SHIPBOARD COMMUNICATIONS SYSTEMS QUALITY MONITORING (QMCS)

In recent years the volume of shipboard communications has increased greatly. This expansion has led to the shipboard installation of sophisticated equipment. Factors such as frequency accuracy and dc signal distortion are critical to the operation of communications systems. These systems demand precise initial lineup and monitoring to ensure satisfactory operations are maintained. System degradation is often caused by many small contributing factors. When these factors are added together, the system becomes unusable.

Scheduled Maintenance

When you perform scheduled, logical checks that ensure continuous, optimum performance of shipboard communications systems, you are doing SCHEDULED MAINTENANCE. In many cases this maintenance prevents outages before they occur. Some of the scheduled checks will include the following:

• Transmitter/receiver frequency
• Transmitter power out
• Receiver sensitivity/bandwidth
• Primary power (voltage, current, cycles)

Electromagnetic Interference (emi)

Many complex electronic systems are installed aboard Navy ships. In modern ships, complex systems with higher power and greater sensitivity are being crowded into a restricted and corrosive area. Figure 3-34 is a Spruance class destroyer with its crowded (compact) communications environment. The ability of these systems to perform their individual functions without interference is known as ELECTROMAGNETIC COMPATIBILITY (emc). Emc is concerned with the structure of the ship and its electrical and electronic system. Compact environment is a major limitation to the effectiveness of a total ship system concept.

Figure 3-34. - Total ship.

Operation of a total ship system in this unique shipboard environment presents a challenge to all concerned. You must always consider the effects that motion, temperature variations, and exposure to adverse elements will have on the performance of the total ship system. This is particularly true on those system components that are mounted topside.

On board ship, you will find much attention is given to keeping the topside cosmetically and mechanically shipshape. It is equally important to keep it electronically shipshape. Minor mechanical problems, such as loose connections, broken bond straps, or rusty junctions can cause serious communications problems. These sources of electromagnetic radiations reduce receiver performance and are known as ELECTROMAGNETIC INTERFERENCE (emi). Sources of emi can be divided into the following broad categories:

Functional. Emi can originate from any source designed to generate electromagnetic energy and which may create interference as a normal part of its operation. The interference may be unintentional or caused by other on board or adjacent platform systems. This interference also may be intentional or caused by electronic countermeasures (ECM). Incidental. Emi can originate from man-made sources. These are sources not designed specifically to generate electromagnetic energy but which do in fact cause interference. Examples of incidental emi sources include power lines, motors, and switches. Natural. Emi can be caused by natural phenomena, such as electrical storms, rain particles, and solar and interstellar radiation. It is recognized by the following audible noises:

• Intermittent impulses of high intensity that are caused by nearby electrical storms
• Steady rattling or cracking caused by distant electrical storms
• Continuous noise of precipitation static caused by electrically charged rain drops
• A steady hiss at high frequencies caused by interstellar noise

Hull-generated. Emi can be caused by the interaction of radiated signals with elements of the hull and rigging of a ship. (The functional signals themselves do not cause interference.)

The following are two general methods by which emi is transmitted:

• Conduction. Undesired energy from one equipment is coupled to interconnecting cables or components of another equipment. This energy is conducted via the wiring in the shielded enclosure that protects sensitive circuits. You will find proper design, adequate isolation, and shielding of cables and equipment can control this problem.
• Radiation. Energy is beamed directly from the transmitting antenna, or source, to the victim receiving antenna. When this interference is picked up by a receiver, you have two solutions. Interfering energy can be eliminated at the source or you can filter, or blank it out at the victim equipment.
• Filtering is far less desirable. Interference may be on the same frequency as the desired signal and will not be eliminated without affecting the reception of all desired signals.

Most unprotected shipboard receivers are susceptible to emi over a frequency range much wider than their normal bandpass. Off-frequency rejection rarely excludes strong, adjacent-channel signals. These signals enter the receiver and degrade receiver performance by being processed along with the desired, tuned signal. Usually, the presence of emi will be apparent to you. It has a bad effect. Upon the desired signal quality, such as that in CROSS-MODULATION where a spurious response occurs when the carrier of a desired signal intermodulates with the carrier of an undesired signal. Extremely strong, off-frequency signals may even burn out the sensitive front-end stages of a receiver. Emi also can degrade overall receiver performance in a less noticeable way. It does this by desensitizing the receiver front end. The noise level is raised and effectively lowers the signal to noise ratio and thus the sensitivity. This causes a decrease in desired signal amplification. For these reasons, shipboard receive systems are designed to include protective circuitry between the antenna and receiver to filter out off-frequency signals. This prevents or limits interference, desensitization, or burnout. Depending upon the system, these protective devices may include filters, multicouplers, preselectors, and so forth. These devices can minimize interference caused by inadequate frequency separation or poor physical isolation between transmit and receive antennas.

Q.35 What is the purpose of QMCS?
Q.36 What is emi?
Q.37 What are the two emi transmission methods?