Logic Signal Sources

Types of Logic Signal Sources

• Pulse Pattern Generator (PPG)
• Data Timing Generator (DTG)

Logic sources are classified into pulse pattern generators and data timing generators. Pulse pattern generators drive a stream of square waves or pulses at very high frequencies from a small number of outputs. the high frequency and fast rise time capabilities of an advanced pulse generator make it an ideal tool to test high-speed digital equipment. The data timing generator generates volumes of binary information. The data timing generator is also called a pattern generator or data generator. It produces the streams of 1s and 0s needed to test computer buses, microprocessor IC devices, and other digital elements. The DTG can be used early in the product development cycle to substitute for system components that are not yet available.

Signal Generator Types

Signal Generators are broadly divided into the following:

• Analog and Mixed Signal Generators
  
  • Arbitrary Generators
    
    • Arbitrary/Function Generator (AFG)
    • Arbitrary Waveform Generator (AWG)

•  Logic Signal Sources (Pulse or pattern generators)

 Each of these types has unique strengths that may make it more or less suitable for specific applications. The arbitrary/function generator (AFG) offers fewer waveform variations than its AWG equivalent, but with excellent stability and fast response to frequency changes. Arbitrary generators answer almost any kind of signal generation needs. In the past, AFGs created their output signals using analog oscillators and signal conditioning. Now they use Direct Digital Synthesis (DDS) techniques to determine the rate at which samples are clocked out of their memory. The typical AFG has several standard waveforms stored in a preprogrammed part of its memory. In general, sine and square waves are the most widely used for many test applications.

Most AFGs offer some subset of the following familiar wave shapes:

• Sine
• Square
• Triangle
• Sweep
• Pulse
• Ramp
• Modulation
• Haversine

The arbitrary waveform generator (AWG) can produce any waveform you can imagine. Whether it is an RF signal to test a GSM or CDMA based telephone handset or a data stream for disk drive characterization. An arbitrary waveform generator (AWG) delivers waveforms based on stored digital data describing the constantly changing voltage levels of an AC signal. though the curve makes their spacing appear to vary. In an AWG, the sampled values are stored in binary form in a fast Random Access Memory (RAM). Some AWGs include separate digital outputs. These outputs fall into two categories: marker outputs and parallel data outputs.

Phase

Inorder to understand Phase, consider a sine wave. The voltage level of sine waves is related to circular motion. One cycle of a sine wave travels through 360 degrees. Phase shift (also known as delay), describes the difference in timing between two signals. Phase is usually expressed in degrees but a time value may be more appropriate in some circumstances. The phase angle of a sine wave shows how much time it has passed. Two waveforms can have similar frequency and amplitude but they may differ in phase. Two waves may be similar in other ways, but the Phase shift describes the time difference or the delay in the waves.

Phase Shift or Delay

Signal Generation Techniques

Signal Generator can be used to create waveforms in many different ways, depending upon the information available about the DUT and its input requirements; whether there is a need to add distortion or error signals, and other variables. 

Following are the three ways to develop waveforms,

1. Create
2. Replicate
3. Generate

Creating brand new signals for testing and circuit stimulus, synthesizing a signal captured from an oscilloscope or logic analyzer, and ideal or stressed reference signals for industry standards with specific tolerances .

Basic Signal Generator Applications

Signal generators fall into three basic categories: verification, characterization, and stress/margin testing. 

1. Testing Digital Modular Transmitters and Receivers
2. Testing D/A and A/D Converters
3. Stressing Communication Receivers

Wireless equipment designers developing new transmitter and receiver hardware must simulate baseband  I&Q signals to verify conformance with emerging and proprietary wireless standards. Some high-performance arbitrary waveform generators can provide the needed low-distortion, high-resolution signals at rates up to 1 gigabit per second (1 Gbps), with two independent  channels, one for the “I” phase and one for the “Q” phase. Sometimes the actual RF signal is needed to test a receiver. Here, arbitrary waveform generators with sample rates up to 200 S/s can be used to  directly synthesize the RF signal.

Newly-developed digital-to-analog converters (DAC) and analog-to-digital converters (ADC) must be exhaustively tested to determine their limits of linearity, monotonicity, and distortion. A state-of-the-art AWG can generate simultaneous, in-phase analog and digital signals to drive such devices at speeds up to 1 Gbps.

Engineers working with serial data stream architectures (commonly used in digital communications buses and disk drive amplifiers) need to stress their devices with impairments, particularly jitter and timing violations. Advanced signal generators save the engineer untold hours of calculation by providing efficient built-in jitter editing and generation tools. These instruments can shift critical signal edges as little as 200 fs (0.2 ps).

Signal Generator

Signal generators are electronic devices that generate repeating or non-repeating electronic signals (in either the analog or digital domains). They are also called as function generators, RF and microwave signal generators, pitch generators, arbitrary waveform generators, digital pattern generators or frequency generators. They are used in testing, troubleshooting, designing, and repairing electronic or electroacoustic devices. The signal generator is exactly what its name implies: a generator of signals used as a stimulus for electronic measurements. Most circuits require some type of input signal whose amplitude varies over time. The signal may be a true bipolar AC1 signal (with peaks oscillating above and below a ground reference point) or it may vary over a range of DC offset voltages, either positive or negative. It may be a sine wave or other analog function, a digital pulse, a binary pattern or a purely arbitrary wave shape. The signal generator can provide “ideal” waveforms or it may add known, repeatable amounts and types of distortion (or errors) to the signal it delivers. Most signal generators today are based on digital technology. Many can fulfill both analog and digital requirements, although the most efficient solution is usually a source whose features are optimized for the application at hand — either analog or digital.

Arbitrary waveform generators (AWG) and function generators are aimed primarily at analog and mixed-signal applications. Digital waveform generators (logic sources) encompass two classes of instruments. Pulse generators drive a stream of square waves or pulses from a small number of outputs, usually at very high frequencies. These tools are most commonly used to exercise high-speed digital equipment. Pattern generators, also known as data generators or data timing generators, typically provide 8, 16, or even more synchronized digital pulse streams as a stimulus signal for computer buses, digital telecom elements, and more.