R$_{B}$ S RFC C$_{2}$ C$_{3}$ C$_{1}$ L Crystal R$_{A}$ R$_{B}$ S RFC R$_{E}$ C$_{2}$ C$_{1}$ L$_{2}$ L$_{1}$ Figure 11-60.  A crystal in an electronic oscillator circuit is used to  tune the frequency of oscillation. Figure 11-59.  A Hartley oscillator uses a tank circuit and a  transistor to maintain oscillation whenever power is applied.  trained avionics technicians. The airframe technician is  responsible for installation, maintenance, inspection, and  proper performance of avionics in the aircraft. Modern aircraft increasingly employs digital electronics in  avionics rather than analog electronics. Transistors are used  in digital electronics to construct circuits that act as digital  logic gates. The purpose and task of a device is achieved  by manipulating electric signals through the logic gates.  Thousands, and even millions, of tiny transistors can be  placed on a chip to create the digital logic landscape through  which a component’s signals are processed. to the tank circuit to compensate for resistance losses. As C$_{2}$  is discharging, it reduces forward biasing and eventually the  transistor becomes reversed biased and cuts off. When the  opposite plate of capacitor C$_{1}$ is fully charged, it discharges,  and the oscillation is in progress. The transistor base becomes  forward biased again, allowing for current flow through the  resistor R$_{E}$, coil L$_{1}$, etc.  The frequency of the AC oscillating in the Hartley oscillator  circuit depends on the inductance and capacitance values  of the components used. Use of a crystal in an oscillator  circuit can control the frequency more accurately. A crystal  vibrates at a single, consistent frequency. When flexed, a  small pulse of current is produced through the piezoelectric  effect. Placed in the feedback loop, the pulses from the  crystal control the frequency of the oscillator circuit. The tank  circuit component values are tuned to match the frequency  of the crystal. Oscillation is maintained as long as power is  supplied.  [Figure 11-60] Other types of oscillator circuits used in electronics and  computers have two transistors that alternate being in the  active mode. They are called multi-vibrators. The choice of  oscillator in an electronic device depends on the exact type  of manipulation of electricity required to permit the device  to function as desired.  Digital logic is based on the binary number system. There are  two conditions than may exist, 1 or 0. In a digital circuit, these  are equivalent to voltage or no voltage. Within the binary  system, these two conditions are called Logic 1 and Logic 0.  Using just these two conditions, gates can be constructed to  manipulate information. There are a handful of common logic  gates that are used. By combining any number of these tiny  solid-state gates, significant memorization, manipulation,  and calculation of information can be performed. The above discussion of semiconductors, semiconductor  devices, and circuitry is only an introduction to the electronics  found in communications and navigation avionics. In-depth  maintenance of the interior electronics on most avionics  devices is performed only by certified repair stations and  The NOT gate is the simplest of all gates. If the input to the  gate is Logic 1, then the output is NOT Logic 1. This means  that it is Logic 0, since there are only two conditions in the  binary world. In an electronic circuit, a NOT gate would  invert the input signal. In other words, if there was voltage at  the input to the gate, there would be no output voltage. The  gate can be constructed with transistors and resistors to yield  this electrical logic every time. (The gate or circuit would also  have to invert an input of Logic 0 into an output of Logic 1.) 11-26 Digital Electronics  The NOT Gate Digital Building Blocks