Signal Formats

The most common signal formats are as follows:

• Non-Return-to-Zero (NRZ)
• Delayed Non-Return-to-Zero (DNRZ)
• Return-to-Zero (RZ)
• Return-to-One (R1)

For NRZ the waveform switches to a “1” and stays at that value until the next cycle boundary, when a valid bit occurs in the cycle. For DNRZ Similar to NRZ, the waveform switches to a “1” after a specified delay time, when a valid bit occurs in the cycle. For RZ the waveform switches to a “1,” then back to a “0” within the same cycle, when a valid bit occurs in the cycle. R1  assumes the cycle begins with a “1”, then switches to a “0” when the bit is valid, then switches back to a “1” before the cycle ends.

Complex Waves

Complex waves include:

• Analog modulated, digitally modulated, pulse-width
• modulated, and quadrature modulated signals
• Digital patterns and formats
• Pseudo-random bit and word streams

Quadrature Modulation

In Signal Modulation waves the amplitude, phase and/or frequency variations embed lower-frequency information into a carrier signal of higher frequency. It gives signals in the form of either speech, data or video. In Analog Modulation the signal varies the carrier’s amplitude and/or frequency. At the receiving end, demodulating circuits interpret the amplitude and/or frequency variations, and extract the content from the carrier. Phase modulation modulates the phase rather than the frequency of the carrier waveform to embed the content. Digital modulation is based on two states which allow the signal to express binary data. In amplitude-shift keying (ASK), the digital modulating signal causes the output frequency to switch between two amplitudes; in frequency-shift keying (FSK), the carrier switches between two frequencies (its center frequency and an offset frequency); and in phase-shift keying (PSK), the carrier switches between two phase settings. In PSK, a “0” is imparted by sending a signal of the same phase as the previous signal, while a “1” bit is represented by sending a signal of the opposite phase. Pulse-width modulation (PWM) is another common digital format; it is often used in digital audio systems. It is applicable to pulse waveforms only. With PWM, the modulating signal causes the active pulse width (duty cycle, explained earlier) of the pulse to vary. Quadrature (IQ) modulation technology is used for building digital wireless communications networks. An in-phase (I) waveform and a quadrature-phase (Q) waveform that is delayed by exactly 90 degrees relative to the “I” waveform are modulated to produce four states of information. An in-phase (I) waveform and a quadrature-phase (Q) waveform are combined and transmitted over one channel, then separated and demodulated at the receiving end. The IQ format delivers far more information than other forms of analog and digital modulation  because it increases the effective bandwidth of the system. A digital pattern consists of multiple synchronized pulse streams. It makes up words of 8, 12, 16, or more bits wide data. The digital pattern generator, specializes in delivering words of data to digital buses and processors via parallel outputs. Digital computers have the inability to produce truly random numbers, therefore Pseudo-random bit streams (PRBS) and pseudo-random word streams (PRWS) are used. Digital video signals can have jagged lines on surfaces that should be smooth. Controlled amount of noise is added to hide these jagged lines from the eye without losing the original information. Serializers or multiplexers are tested using PRWS.

Basic Waves

Following are the Basic types of waves:

1. Sine waves
2. Square and rectangular waves
3. Sawtooth and triangle waves
4. Step and pulse shapes
5. Complex waves

Sine waves are the most commonly known wave shape. Most AC power sources produce sine waves. The sine wave is the result of a basic mathematical function. Graphing a sine curve through 360 degrees will  produce a definitive sine wave image. The power switches at homes deliver power in the form of sine waves. In classrooms, while teaching about waves, teachers usually give examples of sine waves and use them for demonstration. Square and rectangular waves are at the heart of all digital electronics. A square wave is a voltage that switches between two fixed voltage levels at equal intervals, used for testing amplifiers. A rectangular wave has switching characteristics similar to those of a square wave, except that its high and low time intervals are not of equal length. Sawtooth and triangle waves look very much like the geometric shapes they are named for. The triangle wave has a symmetrical rise and fall times while the sawtooth ramps up slowly and evenly to a peak in each cycle, then falls off quickly.They are used to control other voltages in systems. A step wave shows a sudden change in voltage, as if a power switch had been turned on. The pulse is related to the rectangular wave.It is produced by switching up and then down, or down and then up, between two fixed voltage levels. A pulse may represent one bit of information traveling through a computer. A collection of pulses traveling together creates a pulse train. Complex waveforms may include elements of
sines, squares, steps, and pulses. In real life, waves rarely look like the examples in the graphics that we see here.

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

Waveform

According to an English dictionary a waveform is a usually graphic representation of the shape of a wave that indicates its characteristics (as frequency and amplitude).

A waveform is a representation of how alternating current (AC) varies with time, e.g., sine wave. The sine wave represents energy entirely concentrated at a single frequency. An ideal, unmodulated wireless signal has a sine waveform, with a frequency usually measured in megahertz (MHz) or gigahertz (GHz). 

A wave is a pattern of varying quantitative values that repeats over a certain interval of time. They are a periodically repeating phenomena. Signal generators produce electrical (voltage) waves that repeat in a controllable manner. A full repetition of a wave is called a cycle. Waveform graphically represents the activity of the wave and any change in it overtime. Some of the characteristics of waveforms are Amplitude, Frequency, and Phase. Amplitude of a wave measures the strength of the wave from the lowest point that a wave hits to the highest point. In other words it is the maximum disturbance (of a wave) from its undisturbed position. Frequency is the rate at which waveform cycles occur while phase is the time placement of a cycle relative to a reference waveform or point in time.

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 .

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.