Agilent Technologies user service Agilent E1441A Function Generator Tutorial, Appendix C

Appendix C

Agilent E1441A Function Generator Tutorial

The Agilent E1441A is capable of producing a variety of signal waveshapes. You may want to learn more about the internal operations of the instrument in order to achieve the greatest performance from the function generator. This appendix serves that purpose by describing basic signal generation concepts and giving specific details on the internal operations of the Agilent E1441A function generator.

• Direct Digital Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . page 153

• Signal Imperfections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . page 155

• Output Amplitude Control . . . . . . . . . . . . . . . . . . . . . . . . . . page 156

• Attributes of AC Signals . . . . . . . . . . . . . . . . . . . . . . . . . . . . page 157

• Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . page 158

Direct Digital Synthesis

Digital signal processing methods are used in many everyday applications. Whether it is a digital audio compact disc player, an electronic synthesized piano, or a voice-synthesized telephone message system, complex waveforms can be easily created or reproduced using digital signal generation methods.

The Agilent E1441A uses a signal-generation technique called direct digital synthesis or DDS. The basic principle behind DDS is not unlike an audio compact disc. As shown below for digital audio, a stream of digital data representing the sampled analog signal shape is sequentially addressed from a disc. This data is applied to the digital port of a digital-to-analog converter (DAC) which is clocked at a constant rate. The digital data is then converted into a series of voltage steps approximating the original analog signal shape. After filtering the voltage steps, the original analog waveshape will be recovered. The incoming data can be of any arbitrary shape, as long as it matches the requirements of the particular DAC (16 bits for digital audio players).

Figure C-1.

A direct digital synthesis (DDS) signal generator differs from a digital audio player because of its very precise control of the data stream input to the DAC. In a DDS system, the amplitude values for one complete cycle of the output waveshape are stored sequentially in random access memory (RAM) as shown in the figure below. As RAM addresses are changed, the DAC converts the waveshape data into a voltage

Appendix C Agilent E1441A Function Generator Tutorial 153