Decibel (dB)

Decibel is the logarithm unit used to describe a ratio. The ratio may be power, voltage sound or several other things. The decibel is also commonly used as a measure of gain or attenuation. It is dimensionless number.

Mathematically,

                Power ratio in dB             =             10 log₁₀ (P₂/P₁)   

Where P₁ and P₂ are the two power level to be compared.  If power level P₂ is higher then P₁ , the decibel is positive and vice versa. Since

                                P             =             V²/R

The voltage definition of decibel is given by

                Voltage ratio in dB           =             20 log₁₀ (V₂/V₁)   

The Decibel was originally named for Alexandra Graham Bell. The unit was used as a measure of attenuation in telephone cable, i.e. the ratio of the power of the signal emerging from one end of a cable to the power of the signal fed in at the other end. It so happened that one decibel almost equaled the attenuation of one mile of telephone cable.

The decibel is used for a wide variety of measurements in science and engineering, most prominently in acoustics, electronics, and control theory. In electronics, the gains of amplifiers, attenuation of signals, and signal to noise ratio are often expressed in decibels.

Decibel as absolute unit

Decibel can be used to express values of power. All that necessary is to establish some absolute unit of power as a reference. The often used reference units are 1mW and 1W. If 1mW is used as a reference, dBm is expressed as decibel relative to 1mW

                                P(in dBm)            =             10 log P(in mW)

If 1W used as a reference, dBw is expressed as decibel relative to 1W. i.e.

                                P(in dBw)              =             10 log P(in W)

Difference between dBm and dBc

dBm :- It indicates the power measurement relative to 1 milliwatt. dBm is the relative difference to a fixed reference power of 1 mW.

dBc:- It indicates the power relative to the power of the main carrier frequency; typically used to describe spurs, noise, channel crosstalk, and intermodal signals which may interfere with the carrier. dBc is the relative difference to an arbitrary reference power level, the power level of the carrier frequency.

Gas Discharge Tube (GDT)

GDT is a sealed glass-enclosed device containing a special gas mixture trapped between two electrodes, which conduct electric current after becoming ionized by a high voltage spike. It is often considered a slow operating overvoltage protector, but these devices are more intended to handle impulse currents many times higher than faster technologies such as solid states (semiconductor) devices. GDTs can conduct more current for their size than other components. It has a finite life expectancy, and can handle a few very large transients or a greater number of smaller transients.

Working

When a voltage applied across GDT is below breakdown voltage, it remains in high impedance off state condition (i.e. poor conductor). But when the applied voltage exceeds the breakdown voltage, the electric voltage is strong enough to ionize the gas making it low impedance on state (i.e. good conductor) allowing current passing through it to the ground line until voltage reaches normal.  

Use

GDTs are commonly used to help protect sensitive telecom equipment such as power lines, communication lines, signal lines and data transmission lines from damage caused by transient

surge voltages that typically result from lightning strikes and equipment switching operations.

Some terminology related to GDT

Arc Mode: The mode when the GDT is in the low impedance conducting state.

Arc Voltage: The voltage drop across GDT during its low impedance conducting state.

Arc Current: The current that flows through the GDT after breakdown that exceeds the glow to arc transition current during its low impedance conducting state i.e. arc mode.

DC Breakdown Voltage or Sparkover voltage: The voltage at which GDT sparks over or it starts conducting. The DC breakdown voltage is highly dependent upon the time rate of change of applied voltage.

Glow Mode:  The transition state of an energized gas between the high impedance state and the arc mode, in this mode the gas is partially ionized.

Glow Voltage: The voltage across the GDT in the glow state.

Impulse Breakdown Voltage: It represents the device reaction to fast rising voltage transients.

Impulse reset: The maximum DC voltage across the GDT at which the gap clears a given voltage/current condition.

Directive Gain and Directivity of Antenna

The Directive Gain (D_G) is defined as the ratio of radiation intensity due to the test antenna to isotropic antenna (hypothetical antenna that radiates uniformly in all direction).

                       

                                    D_G =     U        =    4πU 

                                                 U₀             P_rad

Where,

U   = radiation intensity due to test antenna, in watts per unit solid angle

U₀  = radiation intensity due to isotropic antenna, in watts per unit solid angle

P_rad =total power radiated in watts

Since U is a directional dependent quantity, the directive gain of an antenna depends on the angle θ and Φ. If the radiation intensity assumes its maximum value then the directive gain is called the Directivity (D₀).

i.e.                               D_G =     U_max    =   4πU_max

                                                   U₀              P_rad

Gain of an Antenna

Power Gain of an antenna is defined as the ratio of its radiation intensity at a point to the radiation intensity that results from a uniform radiation of the same input power.

i.e.                               Gain = 4π x radiation intensity           = 4π (θ, Φ)

                                                    Total input power                     P_in

Whereas, relative gain is defined as the ratio of power gain of the test antenna in a given direction to a power gain of reference antenna (whose gain can calculated or known). In most of cases reference antenna is a lossless isotropic source.

i.e.                               Gain =                  4π (θ, Φ)

                                                 P_in (lossless isotropic antenna)

 When the direction is not stated, the power gain is usually taken in the direction of maximum radiation.

ISO 9000 series

ISO includes following quality standards

1. ISO 9000: Quality management and quality assurance standards- guidelines for selection and use.

2. ISO 9001: Quality systems model for quality assurance in design/development, production,          installation, and servicing.

3. ISO 9002: Quality systems model for quality assurance in production and installation.

4. ISO 9003: Quality systems model for quality assurance in final inspection and test.

5. ISO 9004: Quality management and quality system elements guidelines (provide guideline and suggestion to help management develop and implement the kind of quality management system required in standards 9001 through 9003.

5S Concept

‘5S’ referring to Japanese names for five dimensions of workplace organization. The five ‘5S’ roughly translated into:-

1. SEIRI (proper arrangement and organization)

            Do things in the proper order, eliminate unnecessary things.

2. SEITON (orderliness)

            Specify a location for everything; designate location by number, color coding, name, etc; put things where they belong.

3. SEISO (cleanup)

            Specify recommended procedures for cleanup; follow the procedures; check over all work.

4. SEIKETSU (cleanliness)

            Dust, wash, and maintain equipment; keep equipment and the workplace in the best possible condition.

5. SHITSUKE (discipline)

            Scrutinize practices; expose the wrong ones; learn correct practices and be careful to use them.