Ideal Transistor Switch

A bipolar transistor can be made to approximate an ideal switch.
Consider a common emitter transistor circuit,

A collector resistance R_C is connected from the transistor collector to the supply voltage V_CC.
The emitter terminal of the device is grounded.
For the transistor to simulate a switch, the terminals of the switch are the transistor collector and emitter.
The input voltage or the controlling voltage for the transistor switch is the base-emitter voltage V_BE.
The collector-emitter voltage V_CE is equal to the supply voltage minus the voltage drop across R_C,

V_CE=V_CC-I_CR_C

When the transistor base-emitter voltage is zero or reverse biased, the base current I_B is zero, and the collector current I_C is also zero. The transistor switch is now in its OFF condition.

Since there is no collector current, there can be no voltage drop across the load resistor.
Therefore when I_C=0,

V_CE=V_CC

Thus, when an ideal transistor switch is OFF, its collector-emitter voltage equals the supply voltage.

When the transistor base is made positive with respect to the emitter, a base current I_B flows. The collector current I_C is equal to I_B multiplied by the transistor common-emitter dc current gain h_FE.
i.e.  I_C=h_FE*I_B 
If I_B is made large enough, I_CR_C can become equal to the supply voltage V_CC.

Therefore,

V_CE=V_CC-V_CC=0

Thus, when an ideal transistor switch is ON, its collector-emitter voltage equals zero.

Ideally, it dissipates zero power when ON or OFF.
Transistor power dissipation is given by,

P_D=I_CV_CE

When the switch is OFF, I_C=0,

P_D=0

When the switch is ON, V_CE=0,

P_D=0

The only time power is dissipated is when the device is switching between ON and OFF.

Communication Satellite Link Design

The main controlling factor in the design of links is the frequency of the uplink and downlink (6/4 GHz).

The optimum radio frequency range of satellite communications is determined technically by,

a) absorption, scattering and refraction phenomena rising in the atmosphere.

b) galactic noise and thermal noise emitted by the atmosphere.

c) the feasibility of constructing satellite antenna with gains appropriate to the service area required.

The best frequency range for systems serving fixed earth stations is about 1-10GHz.

The lower limit is set mainly by galactic noise and the size and mass of satellite antennas.

The upper limit is usually set by attenuation due to heavy rain.

In satellite communication link design the important calculation is the power received by the receiving station.

The transmitting source is considered to be located in free space and supposed to radiate the power P_T Watts uniformly in all directions. Normally a directive antenna is used in satellite communication link and the directivity is represented by a finite beam of width θ .

The receiver is characterized by the effective area A_R of its antenna and by the noise temperature T of its low noise amplifier.

The transmitting antenna beam illuminates an area A_O at the receiver.

The distance between transmitter and receiver is d.

Thus, the receiving antenna intercepts the fraction A_R/A_O of the transmitted power. Therefore, the power received by the receiving antenna is given by,

The directivity of antenna is described by its gain as,

which is actually the ratio of the area illuminated by an isotropic antenna to that illuminated by the antenna in question.

The product P_TG_T is called effective isotropic radiated power (EIRP).

The receiving antenna gain G_R is related to its effective area A_R by,

Therefore,


The power attenuation expressed in decibels is,


Thus,

where,

which is called as path loss or free space loss and expresses the signal power attenuation between two isotropic antenna in free space.

The free space loss varies with frequency, the higher the frequency, higher the free loss.

Path loss between a satellite in geostationary orbit and the earth station is 195.6dB and 199.1dB at 4GHz and 6GHz respectively.

In real sense there would be a variety of losses and so instead of L_FS it may be L such that L=L_FS*L_A

where L_A is additional losses given by,

where,
L_FTX = losses between the transmitter output and the transmitting antenna
A_AG = attenuation by the atmosphere and ionosphere
A_Rain = attenuation due to precipitation and clouds
L_POL = losses caused by polarization mismatch between the transmitter and receiver antenna
L_Point = losses caused by antenna depointing
L_FRX = losses between the receiving antenna and the receiver input

Therefore



The gain of an antenna is expressed in terms of actual surface area A by,

where η  is the efficiency of the antenna

The product ηA corresponds to the effective area A_R.

Normally the efficiency of 60% is taken as good though some antenna may achieve efficiency up to 70%.

SET-RESET (S-R) Flip-Flop

S-R flip-flop is an asynchronous sequential circuit.The S-R flip-flop has two inputs, namely SET(S) and RESET(R), and two outputs Q and Q̅.
The two outputs are complement to each other.

 The S-R flip-flop can be easily constructed using two NOR gates connected back to back.

The cross-coupled connections from the output of one gate to the input of the other gate constitute a feedback path.

Flip-Flops

To have a sequential circuit, a storage device is required to know what has happened in the past. The basic unit of storage is the Flip-Flop.

The simplest kind of sequential circuit is a memory cell that has only two states. It can be either 1 or 0. Such two state sequential circuits are called flip-flops because they flip from one state to another and then flop back. A flip-flop is also known as bistable multivibrator, latch or toggle.

Consider the general block diagram of a flip-flop,

It has one or more inputs and two outputs Q and Q̅. The two outputs are complementary to each other. If Q=0 i.e. Reset, then Q̅=1 and
if Q=1 i.e. Set, then Q̅=0.

Types of Flip-flop
S-R flip-flop
D flip-flop
J-K flip-flop
T flip-flop

ghf

Logic Circuits

Types of logic circuits
i) Combinational circuits: The logic circuits whose outputs at any instant of time depend only on the input signals present at that time are called combinational circuits.
For a change in input, the output appears immediately, except for the propagation delay through circuit gates.

 ii) Sequential circuits: The logic circuits whose output at any instant of time depend not only on the present inputs but also on the past outputs are called sequential circuits.
In sequential circuits, the output signals are fed back to the input side.

It consists of a combinational circuit to which memory elements are connected to form a feedback path.

The memory elements denoted by M are devices capable of storing binary information.

The circuit has m number of external inputs denoted by X and n number of outputs denoted by Z.

The signal value at the output of memory elements denoted by y, are referred to as the present state of sequential circuit.

The external inputs X and present state variables y are applied to combinational circuit, which in turn produces the outputs Z and Y, where Y are referred to as next state of sequential circuit.

To have a sequential circuit, a storage device is required to know what has happened in the past. The basic unit of storage is the Flip-Flop.

Types of Sequential Circuits
i) Synchronous or Clocked circuits: In Synchronous Sequential circuits, synchronization is achieved by a timing device called master-clock generator, which generates  a periodic train of clock pulses.

ii) Asynchronous or Unclocked circuits: In an Asynchronous Sequential circuits, events can occur after one event is completed and there is no need to wait for a clock pulse. Therefore, asynchronous circuits are considerably faster than synchronous circuits.

Advantages and disadvantages of Satellite Communications

Until the advent of communication satellites, the long distance communication through space could be done by using cascaded radio relays, very low frequency radio and high frequency or short wave radio. The latter two are inherently low capacity media, suitable only for specialized applications and the cascaded radio relays were limited to overland spans. Thus the satellite has filled a huge void in the sense that has been capable of transmitting high capacities over long distances, either overland or water.

Also because of its unique geometry, it is inherently a broadcast medium with a natural ability to transmit simultaneously from one point to an arbitrary number of other points within its coverage area.

The Advantages of satellite communications are,

1.  Satellite relays are inherently wide-area broadcast i.e. the point-to-multipoint, whereas all the terrestrial relays are point to point.
2. The satellite circuits can be installed rapidly. Once the satellite is in position, the earth stations can be installed and communication may be established in days or even hours. Thus a station may be removed relatively quickly from one location and reinstalled elsewhere. The terrestrial circuits of any kind would require a time consuming installation.
3. The mobile communications can be easily achieved by satellite communications as it has unique degree of flexibility in interconnecting mobile vehicles.
4. The satellite costs are independent of distance whereas the terrestrial network costs are proportional to the distance.
5. In satellite communication, the quality of transmitted signal and the locations of stations sending and receiving information are independent of distance.
6. For search, rescue and navigation efforts satellite offer the advantages which no other systems can offer.

The disadvantages of satellite communications are,

With satellite in position the communication path between the terrestrial transmitter and the receiver is approximately 75000km long. Since the velocity of EM waves is 3*10⁵ km/s,there is a delay of 1/4 second between transmission and reception of a signal. Thus between talks there is an elapse of 1/2 second.
The delay exacerbates echo which actually is caused by an imperfect impedance matching. Though  a variety of echo suppressors have been deployed  in satellite communications, one may still feel the echo.
The time delay of 1/2 second also reduces the efficiency of satellite in data transmission and long file transfers.

General and Technical characteristics of Satellite Communication System

Consider the general structure of a Satellite Communication System,

This consists of a satellite in space that links many earth stations on the ground. The user is connected the earth station through terrestrial network. This terrestrial network may be a telephone switch or a dedicated link to the earth station. 

The user generates the base-band signal that is processed and transmitted to the satellite at the earth station. Thus, the satellite may be thought of as a large repeater in space that receives the modulated RF carriers in its up-link (earth to space) frequency spectrum from all the earth stations in the network, amplifies these carriers and re-transmits them back to the earth in the down-link (space to earth) frequency spectrum which is different from the up-link frequency spectrum in order to avoid the interference.

The signal at the receiving earth station is processed to get back the baseband signal which is then sent to the user through a terrestrial network.

Had there been no difference in the up-link and down-link frequencies, the satellite transmitted signals would have blocked up the up-link received signals and so there would have been no isolation between the transmitter output and the receiver input. 

On the guidelines of WARC-1979, commercial communication satellite use a frequency band of 500MHz bandwidth near 6GHz for up-link transmission and another 500MHz bandwidth near 4GHz for down-link transmission (i.e. 6/4 GHz band). In fact an up-link of 5.725 to 7.075GHz and a down-link of 3.4 to 4.8GHZ is used. 

The 500MHz allocation is usually divided into 12 channels of approximately 40MHz each and the transmit power per channel is typically of the order of 5 to 10W. This allows each of up to 12 transponders to carry one TV channel or 1500 analog FM voice circuits.

This 6/4 band have been the most popular because they offer the fewest propagation problems and also RF components for these bands have been readily available.

Rain attenuation is also not much serious at these bands. Sky noise is also low at 4GHz and so it is possible to build receiving system with low noise at 4GHz.

With the overcrowding of GEO satellites at 6/4 GHz band, 14/12 GHz band is also being used in commercial communication satellites.

A third band where extremely high capacities are potentially available is the 30/20 GHz band.

Consider the basic block diagram of an earth station,

 The baseband signal from the terrestrial network enters the earth station at the transmitter after having processed by the baseband equipment.

After encoding and modulating the baseband signal, it is converted to the uplink frequency.

Then it is amplified and directed to the appropriate polarization port of the antenna feed.

The signal received from the satellite is amplified in a low noise amplifier first and is then down converted to the downlink frequency. It is then demodulated and decoded and then the original baseband signal is obtained.

The isolation of low noise receiver from the high power transmitter is of much concern in the design consideration of earth station.  There may also be satellite/earth terminal mutual interference effects. Other sources of interference include ground microwaves relay links, sun transit effects and inter-modulation products generated in the transponder or earth terminal.

Before 1983, the spacing between two GEO satellites was established at 4⁰ of the equatorial arc and the smallest earth station antenna for the simultaneous transmit-receive operations was 5m in diameter. Now the spacing allowed between two adjacent satellites in space is 2⁰ along the equatorial arc. The close spacing has allowed twice as many satellites to occupy the same orbital arc.

PN Junction Diode

The PN junction is one of the basic building block of integrated circuits. Such a junction can be formed by selective diffusion or ion implantation of N-type dopants into a P-type(or P-type into N-type) semiconductor sample.

The Fermi level in a P-type semiconductor lies close to the valence band edge, while in an N-type semiconductor it lies close to the conduction band edge. this means that P-region has a higher concentration of holes, while N-region has higher concentration of electrons.

When P-region and N-region are brought in close contact, this large concentration gradient at the junction causes diffusion of carriers.
Holes diffuse from P-region to N-region, electrons diffuse from N to P-region. This process results in some uncompensated donor ions (N_D⁺) in N-region and uncompensated acceptor ions (N_A^-) in P-region near the junction.

Consequently, a negative space charge builds in the P-region and a positive space charge in N-region.
This creates a built-in electric field directed from N-region to P-region which gives rise to a drift current. The direction of drift current will be opposite to that of the diffusion current.

An equilibrium condition is reached where there is no net transport of carriers, as the diffusion component is balanced by an equal and opposite drift component of current.

Far from the junction, electron and hole concentration on both sides remain unaffected. Hence the position of valence band and conduction band with respect to Fermi level also remains the same as they were before the junction was formed.

In the space charge region, the conduction and valence band edges bend accounting for the presence of an electric field. The N-region is at higher electrostatic potential than the P-region. This difference is given by qV_bi, where V_bi is contact potential or built-in potential.

The electric field in the bulk quasi-neutral region is zero.
Φ_p and Φ_n are defined as the electrostatic potential of P-type and N-type quasi-neutral region with respect to the fermi level.
Mathematically expressed as,

The total electrostatic potential difference between the P-side and N-side regions at thermal equilibrium is equal to the built in potential. i.e.

                                              V_bi=  Φ_n -Φ_p