Creating an Equalized Wideband Cable Run . . . in which we design an equalized wideband cable run.
Statement of the Problem We’ve been tasked to design a wideband cable run from a shipboard tower to the ship’s radio room. This cable run is 150 feet long, must support 2–18 GHz, and, to overcome the noise figure of the follow-on receiver, must deliver a signal with about 25 dB of gain at the end of the cable run. The gain must be relatively flat with respect to frequency. The major problem we face in this design is the variation of cable attenuation with frequency. In general, RF cable has lower loss at low frequencies than at high frequencies. If we were to simply run the cable directly from the antenna to the receiver, signals at 2 GHz will experience very little loss, whereas the signals at 18 GHz will suffer a great deal of loss. If we allow this situation, then the gain, noise figure, and linearity of the complete system will be a strong function of frequency. We are using a cable with solid outer and inner conductors, which is very good cable but exhibits much variation in its attenuation over the 2–18 GHz frequency range. The attenuation of 150 feet of this cable is 13 dB at 2 GHz and 46.5 dB at 18 GHz. The attenuation, in dB/foot plotted with respect to frequency, is shown in Figure 10-80.
Figure 10-80
We often solve the variation of cable loss with frequency problem with cable equalizers. Equalizers are designed to have higher loss at low frequencies and lower loss at high frequencies—just the opposite of the cable attenuation curve. We insert equalizers into various places in our system cascade to flatten the gain. We have three different types of equalizers available to us in our design:
• 10 dB equalizer: −12.0 dB of gain at 2 GHz and −2.0 dB of gain at 18 GHz
• 15 dB equalizer. −17.5 dB of gain at 2 GHz and −2.5 dB of gain at 18 GHz
• 20 dB equalizer: −23.5 dB of gain at 2 GHz and −3.5 dB of gain at 18 GHz
The attenuation of these equalizers, with respect to frequency, is shown in Figure 10-81. Figure 10-82 shows the architecture of the system, which is due to the physical layout of the ship. Our task is to get any signal in the 2–18 GHz range to the radio room with about 25 dB of gain.
The gain should be relatively flat with respect to frequency. You can use the following parts to build your system, and you have an unlimited number of these parts in your toolbag:
Figure 10-81
Figure 10-82