SPECTRUM WAVEFORM ANALYSIS AND MEASUREMENTS

An analysis of a complex waveform, prepared in terms of a graphic plot of the amplitude versus frequency, is known as SPECTRUM ANALYSIS. Spectrum analysis recognizes the fact that waveforms are composed of the summation of a group of sinusoidal waves, each of an exact frequency and all existing together simultaneously.

Three axes of degree (amplitude, time, and frequency) are important when considering varying frequency. The time-domain (amplitude versus time) plot is used to consider phase relationships and basic timing of the signal and is normally observed with an oscilloscope. The frequency-domain (amplitude versus frequency) plot is used to observe Frequency response - the spectrum analyzer is used for this purpose. Figure 5-8 illustrates the differences between frequency- and time-domain plots. View A illustrates a three-dimensional coordinate of a fundamental frequency (f₁) and its second harmonic (2f₁) with respect to time, frequency, and amplitude. View B shows the time-domain display as it would be seen on an oscilloscope. The solid line, f₁ + 2f₁ is the actual display. The dashed lines, f and 2f₁ are drawn to illustrate the fundamental and second harmonic frequency relationship used to formulate the composite signal f₁ + 2f₁. View C is the frequency-domain display as it would be seen on a spectrum analyzer. Note in view C that the components of the composite signal are clearly seen.

Q.4 A spectrum analyzer is designed to display what signal characteristic?

Figure 5-8A. - Time versus frequencies.

Figure 5-8B. - Time versus frequencies.

Figure 5-8C. - Time versus frequencies.

FREQUENCY-DOMAIN DISPLAY CAPABILITIES

The frequency domain contains information not found in the time domain. The spectrum analyzer can display signals composed of more than one frequency (complex signals). It can also discriminate between the components of the signal and measure the power level at each one. It is more sensitive to low-level distortion than an oscilloscope. Its sensitivity and wide, dynamic range are also useful for measuring low-level modulation, as illustrated in views A and B of figure 5-9. The spectrum analyzer is useful in the measurement of long- and short-term stability such as noise sidebands of an oscillator, residual fm of a signal generator, or frequency drift of a device during warm-up, as shown in views A, B, and C of figure 5-10.

Figure 5-9A. - Examples of time-domain (left) and frequency-domain (right) low-level signals.

Figure 5-9B. - Examples of time-domain (left) and frequency-domain (right) low-level signals.

Figure 5-10A. - Spectrum analyzer stability measurements.

Figure 5-10B. - Spectrum analyzer stability measurements.

Figure 5-10C. - Spectrum analyzer stability measurements.

The swept-Frequency response of a filter or amplifier and the swept-distortion measurement of a tuned oscillator are also measurable with the aid of a spectrum analyzer. However, in the course of these measurements, a variable persistence display or an X-Y recorder should be used to simplify readability. Examples of tuned-oscillator harmonics and filter response are illustrated in figure 5-11. Frequency-conversion devices such as mixers and harmonic generators are easily characterized by such parameters as conversion loss, isolation, and distortion. These parameters can be displayed, as shown in figure 5-12, with the aid of a spectrum analyzer.

Figure 5-11. - Swept-distortion and response characteristics.

Figure 5-12. - Frequency-conversion characteristics.

Present-day spectrum analyzers can measure segments of the frequency spectra from 0 hertz to as high as 300 gigahertz when used with waveguide mixers.