Communications EW - Part 6: Digital Communication (continued)

摘要

Continuing our discussion of digital communication from last month, we'll look at bandwidth required for digital communication and error correction codes.Figure 1 shows what a digital signal looks like on a spectrum analyzer. The fuzzy lump is the main lobe of the transmitted spectrum. The transmit frequency (the carrier) is at the center, and there are distinct nulls on either side. Because the digital data has pseudo-random characteristics, the signal looks noise-like. If you watch the spectrum analyzer screen inrnreal time, the signal within the lobe will constantly undulate. Figure 2 shows the parts of the frequency response. The null-to-null bandwidth of the main lobe is twice the clock rate. That is, each null is the baud rate from the carrier (i.e., if one million bauds per second are sent, the lobe is 2-MHz wide). The side lobes are each one clock rate wide. The drawing shows only the first side lobes, but they continue, with the side lobe peaks diminishing with frequency from the carrier.