•  1) What is 'square law device'?What is it's use?
• 2)What is 'bandpass filter centred'?
• 3)What is the purpose of 'IF stage'?

Hi Hasna!

(1) A square law device is one that produces an output voltage or current that is proportional to the square of its input voltage or current. Square law devices are mainly used in modulators and detectors. Example: The detector diode used in communication systems, FETs used in balanced modulator etc.

(2) The band pass filter used in the modulator is centered at the frequency of the carrier wave.

(3) IF means intermediate frequency. In the receiver, the carrier frequency is shifted to the intermediate frequency by the help of a local oscillator. This shifting is done because the frequency response of amplifiers will be low in the high frequency range. You will study more deeply about these things in your higher grades of electronics engineering.