Electronic and Communications Interview Questions - Part 3
What is a repeater ?
A repeater is an electronic device that receives a signal and retransmits it at a higher level and/or higher power, or onto the other side of an obstruction, so that the signal can cover longer distances without degradation.
What is attenuation ?
Attenuation is the reduction in amplitude and intensity of a signal. Signals may attenuate exponentially by transmission through a medium, or by increments calculated in electronic circuitry or set by variable controls. Attenuation is an important property in telecommunications and ultrasound applications because of its importance in determining signal strength as a function of distance. Attenuation is usually measured in units of decibels per unit length of medium (dB/cm, dB/km, etc) and is represented by the attenuation coefficient of the medium in question.
What is multiplexing ?
Multiplexing (known as muxing) is a term used to refer to a processwhere multiple analog message signals or digital data streams are combined into one signal over a shared medium. The aim is to share an expensive resource. For example, in telecommunications, several phone calls may be transferred using one wire.
What is CDMA, TDMA, FDMA ?
Code division multiple access (CDMA) is a channel access methodutilized by various radio communication technologies. CDMA employsspread-spectrum technology and a special coding scheme (where each transmitter is assigned a code) to allow multiple users to be multiplexed over the same physical channel. By contrast, time division multiple access (TDMA) divides access by time, whilefrequency-division multiple access (FDMA) divides it byfrequency. An analogy to the problem of multiple access is a room (channel) in which people wish to communicate with each other. To avoid confusion, people could take turns speaking (time division), speak at different pitches (frequency division), or speak in different directions (spatial division). In CDMA, they would speak different languages. People speaking the same language can understand each other, but not other people. Similarly, in radio CDMA, each group of users is given a shared code. Many codes occupy the same channel, but only users associated with a particular code can understand each other.
Difference between CDMA and GSM.
These are the two different means of mobile communication being presently used worldwide. The basic difference lies in the Multiplexing method used in the aerial communication i.e. from Mobile Tower to your mobile and vice versa.
CDMA uses Code Division Multiple Access as the name itself indicates, for example you are in a hall occupied with number of people speaking different language. You will find that the one language you know will be heard by you and the others will be treated like noise. In the same manner each CDMA mobile communication takes place with a "code" communicating between them and the other end if one is knowing that code then only it can listen to the data being transmitted i.e. the communication is in the coded form.
On the other hand GSM (Global System for Mobile Communications)uses narrowband TDMA, which allows eight simultaneous calls on the same radio frequency. TDMA works by dividing a radio frequency into time slots and then allocating slots to multiple calls. In this way, a single frequency can support multiple, simultaneous data channels.
What is an Amplifier ?
An electronic device or electrical circuit that is used to boost (amplify) the power, voltage or current of an applied signal is called amplifier.
What is Barkhausen criteria ?
Barkhausen criteria, without which you will not know which conditions, are to be satisfied for oscillations. Oscillations will not be sustained if, at the oscillator frequency, the magnitude of the product of the transfer gain of the amplifier and the magnitude of the feedback factor of the feedback network ( the magnitude of the loop gain ) are less than unity. The condition of unity loop gain -Aß = 1 is called the Barkhausencriterion. This condition implies that | Aß|= 1 and that the phase of - Aß is zero.
Explain Full duplex and half duplex.
Full duplex refers to the transmission of data in two directions simultaneously. For example, a telephone is a full-duplex devicebecause both parties can talk at once. In contrast, a walkie-talkie is ahalf-duplex device because only one party can transmit at a time. Most modems have a switch that lets you choose between full-duplex and half-duplex modes. The choice depends on whichcommunications program you are running. In full-duplex mode, data you transmit does not appear on yourscreen until it has been received and sent back by the other party. This enables you to validate that the data has been accurately transmitted. If your display screen shows two of each character, it probably means that your modem is set to half-duplex mode when it should be in full-duplex mode.