– 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 of the board, and comparing these values to the ones on the schematic. So, by knowing the operation of the circuit you start the process of elimination to find the “suspect” component.

– If there are several “suspects”, and this is not a rare occurrence in complex devices, the testing is divided into groups of components, you start checking in reverse soldering order, this means that you start with components last soldered, because those are the most sensitive components on the circuit like integrated circuits, transistors, diodes, etc.

– Check electrolytic capacitors, since they have a somewhat limited lifespan and leave resistors and block capacitors as last in line since they are sturdy little thingies and can take a lot of beating.

– Grid transformers are tested by measuring the resistance of the copper wire on the primary and secondary coil. Since the primary coil has more curls than the secondary one, and is wound using a thinner wire, it’s resistance is higher, and it’s value lays in range between several tens of ohms (in high power transformers) to several hundreds of ohms, even to kilo ohms (in low power transformers). Coils can be tested in the same way as transformers – through their resistance. Infinite resistance still means disconnected coil.

– DC capacitors should produce an infinite value on the instrument. Exceptions are electrolytic and very high value block capacitors. When the positive end of an electrolytic capacitor is connected to a positive probe of an analog instrument, and a negative end to a negative probe, needle jumps to the lowest value and then gradually comes back towards infinity. This is a proof that the capacitor is ok, and the needle’s movement is the charge stored in the component being discharged. (Even small capacitance components get charged while testing, but their discharge time is very short, so the needle doesn’t have the time to move.)

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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:

1. GSM 900 band (850-915MHz up-link frequency and 935-960MHz downlink frequency)
2. GSM 1800 band or digital cellular system (DCS) 1800 band (1710-1785MHz up-link frequency and 1805-1880MHz downlink frequency)
3. Personal Communication service (PCS) 1900 band (1850-1910MHz up-link frequency and 1930-1990MHz downlink frequency)

GSM Mobile communication system can be intelligently used by electronic devices that can collect some data and send it to the central place using SMS or GSM data call. It’s required In-Vehicle Tracking Systems because GPS (Global Positioning System) can normally only receive location information from the satellites but cannot communicate back with them. Hence we need some other communication system like GSM to send this location information to the central control room. Other technologies can also be used but they are more costly.

GSM Network Architecture

GSM Network consists of three main parts:

• Mobile Station (MS) carried by the subscriber
• Base Station Subsystem (BSS) controls radio link with mobile station
• Network & Switching Subsystem (NSS) mobility management and switching of calls between mobile users, and between mobile and fixed network users.

Mobile Station consists of:

• Mobile Equipment (ME) such as hand portable and vehicle mounted unit
• Subscriber Identity Module (SIM), which contains the entire customer related information (identification, secret key for authentication, etc).

Base Station Subsystem consists of:

• Base Transceiver Station (BTS) defines a cell and is responsible for radio link protocols with the Mobile Station
• Base Station Controller (BSC) controls multiple BTSs and manages radio channel setup, and handovers. The BSC is the connection between the Mobile Station and Mobile Switching Center.

Figure 1. Layout of generic GSM network

Network and Switching Subsystems consists of:

• Mobile Switching Center (MSC) is the central component of the NSS. Operates all switching functions for the mobiles within its  jurisdiction. Interface between mobile and other (including fixed) network. Its functions:
• Manages the location of mobiles
• Switches calls
• Manages Security features
• Controls handover between BSCs
• Resource management
• Interworks with and manages network databases
• Collects call billing data and sends to billing system
• Collects traffic statistics for performance monitoring

Network Databases – Home Location Register and Visitor Location Register together with MSC provides the call routing and roaming capabilities of GSM.

• Home Location Register (HLR) contains all the subscriber information for the purposes of call control, and location determination. There is logically one HLR per GSM network, although it may be implemented as a distributed database.
• Visitors Location Register (VLR) is only a temporary storage while the particular subscriber is located in the geographical area controlled by the MSC/VLR. Contains only the necessary information provision of subscribed services.
• Authentication Center (AuC) is a protected database that stores the security information for each subscriber (a copy of the secret key stored in each SIM).
• Equipment Identity Register (EIR) is a list of all valid mobile equipment on the network.

SMS is one of the unique features of GSM compared to older analog systems. For point-to-point SMS, a message can be sent to another subscriber to the service, and an acknowledgment of receipt is sent to the sender. SMS also can be used in Cell Broadcast mode to send messages such as traffic or news updates. Messages can be stored on the SIM card for later retrieval. SMS is effective because it can transmit short messages within 3 to 5 s via the GSM network and doesn’t occupy a telephony channel. Moreover, the cost savings makes it a worthwhile choice. With SMS transmitting, gathering position data is easy and convenient.

THE GSM UNIT:

The GSM unit contains a GSM module along with a GSM transmitter antenna. The module functions according to its built and the antenna transmits the information to the Base Station wherein this is exposed to further processing. GPS is not a two-way system. It can either receive or transmit but not both. Due to its inability in doing so, GSM systems are used.

The GSM module that we are using in this unit is the SIMCOM SIM300 module. Designed for global market, SIM300 is a Tri-band GSM/GPRS engine that works on frequencies EGSM 900 MHz, DCS 1800 MHz and PCS1900 MHz. SIM300 provide RF antenna interface with two alternatives: antenna connector and antenna pad. The antenna connector is MURATA MM9329-2700. And customer’s antenna can be soldered to the antenna pad.

SMS is one of the unique features of GSM compared to older analog systems. For point-to-point SMS, a message can be sent to another subscriber to the service, and an acknowledgment of receipt is sent to the sender. SMS also can be used in Cell Broadcast mode to send messages such as traffic or news updates. Messages can be stored on the SIM card for later retrieval. SMS is effective because it can transmit short messages within 3 to 5s via the GSM network and doesn’t occupy a telephony channel. Moreover, the cost savings makes it a worthwhile choice. With SMS transmitting, gathering position data is easy and convenient.

We use AT commands to control and program the SIMCOM SIM300 module. The data and control commands are exchanged between the PIC microcontroller and GSM module through the serial interface. There are many groups of AT commands, including: Call Control, Data Card Control, Phone Control, Computer Data Card Control, Reporting Operation, Network Communication Parameter, Miscellaneous, and Short Message Service. We use some of the SMS commands to communicate with the control center. The main AT commands for using SMS are listed below.

• A/                 –  Re-issues last AT command given
• ATD              –  Mobile originated call to dialable number
• ATH              – Disconnect existing connection
• AT+CSCA    – Set the SMS center address. Mobile-originated messages are transmitted through this service center.
• AT+CMGS   – Send SMS command
• AT+CMGF   – Select format for incoming and outgoing messages: zero for PDU mode, one for Text  mode.
• AT+CSMS    – Select message service
• AT+CRES     – Restore SMS settings
• AT+CSCB     – Select cell broadcast SMS messages
• AT+CSDH    – Show SMS text mode parameters

Let’s review an example of how to make a GSM module send and read a sample SMS in Text mode. First, initialize the GSM module with AT commands AT+CSCA and AT+CMGF. Using the former sets the SMS center number to be used with outgoing SMS messages. Remember, the number will be saved on the SIM card just like in normal mobile phones. There are two different modes—Text mode and Protocol Data Unit (PDU) mode—for handling short messages. The system default is PDU mode; however, Text mode is easier to understand. So, use the AT+CMGF=1 command to set the module to the GSM 07.05 standard SMS Text mode. The AT+CMGS command is used to send a short message. The GSM module can receive incoming short messages and save them on the SIM card automatically. You can use the AT+CMGR command to read an incoming short message from the SIM card storage, and then use the AT+CMGD command to delete it when you’re finished. If you want to read an SMS message, then send a AT+CMGR=x command to tell the GSM module which short message you want to read. Next, check the serial port to receive the message from the GSM module.

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The GPS has three components namely:

1. The space segment: consisting of 24 satellites orbiting the earth at an altitude      of 11000 nautical miles.
2. The user segment: consisting of a receiver, which is mounted on the unit whose location has to be determined.
3. The control segment: consists of various ground stations controlling the satellites.

The GPS is owned and operated by the U.S Department of Defense but is available for general use around the world. Briefly, here’s how it works:

1. 21 GPS satellites and 3 spare satellites are in orbit at 10,600 miles above the earth. The satellites are spaced so that from any point on earth, 4 satellites will be above the horizon.
2. Each satellite contains a computer, an atomic clock and a radio. With an understanding of its own orbit and the clock, it continually broadcasts its changing position and time. (Once a day, each satellite checks its own sense of time and position with a ground station and makes any minor correction).
3. On the ground, any GPS receiver contains a computer that “triangulates” its own position by getting bearings from 3 or 4 satellites. The result is provided in the form of a geographic position- Longitude and latitude, for most of the receivers, within 100 meters.
4. If the receiver is also equipped with a display screen that shows a map, the position can be shown on the map.
5. If the 4th satellite can be received, the receiver/computer can figure out the altitude as well as the geographic position.
6. If you are moving, your receiver may also be able to calculate your speed and direction of travel and give the estimated times of arrival to specified destinations.

For a GPS receiver to function, it needs to lock onto satellite signals. Each satellite broadcasts two signals at 1.57542GHz and 1.2276GHz, denoted as L1 and L2, respectively. A satellite specific code, known as the course acquisition (C/A) code, is used to discern satellites. Correlation of the transmitted codes against local codes is needed to locate satellites in frequency space. The 1023 bit C/A code modulates the L1 at 1.023MHz, repeating every millisecond. Accumulation of this 1000Hz data is required for a receiver to operate.

Once the GPS receiver made the calculation, it can tell the latitude, the longitude and the altitude of its’ current position. This doesn’t tell much to the average user. So in order to make use of the GPS receiver more user-friendly many receivers send this data to a program which displays a map and can show the position on it. Geographical Information System (GIS) is a computer-based software capable of handling maps and various details given on the map. Data generated by the GPS use spatial data referenced to the earth. In other words this data is the coordinates of its own position expressed in latitude and longitude. This data needs to be positioned on a map of the area for any useful analysis.

GPS is being used in science to provide data that has never been available before in the quantity and degree of accuracy that the GPS makes possible. GPS receivers are becoming consumer products. In addition to their outdoor use, receivers can be used in cars to relate the driver’s location with traffic and weather information.

THE GPS UNIT:

The GPS unit contains a GPS module along with a GPS receiver antenna. The module functions according to its built and the antenna receives the information from the GPS satellite in NMEA (National Marine Electronics Association) format. This data is then sent to the microcontroller wherein it is decoded to the required format and sent further.

GPS ANTENNA:

The AGA Series GPS antenna is a standard product for the GPS system. The circular polarization improves reception ability. The built-in low noise amplifier with very low DC power consumption enhances an already high performance patch array. The antenna has the following features:

• Low noise figure
• High gain
• Ceramic patch antenna
• Water-tight housing
• Temperature and vibration qualified
• Compact size
• Low cost

GPS MODULE:

CPIT GPS module SA3618/SA3618P (patch on top) is a high sensitivity ULTRA LOW power consumption cost efficient, compact size; plug & play GPS module board designed for a broad spectrum of OEM system applications.

The GPS module receiver will track up to 16 satellites at a time while providing fast time-to-first-fix and 1Hz navigation updates. Its superior capability meets the sensitivity & accuracy requirements of car navigation as well as other location-based applications, such as AVL system. Handheld navigator, PDA, pocket PC, or any battery operated navigation system.

The module communicates with application system via RS232 (TTL level) with NMEA0183 protocol.

Main Features:

• Built-in high performance NMEX chipset.
• Average Cold Start in 60 seconds.
• Ultra Low power consumption.( SA3618 27mA typ @ 3.3V )
• 16 channels All-in-View tracking.
• On chip 4Mb flash memory.
• TTL level serial port for GPS receiver command message Interface.
• Compact Board Size

Technical Specifications:

1. Electrical Characteristics
   • General
     Frequency L1, 1575.42 MHz
     C/A code 1.023 MHz chip rate
     Channels 16
   • Sensitivity
     Tracking -152dBm typ
     Acquisition -139dBm typ
   • Accuracy
     Position 5 meters CEP (90%) horizontal, SA off.
     Velocity 0.1 meters/second
     Time 1 microsecond synchronized to GPS time
2. Dynamic Conditions
   Altitude 10,000 meters max
   Horizontal Velocity 300 kilometers/hour max
   Vertical Velocity 36 kilometers/hour max
   Acceleration 2g, max
   Jerk 4 meters/second3, max
3. Power
   Main power input 3.3 ±5% VDC input.
   Supply Current SA3618 26 mA @ 3.3V (Continuous mode)
4. Serial Port
   Electrical interface one full duplex serial communication, TTL interface
   Protocol message NMEA-0183, version 3.0 optional.
   Default NMEA GGA, GSA, GSV, RMC and VTC. 9600 baud rate, 8 bits data, 1 start, 1 stop, no parity.
5. Recommended External Antenna Specification
   Gain 20dB (including cable loss)
   Noise figure 1.5dB
   Current 3 ~ 30mA
   Operate Voltage 2.5 ~ 2.8V

Serial Interface
Communication to the SA3618 is provided via a serial interface. A 10-pin 1.27mm whole connector is used. Pin 6 (Reset) is the active-low reset input. The SA3618 always requires a reset at power-up, or it will not start properly. An optional onboard reset circuit can be provided. A reset forces the SA3618 processor to reboot, but will not influence other parameters such as hot or cold start. Pin 1 (GPIO [4]) and pin 10 (GPIO [0]) are spare pins that can be used e.g. to control power modes, to indicate SA3618 status, or to force a cold start. They can be left unconnected if desired.
I/O voltage level is set to 2.7V.

PIN DESCRIPTION OF SA3618

GETTING GPS DATA:

After the GPS Module computes the positioning and other useful information, it then transmits the data in some standard format. With differential GPS signal input the accuracy ranges from 1 to 5 m; however, without differential input, the accuracy can be 25 m.

About 60 s after the GPS module is cold booted it begins to output a set of data (according to the NMEA format) though port C once every second at 9600 bps, 8 data bits, one stop bit, and no parity. NMEA GPS messages include six groups of data sets: GGA, GGL, GSA, GSV, RMC, and VTG. We use only the most useful RMC message- Recommended Minimum Specific GNSS Data-which contains all of the basic information required to build a navigation system.

We only need position and time data, so the UTC position, longitude with east west indicator, and latitude with north/south indicator are picked out from the RMC message. All of this data will be formatted into a standard fixed length packet with some other helpful information. Next, this data packet will be transmitted to the control center and stored in the micro controller.

Here’s a sample of how the GPS receiver antenna receives information from the GPS satellite in NMEA format:

NMEA format sample:
The GPS module that we are using in this unit is SA3618 and the GPS receiving antenna used is G-501.

SA3618 NMEA Protocol
The SA3618 software is capable of supporting the following NMEA message
Formats:

* (1): 1sec output 1msg, (3): 3sec output 1msg, 9600 baud rate (Standard output)

General NMEA Format:

The general NMEA (National Marine Electronics Association) format consists of an ASCII string commencing with a. $. Character and terminating with a <CR><LF> sequence. NMEA standard messages commence with .GP. then a 3-letter message identifier. NemeriX specific messages commence with $PNMRX followed by a 3 digit number. The message header is followed by a comma delimited list of fields optionally terminated with a checksum consisting of an asterix .*. and a 2 digit hex value representing the checksum. There is no comma preceding the checksum field. When present, the checksum is calculated as a bitwise exclusive of the characters between the. $. and.*.. As an ASCII representation, the number of digits in each number will vary depending on the number and precision, hence the record length will vary. Certain fields may be omitted if they are not used, in which case the field position is reserved using commas to ensure correct interpretation of subsequent fields. The tables below indicate the maximum and minimum widths of the fields to allow for buffer size allocation.

$GPRMC: This message transfers recommended minimum specific GNSS data. The $GPRMC message format is as shown below

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THE TRANSMITTER UNIT:

The transmitter consists of a code hopping encoder, HCS301 from Microchip Technology. The HCS301 utilizes the KEELOQ code hopping technology, which incorporates high security, a small package security, a small package outline and low cost, to make this device a perfect solution for unidirectional remote keyless entry systems and access control systems.

The HCS301 combines a 32-bit hopping code, generated by a nonlinear encryption algorithm, with a 28-bit serial number and 6 information bits to create a 66-bit code word. The code word length eliminates the threat of code scanning and the code hopping mechanism makes each transmission unique, thus rendering code capture and resend schemes useless. The crypt key, serial number and configuration data are stored in an EEPROM array, which is not accessible via any external connection. The EEPROM data is programmable but read-protected. The data can be verified only after an automatic erase and programming operation. This protects against attempts to gain access to keys or manipulate synchronization values. The HCS301 provides an easy-to-use serial interface for programming the necessary keys, system parameters and configuration data.

Security

• Programmable 28-bit serial number
• Programmable 64-bit encryption key
• Each transmission is unique
• 66-bit transmission code length
• 32-bit hopping code
• 34-bit fixed code (28-bit serial number,
• 4-bit button code, 2-bit status)
• Encryption keys are read protected

Operating

• 3.5V – 13.0V operation
• Four button inputs
• No additional circuitry required
• 15 functions available
• Selectable baud rate
• Automatic code word completion
• Battery low signal transmitted to receiver
• Battery low indication on LED
• Non-volatile synchronization data

Other

• Functionally identical to HCS300
• Easy-to-use programming interface
• On-chip EEPROM
• On-chip oscillator and timing components
• Button inputs have internal pull-down resistors
• Current limiting on LED output
• Low external component cost

The HCS301 is a simple device to use. It requires only the addition of buttons and RF circuitry for use as the transmitter in your security application. The HCS301 will wake-up upon detecting a button press and delay approximately 10 ms for button debounce. The synchronization counter, discrimination value and button information will be encrypted to form the hopping code. The hopping code portion will change every transmission, even if the same button is pushed again. A code word that has been transmitted will not repeat for more than 64K transmissions. This provides more than 18 years of use before a code is repeated; based on 10 operations per day. Overflow information sent from the encoder can be used to extend the number of unique transmissions to more than 192K. If in the transmit process it is detected that a new button(s) has been pressed, a RESET will immediately occur and the current code word will not be completed. Please note that buttons removed will not have any effect on the code word unless no buttons remain pressed; in which case the code word will be completed and the power-down will occur.

Code Word Format:

The HCS301 code word is made up of several parts. Each code word contains a 50% duty cycle preamble, a header, 32 bits of encrypted data and 34 bits of fixed data followed by a guard period before another code word can begin.

Code Word Organization:

The HCS301 transmits a 66-bit code word when a button is pressed. The 66-bit word is constructed from a Fixed Code portion and an Encrypted Code portion. The 32 bits of Encrypted Data are generated from 4 button bits, 12 discrimination bits and the 16-bit sync value. The encrypted portion alone provides up to four billion changing code combinations. The 34 bits of Fixed Code Data are made up of 2 status bits, 4 button bits and the 28-bit serial number. The fixed and encrypted sections combined increase the number of code combinations to 7.38 x 1019.

Code Word Completion:

The code word completion feature ensures that entire code words are transmitted, even if the button is released before the code word is complete. If the button is held down beyond the time for one code word, multiple code words will result. If another button is activated during a transmission, the active transmission will be aborted and a new transmission will begin using the new button information.

THE RECEIVER UNIT:

The receiver unit consists of the RX3400, which is a low power ASK receiver IC, and is suitable for use in a variety of low power radio applications including remote keyless entry. The RX3400 is based on a single-conversion, super-heterodyne receiver architecture and incorporates an entire phase-locked loop (PLL) for precise local oscillator generation.
Features

• Extremely low power operation
• Low external part count
• Receiver input frequency: 290 – 460 MHz
• On-chip VCO with integrated PLL using crystal oscillator reference
• PLL power down feature
• Integrated IF and data filters
• SSOP-24 package (0.64 mm pitch)

Functional Description

The RX3400 ASK receiver IC incorporates an LNA; mixer; PLL-based local oscillator including VCO, fixed divider (÷ 64), reference crystal oscillator, phase-frequency detector (PFD), and charge pump; IF filter; logarithmic amplifier; data filter; peak detector; and 1- bit comparator and is capable of demodulating ASK input signals.

PLL Power-down Function

The PLL portion of the IC can be powered up and down through the control of the PD input (pin 14). During PLL power down operation (pin 14 pull low), the reference crystal oscillator, fixed VCO divider, PFD, and charge pump are all shut off and the current consumption of the IC drops by approximately 600mA. The VCO circuitry remains on and may be configured to operate as a buffer amplifier for an external SAW-based oscillator.

1-bit Comparator

The integrated 1-bit comparator operates as a data slicer and “squares up” the data filtered RSSI output from the logarithmic amplifier. The decision threshold voltage level for the 1-bit comparator is stored on an external capacitor connected to the ASKREF pin.

The post RF UNIT first appeared on Student Projects.

High state is represented by a M in modulated signal and low state is represented by a wave which appears like W in modulated signal DPSK encodes two distinct signals of same frequency with 180 degree phase difference between the two. This experiment requires two 180 degree out of phase carrier and modulating signals. Sine wave from oscillator is selected as carrier signal. DSG converts DC input voltage into pulse trains. These pulse trains are taken as modulating signals. In actual practice modulating signal is digital form of voice or data. Sine wave is selected as carrier and 180 degree phase shift is obtained using Opamp as shown in figure below. Different methods are used to demodulate DPSK. The analog scheme is the PLL (Phase Locked loop).

DPSK Modulation:

In DPSK, during HIGH state of the modulating signal flead signal is allowed to pass and during LOW state of the modulating signal flag signal is allowed to pass.  Figure below shows DPSK [10] modulator circuit. The Opamp is tied in the inverting amplifier mode.  The closed loop voltage gain of the Opamp is given by

RF + rDS (on) 3
AV(CL)  =   – ———————
RI + rDS (on) 1,2

Where:      rDS (on) 3   is the drain- source resistance of Q3 FET
rDS (on) 1,2  is drain-resistance of  the conducting FET(Q1 or Q2)

The drain source resistance is of the order of 100Ω which is very small compared to RF and RI.

Hence

RF
AV(CL)  =   –  ——
RI

DPSK Demodulation:

DPSK Demodulation [12,13 & 14]is done with PLL IC 565[3 4 5]. DPSK [10] signal is given as input at DPSK input terminal of PLL as shown in the figure below.
A capacitor C is connected between pin7 and power supply forms first order low pass filter with an internal resistance 3.6KW, The capacitor C should be large enough to eliminate variations in the demodulated output voltage in order to stabilize the VCO frequency. The cut-off frequency of Low pass filter is made equal to carrier frequency. The cutoff frequency of low pass filter is given by

1
fH    = ———-
2pRC

R = 3.6KW, fH = 18.7KHz

The value of C designed by

1
C  =   ———-
2pRfH

1
C  =   ———————   =  2.3nF
2px3.6Kx18.7K

C selected is 3nF

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