synthesized function/signal generator design. Advances in digital technology have made thesesynthesized function/signal generators truly versatile. Synthesized sine wave output up to 30 MHz andother functions such as pulse, ramp, triangle, etc., up to 100 kHz, all with a resolution of 1Hz, areavailable in contemporary synthesized function generators.Figure 16.31 shows one such synthesized function generator (Fluke 271 DDS function generator)that employs direct digital synthesis for achieving a high level of stability. It offers sine, square, triangleand ramp outputs of up to 10 MHz.16.15.6 Arbitrary Waveform GeneratorThe arbitrary waveform generator (AWG) is a signal source that is used to generate user-specifiedcustom analogue waveforms. Using a custom stimulus waveform and measuring the response waveformprovides realistic characterization of the device or system under test. The contemporary AWG allowsgeneration of almost any conceivable waveform.Direct digital synthesis again is the heart of an arbitrary waveform generator. Figure 16.32 shows thehardware. It looks very similar to the one shown in Fig. 16.29. The sequential amplitude values of thewaveform to be generated are stored in the RAM. The size of the RAM decides the number of samplesthat can be stored, which in turn decides the maximum number of samples into which one period ofthe desired waveform can be divided. These sample values can be entered into the RAM from thekeyboard. Once the sample values are loaded into the RAM, they can be stepped through at a repetitionrate governed by the frequency word input to the phase accumulator in the same way as explained inthe case of a frequency synthesizer. The complexity of the waveform that can be synthesized by thisprocess is limited by the size of the RAM. As a rule of thumb, a minimum of about 3–4 samples percycle of the highest frequency in the waveform should be used. This is intended to eliminate aliasing.Figure 16.33 shows a typical arbitrary waveform possible in a typical arbitrary waveform generator.Figure 16.31Synthesized function generator. Reproduced with permission of Fluke Corporation.