# Digital Electronics

This is a branch in electronics that studies machines whose entire operation is governed by two voltage states that is off (0) and on(1). Before now, most of the electronics lessons are analog electronics; this is the main branch in electronics that studies machine whose operations are not concerned with two fixed voltage level rather based on varying voltage level.
Analogue Electronics demonstration: if you build a system, and this system can
1 Function using 1.5v battery, 3v battery, 4.5volt battery, 7v battery.
2 At various sections voltage level can be increased or decrease for good operations.
3 The system output level is directly affected by the input level.
4 The data understood by this system is like sine wave.

That system that possesses the above four characteristics is an analogue system. Example, making light brighter by increasing voltage or making sound louder by increasing voltage or making your electric toy car run faster by increasing voltage are all analogue system. Everything in our natural world works based on analogue operations.
Fig 1. In Analogue system the voltage is not fixed

Digital Electronics demonstration: if you build a system, and this system can
1 Function using single voltage level in example 5v only.
2 At various sections voltage level are never increased or decreased.
3 The system output voltage level is not directly affect by the input voltage level.
4 The data understood by this system is like square wave.
Then that system is said to be a digital system, example is the computer systems and embedded systems. It is because of the systemâ€™s fixed voltage level that make us say it works based on zeros (0) and one (1). Where zero means it lowest fixed voltage and 1 means its highest fixed voltage.
NOTE: the zeros and 1s that is always discussed as the language of computers are never imaginary numbers rather these are fixed voltage levels upon which a given digital system functions. In reality, if light emitting diode comes in contact with 1 the light will surely glow (ON), and if it comes in contact with 0 the light will be turned OFF.

DIGITAL SYSTEMS
In mathematics and mathematical logic, Boolean algebra is the branch of algebra in which the values of the variables are the true and false, usually denoted 1 and 0 respectively. Instead of elementary algebra where the values of the variables are numbers, and the prime operations are addition and multiplication. Digital systems are electronic circuits capable of solving all possible Boolean algebra; upon which we now have computers and other complex robotic systems.
Fig 3. REAL LIFE NAND GATE EXTERNAL AND INTERNAL VIEW

Full understanding of digital electronics has to go in steps; this is because using only two voltage states to make a system useful to man involves lots of logics and manipulations. To help beginners understand this digital systems, its operations has been broken down into seven (7) LOGIC SYSTEM called the LOGIC GATES. These seven systems can be called the elements of digital electronics, because full mastering of them will make you fully understand the principles and operations of DIGITAL ELECTRONICS. Internally, each of the LOGIC gates is intelligently implemented using basically diodes, resistors and transistors to achieve their various switching operations. Example, fig 4, shows the real components making up an AND GATE and chosen circuit symbol adopted to make its drawing easier. Therefore, all GATES drawn here simply represent the schematic and not the real life circuit. Therefore, each of the GATE can do what its truth table says because of complex circuits that make each of them up.

LOGIC GATES
These are logical digital electronic components that have its output voltage controlled by its input voltage. They are sold in the market, and are found as Integrated circuits. It should be noted that the input simply switches on/off the output, and not that it is the same input that is coming out as output. No matter the number of input the gate has, the output voltage will never exceed a fixed voltage level. Real life testing of the logic gates output, based on input requires making a circuit connection as the fig 5. below.

FIG 5. REAL LIFE TEST CONNECTIONS FOR LOGIC GATES
The above figure is called an AND GATE; it has six inputs and a single output. Connecting all the six input to 5volts power supply does not make the output to be 30volts. The work of the input is simply to make the output 0volt or 5volts.
NOTE: for the purpose of simplicity, we shall no longer use 0volt or 5volts in describing input or output state even though in reality that is what it is; we shall henceforth use 0 or 1. 0 means 0volt and 1 means 5volts. In some cases we will also use HIGH to signify 5volts and LOW to signify 0volt. Fig 5, shows the real life test connections circuits for every logic gate. This real life connection will not be used in explaining all the gates for simplicity reasons, but you should know that the connection for testing gate behaviors must be done as it is in fig 5.

NOR GATE

OR gate has its output HIGH or 1, if any of the input is HIGH or 1. If in your circuit design, you are required to implement a system that turns ON if any of the input is turned ON then OR GATE is required. Example, if you are required to monitor two separately located doors at the same time and turn on SIREN (ALERT) if someone opens any of the doors then OR GATE will be used.

Algebraically, OR GATE has mathematical notation.
A + B = Q

In advance topics in digital electronics this mathematics takes deeper understanding of digital systems which is beyond the scope of this lesson.

NOR GATE
NOR GATE is a direct opposite of OR GATE. The only condition for its output to be HIGH is that the two inputs must be 0s or LOW.

AND GATE
The output of AND gate can only be made 1 or HIGH if and only if the inputs are made HIGH or 1. So if you are given a task to build a system that turns ON only when all of its inputs are HIGH, then AND GATE must be employed.
Algebraically, AND GATE has mathematical notation.
A . B = Q

NAND GATE

The output of NAND gate can only be made 1 or HIGH if and only if the inputs are made LOW or O. This gate is a direct opposite of an AND GATE. In real sense, NAND gate is actually an AND GATE whose output is passed through a NOT GATE.

NOT/INVERTER GATE
NOT or INVERTER GATE has its output always the opposite of its input; that is if the input is LOW (0) the output will be HIGH (1) and vice versa.

EXCLUSIVE OR GATE
Exclusive OR (XOR) GATE, the condition for this gate to be HIGH is that the two inputs should never be the same. Once the two inputs are exactly the same the output will be LOW (0).

EXCLUSIVE NOR GATE
EXCLUSIVE NOR (XNOR) GATE is a direct opposite of Exclusive OR gate. The output is HIGH when both input is similar, that is if both inputs are Zeros (0s) the output will be 1(HIGH). And when the two input are both 1s (HIGH) the output also is HIGH.

To finally build a complex digital electronic device it involves combining these logic gates as many times as possible until the require result is achieved. This involves using Boolean algebra solution to simplify and resolve the required logic. This mathematical resolution of logic gate for system design is beyond this lesson.

### SATELLITE COMMUNICATION SYSTEMS

A communications satellite is an artificial satellite that relays and amplifies radio telecommunications signals via a transponder; it creates a communication channel between a source transmitter and a receiver at different locations on Earth.
What are the types of satellite orbits?
There are at least three special types of orbits employed by modern satellites. These are;
1. the Geostationary Orbits,
2. Low-Earth Orbits and
3. Molniya types.
A Geostationary Orbit satellite provides an illusion that it is stationary in a fixed location in space. Actually, it revolves around the planet once a day in the equator. It is useful for telecommunication devices especially those that rely on stationary antennas since there is no need to install special equipments for such facilities just to track the satellite.

Meanwhile, Low-Earth Orbit satellites are located at least 400 Km above the Earth. Because of their lower altitude with respect to the ground, they can circle the Earth in only about 90 minutes. These satellites are less expensive and require less energy to receive and transmit data.

On the other hand, the Molniya Orbit group of satellites operates at a certain inclined position suitable for Northern Altitudes. These Comsats were designed so that they will take more time servicing Northern latitudes. Molniya satellites are usually used for Television broadcast transmissions and telephone relays over the Russian state.

### Satellite communications basics

When used for communications, a satellite acts as a repeater. Its height above the Earth means that signals can be transmitted over distances that are very much greater than the line of sight. An earth station transmits the signal up to the satellite. This is called the up-link and is transmitted on one frequency. The satellite receives the signal and retransmits it on what is termed the down link which is on another frequency.
The circuitry in the satellite that acts as the receiver, frequency changer, and transmitter is called a transponder. This basically consists of a low noise amplifier, a frequency changer consisting a mixer and local oscillator, and then a high power amplifier. The filter on the input is used to make sure that any out of band signals such as the transponder output are reduced to acceptable levels so that the amplifier is not overloaded. Similarly the output from the amplifiers is filtered to make sure that spurious signals are reduced to acceptable levels. Figures used in here are the same as those mentioned earlier, and are only given as an example. The signal is received and amplified to a suitable level. It is then applied to the mixer to change the frequency in the same way that occurs in a superheterodyne radio receiver. As a result the communications satellite receives in one band of frequencies and transmits in another.
In view of the fact that the receiver and transmitter are operating at the same time and in close proximity, care has to be taken in the design of the satellite that the transmitter does not interfere with the receiver. This might result from spurious signals arising from the transmitter, or the receiver may become de-sensitised by the strong signal being received from the transmitter. The filters already mentioned are used to reduce these effects.
Signals transmitted to satellites usually consist of a large number of signals multiplexed onto a main transmission. In this way one transmission from the ground can carry a large number of telephone circuits or even a number of television signals. This approach is operationally far more effective than having a large number of individual transmitters. Obviously one satellite will be unable to carry all the traffic across the Atlantic. Further capacity can be achieved using several satellites on different bands, or by physically separating them apart from one another. In this way the beamwidth of the antenna can be used to distinguish between different satellites. Normally antennas with very high gains are used, and these have very narrow beamwidths, allowing satellites to be separated by just a few degrees.