Short notes about Circuit troubleshooting.
- Check the circuit for mechanical failures, like disconnected wire, broken vias on the board, and connections between two adjacent vias which are not to be connected.
- You should compare values of components and their connections with values and connections between components on the schematic.
- Measuring DC voltages at certain points [...]

GSM – AN OVERVIEW
GSM, Global System for Mobile communications, is today the most successful digital mobile telecommunication system. This second-generation (2G) system provides voice and limited data services and uses digital modulation with improved audio quality.
The different versions of GSM are:

GSM 900 band (850-915MHz up-link frequency and 935-960MHz downlink frequency)
GSM 1800 band or digital cellular [...]

GPS – AN OVERVIEW
The GPS (Global Positioning System) is a “constellation” of 24 well-spaced satellites that orbit the earth and make it possible for people with ground receivers to pinpoint their geographic location. The location accuracy is anywhere from 100 to 10 meters for most equipment. Accuracy can be pinpointed to within 1 meter with [...]

The RF section consists of an RF transmitter and an RF receiver.  The RF transmitter mainly consists of a remote key that contains a code hopping encoder, HCS301 along with 2 buttons and two LEDs. The RF receiver section consists of a Low Power ASK receiver IC. The key encoder will generate the company specific [...]

Differential phase shift keying (DPSK), a common form of phase modulation conveys data by changing the phase of carrier wave. In Phase shift keying, High state contains only one cycle but DPSK contains one and half cycle. Figure illustrates PSK and DPSK Modulated signal by 10101110 pulse sequence