Analog signal Digital signal Volts Time Figure 11-4.  Analog signals are continuous voltage modified by all  external events including those that are not desired called noise.  Digital signals are a series of voltage or no voltage that represent  a desired event.  Figure 11-3.  An analog signal displayed on an oscilloscope is a  continuous curve.  state devices. Today, digital electronic circuits dominate  modern avionics. A brief look at various electron control  valves used on aircraft follows. A significant advantage of digital electronics over analog  electronics is the control of noise. Noise is any alteration of  the represented real-world phenomenon that is not intended  or desired. Consider the operation of a microphone when  understanding noise. A continuous analog voltage is modified  by a voice signal that results in the continuous voltage varying  in proportion to the volume and tone of the input sound.  However, the voltage responds and modifies to any input.  Thus, background sounds also modify the continuous voltage  as will electrostatic activity and circuitry imperfections. This  alteration by phenomenon that are not the intended modifier  is noise.  Electron control valves are an essential part of an electronic  circuit. Control of electron flow enables the circuit to produce  the desired outcome. Early aircraft made use of vacuum tubes  to control electron flow. Later, transistors replaced vacuum  tubes. Semiconductors used in transistors and integrated  circuits have enabled the solid-state digital electronics found  in aircraft today. Electron control valves found in the analog circuits of early  aircraft electronics are constructed of vacuum tubes. Only  antique aircraft retain radios with these devices due to their  size and inability to withstand the harsh vibration and shock  of the aircraft operating environment. However, they do  function, and a description is included here as a foundation for  the study of more modern electronic circuits and components. During the processing of digitized data, there is little or  no signal degradation. The real-world phenomenon is  represented in a string of binary code. A series of ones and  zeros are electronically created as a sequence of voltage or no  voltage and carried through processing stages. It is relatively  immune to outside alteration once established. If a signal is  close to the set value of the voltage, it is considered to be that  voltage. If the signal is close to zero, it is considered to be  no voltage. Small variations or modifications from undesired  phenomenon are ignored.  Figure 11-4  illustrates an analog  sine wave and a digital sine wave. Any unwanted voltage will  modify the analog curve. The digital steps are not modified  by small foreign inputs. There is either voltage or no voltage.  A diode acts as a check valve in an alternating current (AC)  circuit. It allows current to flow during half of the AC cycle  but not the other half. In this manner, it creates a pulsating  direct current (DC) with current that drops to zero in between  pulses. A diode tube has two active electrodes: the cathode  and the plate. It also contains a heater. All of this is housed  in a vacuum environment inside the tube.  [Figure 11-5]  The  heater glows red hot while heating the cathode. The cathode  Early aircraft were equipped with radio communication  and navigational devices that were constructed with analog  electronic circuits. They used vacuum tubes that functioned  as electron control valves. These were later replaced by solid- 11-4 Diodes  Analog Electronics Vacuum Tubes Electron Control Valves Noise