Introduction To High-Frequency Analog Signals

Overview

In the following article, learn how factors such as bit resolution and sampling rate affect the ability of a signal generator to produce an ideal analog signal. This tutorial is part of the National Instruments Signal Generator Fundamentals series. Each tutorial in this series will teach basic concepts about the architecture, features, or applications of signal generators. While the fundamental building block of analog signal generation is a Digital-to-Analog Converter (DAC), the process of generating advanced analog signals is much more complex. In this article, we will discuss the factors that affect signal quality including: bit resolution, bandwidth, sample rate, interpolation, and filtering.

Digital to Analog Conversion Characteristics

National Instruments signal generators, like many in the industry, utilize off the shelf Digital-to-Analog Converters (DAC's) from vendors such as Analog Devices. While many factors affect the quality of a waveform from a signal generator, three fundamental characteristics profoundly affect its performance when generating an analog waveform. These three characteristics are: bit resolution, bandwidth, and sampling rate.

Bit Resolution
The bit resolution of a signal generator is limited by the bit resolution of the DAC that it uses. This specification of a signal generator is important because it affects the amplitude range of the device. Fundamentally, a DAC is able to approximate the actual analog signal by outputting series of voltages levels at discrete analog levels. The bit resolution of an ADC is important because it determines the number of discrete voltage levels that are possible, and thus the voltage difference between each one.

An example, consider a 3-bit DAC. With three bits of resolution, this DAC divides its vertical range into 23 or 8 discrete analog voltage levels. These 8 discrete voltage levels are divided across the entire vertical range and the voltage difference between each level is similar. Thus, when a 3-bit DAC is configured with an output range of 0-10 V, each level accounts for (10 V / 8) = 1.25 volts. Thus, at this range, a 3-bit DAC cannot generate voltage differences smaller than 1.25 V.