the signal applied to it. These spurious frequencies are called “external cross-modulation.” These spurious frequencies (sum, difference, and harmonics) can be expected to cause interference problems when the combined product of their field strengths exceed 1 millivolt. A common example of this action is the entry of a strong off-frequency RF voltage into the mixer stage of a superheterodyne receiver. By the time the interfering signal has passed through the preselector stages of the receiver, it has undergone distortion by clipping. Therefore, the interfering signal is essentially a rectangular wave that is rich in harmonics. Frequency components of the wave beat both above and below the local oscillator frequency and its harmonics, and produce, at the output of the mixer, signals that are acceptable to the IF amplifier. POWER LINES Alternating current power sources have already been briefly discussed as broadband interference. Even though they are conducting a nearly sinusoidal waveform, ac signals on power lines are capable of interfering with audio signals in receivers. In such cases, only the power-line frequency appears. However, where multiple sources of ac power are present, these signals are capable of being mixed in the same manner as discussed under receiver oscillators. Sum and difference frequencies appear. In at-powered equipment, ac hum can appear at the power frequency or at the rectification ripple frequency. The rectification ripple frequency is twice the power frequency times the number of phases. Normally, aircraft systems use only single- and three-phase sources at a nominal 400 Hz. Full-wave rectification with single-phase 400-Hz power gives a ripple frequency of 800 Hz. A three-phase source would give a 2400 Hz ripple. This ripple produces interference varying from annoyance to complete unreliability of equipment, depending upon the severity and its coupling to susceptible elements. Openings in the outer shields of equipment are necessary for the entrance of power leads, control leads, mechanical linkages, ventilation, and antenna leads. Interference entering these openings is amplified by various amounts, depending upon the point of entry into the equipment’s circuits. Coupling between the entry path and the sensitive points of the receiver can be in any form. CONDUCTIVE COUPLING Interference is often coupled from its source to a receiver by metallic conduction. Normally, this is done by way of mutual impedance, as shown in figure 10-1. Note in the figure that A is the power source, B the receiver, and C the interference source. The interference is maximum at the interference source (C), and attenuates rapidly to a relatively low value at the battery (A). This occurs because of the very low impedance of the battery. It is apparent from the size of the arrows that the nearer the power tap of the receiver (B) to the interference source (C), the greater the amplitude of interfering current in the BC loop. INDUCTIVE-MAGNETIC COUPLING Every current-carrying conductor is surrounded by a magnetic field whose intensity variations are faithful reproductions of variations in the current in the conductor. When another parallel conductor is cut by the lines of force of this field, the conductor has a current induced into it. The amplitude of the induced current depends upon the following factors: The strength of the current in the first conductor The nearness of the conductors to each other The angle between the conductors The length through which the conductors are exposed to each other INTERFERENCE COUPLING Learning Objective: Identify the various types of electrical interference caused by coupling, and recognize the means used to reduce the interference. Figure 10-1.-Path of conducted interference. 10-6