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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Chips for radio modems. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Civil radio communications Data transmission over a radio channel over short distances is becoming increasingly common in everyday life. “Radio keys” for car alarms and remote control of various objects have already become common, “radio mice” and “radio keyboards” of computers, etc. are gaining popularity. The time has come for wireless networking of computers. This article will introduce readers to specialized microcircuits designed to solve such problems. Until recently, everyone who first saw the "rear" side of the system unit of an operating computer was amazed by the web of wires and cables attached to it, which went to a considerable number of devices interacting with the computer. The introduction of the USB bus, which bypasses all devices in series, simplifies the cable network, but does not completely solve the problem. Attempts to use infrared radiation for communication between a computer and its periphery are not very successful, since line of sight is required between the source and receiver of infrared rays, and the real range of reliable communication does not exceed two meters. In addition, competing hardware manufacturers have not developed a single data exchange protocol. Therefore, the presence of an IrDA adapter in your computer does not guarantee the ability to communicate with it with any of the IrDA-equipped devices. Recently, the idea of organizing a "near" connection between computers located in one or neighboring rooms and devices interacting with them (printers, scanners, modems, etc.) via a radio channel has been gaining more and more development. However, despite the apparent simplicity and obviousness of such an approach, there are so many difficulties in its implementation that the problem still cannot be considered solved. At least, the goal proclaimed by some developers "we add one chip to each computer and peripheral device - and it's in the bag" is still very far away. Nevertheless, the process has begun. Attempts are being made to develop common technologies and protocols for "local" computer radio communications. The most famous of these are Bluetooth, IEEE 802.11, UWB and Nome RF compete with each other. To identify the winner, evaluating in practice the declared advantages and disadvantages of the proposed technologies, will be in the near future. In the meantime, manufacturers of nodes required for communication over any protocol - microcircuits of microwave transceivers (transceivers) - focusing on one of the protocols, nevertheless, lay the possibility of using others. In this article, we will talk about some of these microcircuits. Norwegian firm BlueChip Communications AS produces single-chip microcircuits of BCC418 and BCC918 radio transceivers, which are characterized by micro-power consumption of energy, the ability to operate in a wide temperature range (from -40 to +85 ° C) and are intended mainly for digital data exchange in radio networks in the 400 and 900 MHz bands. The main applications for these transceivers are remote sensors used in industry, security systems and medicine. In addition, they can be used in environmental monitoring systems, low-speed computer radio networks, remote barcode readers, bi-directional paging, etc. The microcircuits are similar in internal structure and parameters, they are produced in TQFP-44 plastic packages (dimensions 12x12 mm) with a four-sided pin arrangement and differ only in that BCC418 covers the range of 300..600 MHz, and BCC918 - 700..1100 MHz. The operating frequency and other modes of operation of transceiver microcircuits are set using an 80-bit command entered by a serial binary code into a special microcircuit register. To ensure the flexibility of using these microcircuits, it is possible to program eight levels of transmitter output power (interval - 3 dB, maximum level - 10 mW), two (for BCC418) or four (for BCC918) gain values of the receiver input stages (allows you to reduce the sensitivity by 25 ..33 dB), as well as four LPF bandwidths (10, 30, 60 or 200 kHz). Other features of the construction of these transceivers include the use of a direct frequency conversion method in the receiver, the presence of a two-channel frequency synthesizer with an external PLL loop that provides a very dense frequency grid (hundreds of hertz), outputs of the LockDet capture detector and the level of the received RSSI signal, as well as a built-in tunable seven-pole elliptical gyratory low-pass filter of the receiver. To transmit information, carrier frequency shift keying (FSK) is used with a deviation, which is selected in accordance with the required data reception / transmission rate. The maximum baud rate supported by the BCC transceiver chips is 128 kBaud. For speeds of 9,6 kBaud and less, the recommended deviation is ±25 kHz. With a receiver sensitivity of -105 dBm and omnidirectional antennas, this guarantees a communication range in open space up to 700 m. Rated supply voltage - 3 V. Current consumption in transmit mode - no more than 50 mA, in receive mode - 8 mA, in standby mode - less than 2 uA. The transmitter master oscillator and the receiver local oscillator are a frequency synthesizer consisting of a voltage controlled oscillator (VCO), two programmable frequency dividers and a PLL. To stabilize the frequency of the synthesizer, it is recommended to use a high-quality quartz resonator with a frequency of 10 MHz. Depending on the required data transfer rate, the BCC transceiver microcircuits provide the possibility of using one of four methods for manipulating the transmitter frequency - by changing the division factor of one of the synthesizer counters, switching between two programmed frequency dividers, modulating (pulling) the frequency of the reference quartz resonator or direct VCO modulation. The receiving part is made according to the scheme of direct frequency conversion and contains a digital frequency detector. Demodulation is performed by comparing the phases of the received signal in the in-phase I and quadrature Q channels. If in channel I it lags behind Q, the signal frequency is higher than the local oscillator frequency, if it is ahead, it is lower than it. The so-called "jitter" inherent in such schemes (front jitter) of the received data, as a rule, does not create any problems when receiving digital data, however, its value must be taken into account in cases where the moment of arrival of the signal front is important. The jitter decreases with increasing frequency deviation ΔF, while its maximum value does not exceed 1/(4ΔF). The PLL tunes the local oscillator to the center frequency of the signal, so the transmitted code sequence must contain an equal number of logical zeros and ones in order to avoid glitches. This requirement, common for digital communication systems, must be taken into account when choosing a method for encoding transmitted data. BlueChip Communications recommends using the Manchester or 4BXNUMXB block code for this purpose. To control the operation of the PLL in VSS transceivers, it is possible to use the specially provided LockDet output - a capture detector. The constant voltage at the RSSI output is proportional to the logarithm of the signal power at the receiver input, and this dependence is maintained over a dynamic range of about 70 dB. A typical circuit for switching on the BCC418 chip is shown in fig. 1. Varicap D1 and its environment - VCO and PLL elements. The quartz resonator ZQ1, as already mentioned, sets the exemplary frequency. The inductors and most of the capacitors on the right side of the circuit are included in the microwave circuit for matching the input and output of the transceiver with the WA1 antenna. The R15D3L3D2 circuit is used to switch the antenna to the receiver input or transmitter output of the transceiver chip.
On the basis of BCC418 and BCC918 microcircuits, RFB433, RFB868 and RFB915 microwave modules are produced, built according to schemes similar to the one discussed above (Fig. 1). They are approximately 25x25x3 mm in size and have terminals adapted for surface mounting. The modules are optimized (configured by the manufacturer) for a transmission rate of 19,2 kBaud and operation, respectively, in the ISM bands 433,4 ... 434,4 MHz, 868,8 ... 869 MHz and 903 ... 927 MHz, while they can operate over a wider frequency range. A matched antenna (with a feeder impedance of 50...100 Ohm) can be connected directly to the modules, without additional microwave elements. The abbreviation ISM is used to designate ranges designed to work on the radiation of equipment for industrial (Industrial), scientific (Scientific) and medical (Medical) purposes. In Europe and the USA, no license is required to operate on these bands. BlueChip Communications offers hardware developers debug boards (Evaluation Kits, a set of 2 pieces) containing a microwave module, a printed antenna and a PIC16LC63A microcontroller. Using the software supplied with the boards, it is possible to organize two-way data transfer between two computers remote at a distance of up to 300 m. and to an external matched antenna. The radio modem is configured for a data rate of 433 kBaud and uses ten operating frequencies in the range of 232...6 MHz, automatically scanned at a rate of 9 ms. Other companies also produce ISM transceivers. For example, Texas Instrument manufactures TRF6900 and TRF6901 chips in the PQFP-48 package. The first of them covers the frequency band 850...950 MHz, the second - 860...930 MHz. Transmitter power - 3 mW, receiver noise figure - 3,3 dB. The external digital transceiver interface is oriented to the MSP430 microcontroller of the same company. The American company Atmel Corporation, known for its memory chips and microcontrollers, did not stand aside. . Having joined the Bluetooth Association (by the way, the name comes from the nickname of King Harald, who ruled Denmark and Norway in the 76th century), she developed a number of microcircuits in support of this protocol. The most complex of them is the AT511C176 protocol controller. Suffice it to say that it is made in a 32-pin package, contains a 7-bit ARM256TDMI RISC computing core, and to perform all the functions provided by Bluetooth, it requires XNUMX KB of external RAM and the same amount of FLASH or other non-volatile memory. To communicate with a computer, the AT76C511 chip is equipped with three different interfaces: USB, PCMCIA and a UART 16550 emulator. In the future, it is planned to release simplified versions, each of which will have only one interface. The controller organizes radio communication by "commanding" the microwave module - the T2901 microcircuit of the same company. Communication is conducted on 79 fixed frequencies in the range of 2400...2500 MHz. According to the Bluetooth protocol, the operating frequency changes abruptly every 625 µs, and the law of change is known to the subscribers who have established a connection, and is unpredictable for others. As a result, two or more communication channels, operating simultaneously in the same frequency band, do not interfere with each other. Rare failures caused by random short-term coincidence of transmitter frequencies are quickly eliminated by the multi-level system of error-correcting data coding and error correction provided by the protocol. True, the "net" data exchange rate of 1 Mbps as a result is reduced by approximately 20%. A typical circuit for switching on the T2901 chip is shown in fig. 2, the numerous 4,7 pF bypass capacitors connected to all power and control pins are not shown. The reference frequency signal is applied to pin 1 (CLK). It is possible to programmatically select one of four possible values. Transmitter power - 1 mW. The information is transmitted by carrier frequency shift keying with a nominal deviation of ±160 kHz. The modulating signal can be pre-filtered using the built-in Gaussian low-pass filter. This filter is turned on and off by switch SW1.
The receiver in this case is a conventional superheterodyne with an intermediate frequency of 111 MHz. Its noise figure is 12 dB. Selectivity is provided by the SAW filter F1, oscillatory circuits with coils L2 and L3 - elements of the IF and frequency discriminator. Transistor Q1 is part of the internal voltage regulator. The current consumed by the microcircuit is almost independent of the receive / transmit mode, amounting to approximately 60 mA, and only in the standby mode decreases to tens of microamperes. An interesting feature of the T2901 microcircuit device is that the transmitter signal is generated at a double frequency (4800 ... 5000 MHz), which is divided by two before being sent to the output. The receiver's demodulator also operates at half the intermediate frequency, 55,5 MHz. To increase the output power and sensitivity of the T2901 transceiver, Atmel offers additional microcircuits for a microwave power amplifier (T7023) and a similar amplifier combined with a low-noise input (T7024). Their feature is the presence of a special input for adjusting the output power, which allows you to smoothly turn on and off the transmitter, set the minimum power level of the emitted signal sufficient to maintain communication. These measures minimize the interference created by other communication channels operating in the same range. The output power of both microcircuits is 200 mW, the noise figure of the T7024 microcircuit is no more than 2,3 dB. Author: A. Dolgiy, Moscow
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