REMOTE KEYLESS ENTRY SYSTEM RECEIVER DESIGN

Transcription

1 INTRODUCTION: REMOTE KEYLESS ENTRY SYSTEM RECEIVER DESIGN Remote keyless entry (RKE) has captivated automobile buyers, as evidenced by the popularity of RKE on new automobiles and as an aftermarket item. RKE systems meet requirements such as range, battery life, reliability, cost, and regulatory compliance. Remote keyless entry (RKE) systems have become extremely popular. The installation rate for RKE systems in new vehicles is more than 80% in North America and more than 70% in Europe. Besides the obvious advantages of convenience, RKE-actuated vehicle-immobilization technology minimizes car theft. European automakers are incorporating the technology in vehicles in cooperation with insurance companies, who in turn, require it as a condition for acquiring auto insurance. That trend began in Germany, and is expected to spread throughout Europe within a few years. Most of these systems employ one-way (simplex) communications. But second- and third-generation systems may talk back to the key, telling that the car needs gas or more pressure in the left front tire. An RKE system consists of an RF transmitter in the key-fob (or key) that sends a short burst of digital data to a receiver in the vehicle, where it is decoded and made to open or close the vehicle doors or the trunk via receiver-controlled actuators. The wireless carrier frequency is currently 315MHz in the US/Japan and MHz (ISM band) in Europe. In Japan the modulation is frequency-shift keying (FSK), but in most other parts of the world, amplitude-shift keying, or ASK is used. The carrier is amplitude modulated between two levels: To save power, the lower level is usually near zero, producing complete on-off keying (OOK). BLOCK DIGRAM BUTTONS MICRO- CONTROLLER RF Tx LIGHTINGS LOCK &UNLOCK RELAYS MICRO- CONTROLLER RF Rx

2 Typical RKE systems include a microcontroller in the key or key fob. You unlock the car by pressing a pushbutton on the key that wakes up the microcontroller. The microcontroller sends a stream of 64 or 128 bits to the key's RF transmitter, where it modulates the carrier and is radiated through a simple printed-circuit loop antenna. (Though inefficient, a loop antenna fabricated as part of the PC board is inexpensive and widely used.) In the vehicle, an RF receiver captures that data and directs it to another microcontroller, which decodes the data and sends an appropriate message to start the engine or open the door. Multi-button key-fobs give the choice of opening the driver's door, or all doors, or the trunk, lightings etc. The digital data stream, transmitted between 2.4kbps and 20kbps, usually consists of a data preamble, a command code, some check bits, and a "rolling code" that ensures vehicle security by altering itself with each use. Without this rolling code, your transmitted signal might accidentally unlock another vehicle or fall into the hands of a car thief who could use it to gain entry later. Several major objectives govern the design of these RKE systems. Like all mass-produced automotive components, they must offer low cost and high reliability. They should minimize power drain in both transmitter and receiver, because replacing batteries in a key-fob is a nuisance and recharging the car battery is a major nuisance. In addition to these requirements, the RKE system designer must juggle receiver sensitivity, carrier tolerance, and other technical parameters to achieve maximum transmission range within the constraints imposed by low cost and minimum supply current. Future developments may also include the technology for tire-pressure sensing (TPS). Like passive RKE, TPS is available at this time only for some trucks and luxury automobiles. TPS systems have much in common with RKE. Circuitry very similar to that of an RKE key-fob resides in the valve stem of each tire, along with a sensor for tire pressure and temperature. Regular transmissions from each tire to a receiver in the vehicle (quite similar to an RKE receiver) then provide the driver with an early warning of any problem developing with the tires. Comfort settings such as driver's preferred seat and steering-wheel position, incar temperature and entertainment requirements, can be stored in the key effective solution against car theft Increased user convenience. Typical applications are high volume low-cost and low-power data transmission systems such as: interconnections of computer peripherals, wireless distributed sensors, domestics, and biomedical circuits. TPS and RKE have so much in common (short range, simple modulation, need to conserve power, etc.), that future systems will probably save cost by sharing and consolidating circuit functions. RKE may, or may not, evolve into a half-duplex system that informs the driver about the state of the car and its need for gas, oil, etc all before the door is opened. It is more likely that RKE, if proven sufficiently robust and reliable, will eventually obsolete the key and its associated door hardware.

3 RECEIVER DESIGN: The super-regenerative receivers is one the oldest receiver technologies which is still widespread for many radio applications. SRRs are used in very large volume applications such as perimeter detection alarms and remote meter reading devices. Perhaps the most well know application is in remote keyless entry devices used for automobile door, garage door and gate openers. This project provides a reference design for a modern, high performance, low-cost SRR suitable for keyless entry applications. Super-regenerative actually refers to the type of detector that the receiver uses to detect and demodulate the base-band signal. The regenerative receiver must not have been very popular with the average consumer, however, who was more interested in listening to the radio broadcast than fiddling with the technology. The regenerative receiver required that the operator constantly adjust the feedback or regen knob in order to bring the detector to the verge of oscillation. Too much feedback would make the receiver would howl violently, and too little weak signals would not be received. The SRR solved this problem by automatically increasing the gain of the detector stage until oscillation just started. As soon as the oscillatory condition started the bias conditions w ould change and the oscillation w ould cease or quench. T his process would continuously bring the detector into its most sensitive region at a rate that was higher than the audio frequency range (hence the term super as in supersonic) but lower than the radio frequency range. The major advantage then, as it is today, of SRR is its low cost. With the exception of single solid state diode detector, the SRR remains the lowest cost receiver another; significant advantage of the SRR is its low power consumption. An externally quenched SRR operating with a current controlled RF stage can be operated at 3 Volts with less than 10uA average current drain. This type of power consumption would allow battery powered receivers to operate in the field for many years. The super-regenerative detector stage operates simultaneously at three frequencies: one is typically a VHF radio frequency, another is the super sonic quench frequency, and the third is the audio or base-band data frequency. The RF frequency of interest (433MHz) is amplitude modulated and will have a signal level -30 to -100dBm.The block diagram of SRR described in this paper has the following major blocks: T he input buffer R F am plifier. The super-regenerative detector. Base band audio frequency amplifier. Voltage comparator.

4 BLOCK DIAGRAM OF THE RECEIVER Antenna IN RF Buffer Amplifier RF Detector Amplifier Voltage Comparator db DET LPF * 50 LPF VC 185 KHz Quench Signal DESCRIPTION: The input buffer RF amplifier establishes the noise figure for the receiver which is an important parameter in determining the ultimate sensitivity and range performance. The RF buffer amplifier also isolates the following RF detector stage from the antenna. Isolation is required in order to prevent RF energy created by the detector RF oscillations from being radiated by the antenna and violating FCC part 15 regulations. Further more, the buffer amplifier prevents impedance variations in the antenna caused by changes in the nearby environment from pulling the RF detector frequency and subsequently resulting in a loss of sensitivity. The gain provided by this stage is should be from 10 to 20dB. The output of the RF buffer amplifier is coupled to the super-regenerative RF stage. The detector essentially functions as an RF amplifier with and AGC port. A tuned circuit provides positive feedback at the RF frequency from the output to the input of the detector. Added to the feedback signal is the coupled RF input from the buffer amplifier. The AGC port is driven from a triangle waveform derived from the external quench oscillator and low-pass filter. The gain of the detector is varied by the AGC signal from zero to its maximum value. At some point, as the gain is increased, the Bark-Hansen criterion will be satisfied and the detector will break into oscillation.

5 DETAILED CIRCUIT DESCRIPTION: Q1 is input RF amplifier/buffer transistor. The antenna signal is coupled by C1 directly to the base of Q1 without the benefit of any tuned circuits. R1, R2, and R3 determine the bias conditions for Q1. R4 is the output load and sets the broadband band gain of this stage. This stage provides approximately 19dB of gain at the operating frequency. AM Model and RF Input Buffer Amplifier The ramping triangle-wave applied to Q2 varies the gain of the detector until sufficient gain is achieved to sustain oscillation at the RF frequency. RF Buffer Amplifier and Detector

6 The onset of oscillation will occur as the gain is increased by the bias ramping. That is, the time that the detector stage is in oscillation is dependent on the quench signal and the received RF signal strength. The length of time that the detector stage is oscillating effects the average value of current in the collector circuit. This collector current develops a voltage drop across collector resistor R8. The voltage drop across R8 will be proportional to the modulation on the input signal. This detected modulation signal is coupled by C6 to the lowpass network, which removes most of the large quench signal which will also be present. Base-band Amplifier and Voltage Comparator The low-pass network has a -3dB corner frequency of 3 KHz which is a suitably low enough assuming the keyless entry code will be approximately 500 to 1000 bits per second. This signal is applied to the inverting input of the opamp. The base-band signal is applied to the non-inverting input. A small amount of positive feedback is provided to add hysteresis to the operation of the comparator. This will help reduce noise in the output as the base-band signal transition the threshold value. The first base-band amplifier is U1A and provides approximately a gain of 50. The output of U1A is further filter by an additional low-pass filter consisting of R and C which has a 3dB corner frequency of 16KHz. Following the first baseband amplifier and low-pass filter is a voltage comparator which is used to detect the data and convert it into appropriate levels suitable for use by the digital data and clock recovery circuit. This compares the incoming base-band signal against the average of the base-band signal over a relatively long time period. R and C integrate and hold the dc average of the base-band signal (and noise) with a time constant of 100mS.

7 mag(detectedsignal[1]) squarewaveoutput mag(amplifiedoutput[1]) mag(input[1]) mag(input[1]) ADS Simulation: The individual stages are simulated using Envelope Detection and transient analysis. The results are shown below Amplified signal Output time, msec time, msec Square signal time, msec Detected SIgnal time, msec

8 Work Done : Each block was designed and simulated using ADS2006A.The appropriate simulations are done separately for each stage of the receiver. The combined circuit and the entire simulation is done using Envelope Detection. Further work includes constructing the layout and EM simulation.the combined circuit diagram is given below.

9 SIMULATION RESULTS:

10 CONCLUSION: The receiver for remote keyless entry system is designed and simulated using Advanced Design System 20006A and the specifications are met. The Software provided the in-built system models for system verification. For example the AM modulated model