I was planning to interface an RF module to my PIC16F877A microcontroller and was exploring the web. I came across many modules which use 433 MHz as the standard frequency. Why is it so? Can't we change the frequency as per our requirements?
The usage of frequencies is restriced by authorities in each country. The 433 MHz ISM band is free to use in many countries and thus very popular.
This is because traditionally, the 433.050 - 434.090 MHz band could be used without license in many countries. For example it is license-free in all European countries. This is nice, since applying for a license is bureaucratic, has to be done nationally, involves a yearly fee, and is restricted to a geographic area.
However, the European Union has tried to harmonize the use of radio frequencies, adding some restrictions to the 433MHz band:
"Short range devices" (generic radio) using 433.050 to 434.040 MHz are not allowed to use an output power greater than 1mW (or 10mW in case they use 10% "duty cycle"). They are allowed to use 10mW from 434.040 to 434.900.
National exceptions to this still exist, and unfortunately radio amateurs are excluded from these power requirements.
And in other countries in North America and Asia, you can't use 433MHz at all. In some countries it is a restricted band for RFID container identification systems only.
Taking into account that whole automatic branch uses 433MHz I don't recommend You to use this frequency. I have done my engineering project with this modules: http://www.ebay.com/itm/433MHz-Radio-Transceiver-Transmitter-Sender-Module-Remote-Arduino-/252184498733?hash=item3ab75e122d:g:MyAAAOSw2xRYWe46 so I know what I'm talking about. If You want to save Your time, use any other frequency, for example 868MHz or 2.4GHz. 433MHz is full of noises. For instance if Your neighbor uses wireless thermometer be sure that You can receive nothing.