SPECTRUM ANALYZER APPLICATIONS

Figure 5-13 shows a typical spectrum analyzer. The previously mentioned measurement capabilities can be seen with a spectrum analyzer. However, you will find that the spectrum analyzer generally is used to measure spectral purity of multiplex signals, percentage of modulation of AM signals, and modulation characteristics of fm and pulse-modulated signals. The spectrum analyzer is also used to interpret the displayed spectra of pulsed rf emitted from a radar transmitter.

Figure 5-13. - Typical spectrum analyzer.

COMPLEX WAVEFORMS

Complex waveforms are divided into two groups, PERIODIC WAVES and NONPERIODIC WAVES. Periodic waves contain the fundamental frequency and its related harmonics. Nonperiodic waves contain a continuous band of frequencies resulting from the repetition period of the fundamental frequency approaching infinity and thereby creating a continuous frequency spectrum.

MODULATION MEASUREMENTS

In all types of modulation, the carrier is varied in proportion to the instantaneous variations of the modulating waveform. The two basic properties of the carrier available for modulation are the AMPLITUDE CHARACTERISTIC and ANGULAR (frequency or phase) CHARACTERISTIC.

Amplitude Modulation

The modulation energy in an amplitude-modulated wave is contained entirely within the sidebands. Amplitude modulation of a sinusoidal carrier by another sine wave would be displayed as shown in figure 5-14. For 100% modulation, the total sideband power would be one-half of the carrier power; therefore, each sideband would be 6 dB less than the carrier, or one-fourth of the power of the carrier. Since the carrier component is not changed with AM transmission, the total power in the 100-percent-modulated wave is 50% higher than in the unmodulated carrier. The primary advantage of the log display that is provided by the spectrum analyzer over the linear display provided by the oscilloscopes for percentage of modulation measurements is that the high dynamic range of the spectrum analyzer (up to 70 dB) allows accurate measurements of values as low as 0.06%. It also allows the measurements of low-level distortion of AM signals. Both capabilities are illustrated in figure 5-15, view A, view B, and view C. The chart in figure 5-16 provides an easy conversion of dB down from carrier into percentage of modulation.

Figure 5-14. - Spectrum analyzer display of an AM signal.

Figure 5-15A. - Spectrum analyzer displays of AM signals.

Figure 5-15B. - Spectrum analyzer displays of AM signals.

Figure 5-15C. - Spectrum analyzer displays of AM signals.

Figure 5-16. - Modulation percentage versus sideband levels.

NOTE: Anything greater than -6 dB exceeds 100% modulation and produces distortion, as shown in figure 5-16.

In modern, long-range hf communications, the most important form of amplitude modulation is ssb (single-sideband). In ssb either the upper or lower sideband is transmitted, and the carrier is suppressed. Ssb requires only one-sixth of the output power required by AM to transmit an equal amount of intelligence power and less than half the bandwidth. Figure 5-17 shows the effects of balancing the carrier of an AM signal. The most common distortion experienced in ssb is intermodulation distortion, which is caused by nonlinear mixing of intelligence signals. The two-tone test is used to determine if any intermodulation distortion exists. Figure 5-18 illustrates the spectrum analyzer display of the two-tone test with the modulation applied to the upper sideband input.

Figure 5-17. - Double sideband carrier suppressed.

Figure 5-18. - Two-tone test.

Q.5 What is the advantage of single-sideband (ssb) transmission over AM transmission?