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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Encoder and decoder of the radio channel of the guard. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Automobile. Security devices and alarms

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Many radio amateurs and motorists already know that now the internal affairs bodies are allowed to mount on a car and operate electronic watchdog devices with a radio channel.

Unlike the widely used sound signal guards, the watchman with a radio channel does not give an alarm to the entire district, but only to the owner (although, if necessary, he is able to duplicate the radio signal with loud sound and light signals).

Having received an alarm signal over the radio channel, the owner takes actions corresponding to specific circumstances, in particular, calls the police and reports an attempt to open the car or dismantle its components. If, despite the measures taken, the theft did occur, then there remains a real possibility of finding the car in "hot pursuit" by police officers equipped with the necessary equipment.

The radio channel autoguard consists of two blocks - transmitting and receiving.

The transmitting unit includes the guard itself with the necessary set of sensors, an encoder and a transmitter with a radiating antenna. This block is mounted on the car. The power source can be either an on-board battery or its own built-in battery.

The receiving unit consists of a receiving antenna, a receiver, a decoder and an audible alarm generator. This unit is made either as a self-powered miniature pocket design or as a desktop high-sensitivity mains-powered receiver.

In the general case, when the auto-guard is triggered, the transmitter begins to emit a radio signal modulated by a pulse code generated by the encoder. The receiver with the decoder extracts "its" code signal from the mass of on-air signals and turns on the generator of alarm signals.

In reality, there can be many options for organizing a radio channel due to the variety of practical tasks. But in all cases, the parameters of the radio channel must meet the technical requirements established by the State Telecommunications Inspectorate. Here are the main ones:

  • Operating frequency, kHz ....... 26 945
  • Permissible relative deviation of the frequency of the transmitter and the local oscillator of the receiver, no more ....... 50 * 10 ^-6
  • Emission class......AID; FID; POD
  • Carrier transmitter power, W, no more.......2
  • Transmitter frequency deviation, kHz, no more.......3
  • Radiation bandwidth, kHz, not more than -30 dB.......12
  • The accuracy of the initial setting of the frequency of the transmitter and the local oscillator of the receiver, not worse ....... 20 * 10 ^-6
  • The level of spurious emissions, dB, not more than.......-40

According to the "Radio Regulations" of the International Telecommunication Union (vol. 1, "Radio and Communication". M., 1985), it is customary to designate radiation classes with three symbols. The first - a letter - indicates the type of modulation of the main carrier. The second - a figure - on the nature of the signals modulating the main carrier. The third - a letter - the type of transmitted information.

In relation to our case, the letter A denotes two-sideband modulation, the letter F - frequency, P - a sequence of unmodulated pulses. The number 1 corresponds to a variant with one channel containing quantized or digital information without the use of a modulating subcarrier (excluding time division of channels), and the number 0 corresponds to the absence of a modulating signal. And, finally, the letter D is assigned to the case of the transmission of digital information, telemetry signals, telecontrol.

It is easy to see that the requirements presented here relate mainly to the transmitter. This is understandable - after all, the possibility of joint simultaneous operation of several security systems will largely depend on its quality. The characteristics of the receiver can be any, as long as it provides reliable communication in specific operating conditions and would not itself be a source of interference.

The listed requirements, apparently, are not final, and as the development of this technique will be refined.

The most complex nodes of the radio channel are the encoder and decoder. Therefore, the editors decided, following the tradition, to begin acquaintance with the radio channel guard with an article about these nodes. In the future, it is planned to publish descriptions of the remaining nodes of the radio guard.

The introduction of a radio channel into an electronic security alarm system dramatically expands its capabilities and will require the designer to solve a difficult task - to ensure the reliable selection of one radio signal among many others, including signals of a similar purpose.

To do this, it would seem that it is enough to find a "quiet" section in one or another radio band and radiate only one carrier in it. Then the disappearance of the carrier will serve as an alarm signal. Or vice versa - the appearance of a carrier will be an alarm signal. Such a radio system is quite simple to implement. However, it turns out to be unusable. Firstly, because there are practically no "quiet" sections in the modern radio spectrum; secondly, it is not protected in any way from blocking by even the most primitive means, from interference that provokes false calls, it will quickly disappoint its creator; thirdly, such use of the air will probably come into conflict with the legislation on radio communications*.

Another way is to modulate the carrier with a tone signal. But here, too, the difficulties of creating filters with the necessary selectivity and accurate in their frequency position do not allow placing any significant number of channels in the radio receiver passband: usually no more than 10-15, which means the same number of protected objects. Low, of course, and noise immunity of such systems.

The carrier can also be modulated (keyed) with a pulsed signal. Such encryption systems are used, but for the most part in very simple forms: a variety of signals is achieved by varying the pulse width (PWM), their number, etc. The capabilities of such systems are also relatively small, especially in strictly time-limited transmissions.

One of the possible principles for constructing a cipher signal with a large combinatorial "capacity" is that the time allotted for transmission is divided into equal intervals - familiarity, each of which corresponds to either 0 or 1. If for 1 we take the presence of high-frequency radiation in transmitter antenna, and for 0 - its absence, then such a cipher signal will look like a very short radio telegraph message.

In a binary sequence consisting of n character spaces, 2 "different cipher messages can be placed. True, in addition to the information part itself, such a message usually contains auxiliary bits (start, for example), which simplify its decryption.

Figure 1 shows a schematic diagram of an encoder that implements this principle.

Encoder and decoder of the radio channel of the guard
Fig.1 (click to enlarge)

The encoder contains a low-frequency oscillator stabilized by quartz (DD5.3, DD5.4, ZQ1), a trigger (DD4.3. DD4.4), which changes its state when the watchdog node is triggered (with at least a short-term high level at the "Signal" input ), node switching the system to standby mode (SB1, DD4.1, DD4.2) and the counter DD1, which controls the operation of the switches DD2 and DD3.

One or another cipher combination is dialed by connecting the information inputs of the switches D02, DD3 with a positive power wire or with a common wire. The initial (zero) familiarity of the cipher combination is always occupied by one - the start bit (a high level is applied to pin 14 of the DD2 switch). Acquaintances 1,2,..., 14 (according to the numbers of the terminals of the bundle) follow in time in this order.

The encoder controls the operation of the radio transmitter with signals from the output of elements DD5.2 and DD6.4. When a low level appears at the output of the DD5.2 element, the transmitter is powered on. A diagram of one of the variants of the power-on node is shown in Fig. 2.

Encoder and decoder of the radio channel of the guard
Ris.2

The signals from the output of the element DD6.4 control the operation of the high-frequency path of the transmitter. The manipulating signal can be fed into the emitter circuit of the transistor of the intermediate or output stage through the buffer transistor VT2 (Fig. 3).

Encoder and decoder of the radio channel of the guard
Ris.3

The transmission of the cipher combination is possible only in the "Code" position of switch SA1. The position "Continuous emission" is intended for control of the mode and settings of the transmitter.

In armed mode, the "Signal" input is low; trigger DD4.3, DD4.4 by pressing the button SB1 is set to state 0, in which the clock generator is inhibited, and the counter DD1 goes to the zero state, in which there is a low voltage at its outputs. As a result, the output of the switch DD2 is low (as at the input of XO), and the output of the switch DD3 is in a state of high resistance. Transmitter power and manipulator are off.

After the watchdog node is triggered, the level at the "Signal" input changes from zero to one, the trigger DD4.3, DD4.4 switches to state 1, the transmitter is powered on and the clock generator starts to work. The counter DD1 and the switches generate a cipher combination of pulses corresponding to the position of the jumpers of the contact field X1. This cipher combination through the opened element DD6.4 enters the transmitter's manipulator.

In an encoder with a "clock" quartz resonator in a clock generator, the duration of one familiarity will be approximately equal to 1,95 ms. The duration of the entire cipher combination is 30 ms, the pauses between them are about 470 ms. The duration of the pause is determined by the lifetime of the high-level signal at the output of the diode-resistor assembly VD1 - VD4.R9. By eliminating, for example, the VD4 diode, the duration can be reduced to about 220 ms. The total number of possible cipher combinations is 2^14 = 16384.

To work at a higher speed, you only need to replace the "clock" quartz resonator with a higher frequency one. However, this will obviously lead to a corresponding expansion of the bandwidth occupied by the radio channel, up to the exit from the allowed boundaries, and the insufficiency of the FSS bandwidth of the radio receiver.

The current consumed by the encoder in standby mode at a supply voltage of 9 V does not exceed 1...2 μA. The signal amplitude of the security node should not be less than 4 V. The encoder remains operational when the supply voltage drops to 5 V.

The selection of "own" cipher signal against the background of various kinds of interference in the communication channel is assigned to the decoder. Its schematic diagram is shown in Fig.4.

Encoder and decoder of the radio channel of the guard
Fig.4 (click to enlarge)

The decoder consists of a clock generator, assembled on the elements DD5.3, DD5.4 and stabilized by a quartz resonator ZQ1 (at the same frequency as the quartz resonator of the encoder), a trigger DD4.1, DD4.3, switched by the front of the on-air signal, a comparator DA1 , amplifying and shaping this signal, the node for switching the decoder to standby mode (SB1, R7, C3, DD6.1) and the counter DD1, which controls the operation of the switches DD2 and DD3, just like in the encoder.

In addition, the decoder includes a node for comparing the cipher combination received from the air with the one installed in the decoder. The comparison node is assembled on the elements DD5.2, DD6.2, DD7.1, DD7.2, DD7.3.

The decoder is put into standby mode by pressing the SB1 button, while a high-level pulse occurs at the output of the DD6.1 element, setting the trigger DD4.1, DD4.3 to state O and resetting the counter DD1. Element DD5.1 ​​closes and does not pass the pulses of the operating clock generator to the input C of the counter DD1, due to which its outputs remain low.

As soon as the output of the inverter DD4.4 appear pulses of the cipher combination received from the air, the trigger DD4.3, DD4.1 switches, the element DD5.1 ​​opens and the counter DD1 starts counting the pulses of the clock generator. Switches DD2, DD3 produce an exemplary cipher combination of pulses corresponding to the position of the jumpers of the contact field X1.

The actual comparison of the ethereal and exemplary cipher combination occurs on the element DD7.3. It goes bit by bit, starting from the start bit, followed by gating the result with the element DD6.2. The strobe pulse taken from the output of the DD7.2 element occupies the second quarter of each familiarity, which makes it possible to neglect some advance of the received cipher combination in relation to that installed in the decoder and the disparity in the frequency values ​​​​of the clock generators of the encoder and decoder.

The first mismatch of the cipher combination switches the decoder to its original state. If the cipher combinations are identical, a high level appears at the output 2 ^ 10 of the counter DD1.

This signal includes an alarm signaling unit, the scheme of which is shown in Fig.5.

Encoder and decoder of the radio channel of the guard
Fig.5 (click to enlarge)

The signal node consists of two generators: one, assembled on the elements DD1.1, DD1.2, operates at a frequency of 007 ... 5 Hz, and the other - DD0,5, DD1 - at a frequency of 1.3.. .1.4 kHz. As a result of the joint work of both generators, the BF1 acoustic piezo emitter reproduces short disturbing tonal bursts, alternating with pauses of the same duration. If a high volume of the alarm signal is needed, instead of the piezoelectric BF2, a power amplifier is turned on based on the transistor VT1 loaded with the dynamic head BF1. Head power - not less than 1 W, resistance - 2 Ohm.

The current consumed by the decoder and the signal node in standby mode at a supply voltage of 9 V is 1,2 mA. In the alarm mode, the decoder consumes 5 mA if the sound emitter is a piezoelectric element, and 60 mA if the sound emitter is a dynamic head 0,5 ГДШ-9. The decoder remains operational when the supply voltage drops to 5 V.

The signal at the input of the decoder (at the output of the detector of the radio receiver) must have a positive polarity and an amplitude of at least 150 mV.

See other articles Section Automobile. Security devices and alarms.

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