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.

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.