-3db !!?? what is it?

Lets consider an example:
To get frequency upto 100Hz and to reject all other higher frequencies, what will you do?
You will use a low pass filter of cut-off frequency 100Hz?! right ?
So now at the input of this filter you send a random signal of power say P (in watts).
In ideal case, at the output of the filter, you will expect to get signal which consists only 0-100 Hz frequency signals of power P, implying that the filter succeeded in attenuating all signals of frequency greater than 100Hz to 0 power and retaining full power of all signals with frequency ranging from 0-100Hz. But this does not happen in practical case, the signal begins to attenuate gradually even before 100Hz and attenuates sharply after 100Hz. Infact, for 100Hz (cut-off frequency), the power of the output signal is half the power of signal at the input of the filter ie., P/2 watts.
We know,

Plot of Gain vs Frequency for a low pass filter

Power Gain = Output Power / Input Power
Power Gain in db = 10 log (Output Power / Input Power)

what did you except from your low pass filter !

You expected that for all frequencies from 0-100Hz;
Output Power = Input Power.
or in terms of Gain:
Gain in db = 10 log (1)
               = 0 db

what did you actually get from your low pass filter !

For 100 Hz frequency, you found:
Output Power = Input Power/2
or in terms of Gain: 
Gain in db = 10 log (1/2)
               = -3 db

Conclusion:
Saying, at cut-off frequency gain is -3db, is just another way of saying that for the cut-off frequency, power of signal at the output is reduced to half the power supplied at the input. 
(This statement is valid for all kinds and types of filter at cut-off frequency)

Happy Learning !

p.s: Hope this cleared your concept ! If not, do comment what you aren't still clear about. Will be glad to give my best try.

Understanding TCP/IP addressing and subnetting basics

The following link clearly describes the entire concept. :)

Understanding TCP/IP addressing and subnetting basics

Happy Networking !! :)

p.s: will upload the gist of it soon, till then you can go through the entire article, it is interesting and clear in approach and not much time consuming.

Understanding frequency domain concept

French mathematician and physicist Baron Jean Baptiste Joseph Fourier (1768-1830) 

realized that any complex waveform could be decomposed into a group of sinusoids of 

different frequencies and amplitudes.

What does this exactly mean ?

lets take an example:

A sine wave with a period of 1 millisecond, has Frequency of 1000 Hz.Then its time domain and frequency domain graphs are given.We notice the frequency domain graph gives the amplitude and the frequency content of the wave, which is only a single frequency in this case.

Frequency Content:
Suppose we add another sine wave of the same amplitude with a frequency of 2000 Hz to our 1000 Hz sine wave.The complex wave represents the sum of Frequencies 1 and 2 as they rise and fall at different times with different slopes. 

Now, think what Ol’ Joe Fourier said. The complex waveform is comprised of 

simple sine waves of various amplitudes. 

Surprised? Isn’t it easier to see the complex waveform in the frequency domain.

Time domain: What we see is a waveform that varies in amplitude as time moves.
Frequency Domain: What we see is a waveform that varies in amplitude corresponding to its frequency content.

The spectral display gives us much more useful information than the time domain display 

(oscillograph). It can tell us which frequencies are dominant and which are not.

The continuous band of high amplitude will tell us the dominant frequency band etc. It is important to remember that each vertical bar is a single sine wave.
You might be thinking, that there may be a complex wave where one frequency content exists for a certain period of time and then other frequency content is present for another period of time; for example, the image below shows a signal with four frequency contents 100Hz, 50Hz, 25Hz and 10Hz  Will fourier transform of such a signal will be, same as when all the frequencies were present throughout the signal?? Yes, absolutely! as the figures given below. 

Thus a signal in frequency domain, obtained from Fourier transform does NOT give any information, as to when a frequency component was present in time. 

Important: Remember that the two figures (time and frequency domain representation) represent the same thing: a complex waveform, first in the time domain and second in the frequency domain. Do not try to relate what you see in the time-domain display to what you see in the frequency-domain display. They are totally different domains. 





Happy learning !! :)

FLIP FLOP FLIP FLOP ;)

Flip Flop is a bi-stable multivibrator, used to store 1 bit information.

A multivibrator is an electronic circuit used to implement a variety of simple two-state systems such as oscillators, timers and flip-flops. It is characterized by two amplifying devices (transistors, electron tubes or other devices) cross-coupled by resistors or capacitors.
bistable, in which the circuit is stable in either state. The circuit can be flipped from one state to the other by an external event or trigger.
Latch is different from Flip Flop as it is asynchronous(no clock input) and is level triggered whereas,
Flip Flop is synchronous(has clock input) and is edge triggered.
RS(Reset-Set), JK, T(toggle), D(Delay), Master Slave etc

Handy Note below gives an overview of the effect on the status of the output pin with the clock pulse depending on the status of flip flop inputs :

SR Flip Flop

00 - no change

01 - 0 (reset)

10 - 1 (set)

11 - not allowed

JK Flip Flop

00 - no change

01 - 0 (reset)

10 - 1 (set)

11 - T (toggle)

D Flip Flop

0 - 0

1 - 1

T Flip Flop

0 - no change

1 - T (toggle)

Master Slave Flip Flop

can be D,SR,JK,T.. only that output is obtained on the falling edge of the clock in the case of positive edge triggered flip flop.

Master Slave flip-flop is created by connecting two flip flops in series, and inverting the enable input to one of them. It is called master–slave because the second flip flop in the series only changes in response to a change in the first (master) flip flop.

The term pulse-triggered means that data is entered on the rising edge of the clock pulse, but the output does not reflect the change until the falling edge of the clock pulse.

Have a Happy Day :)

p.s: this is an extremely limited information about flip flops, only to present an overall easy to memorize picture.