CDMA

Code division multiple access (CDMA) adalah sebuah bentuk pemultipleksan (bukan sebuah skema pemodulasian) dan sebuah metode akses secara bersama yang membagi kanal tidak berdasarkan waktu (seperti pada TDMA) atau frekuensi (seperti pada FDMA), namun dengan cara mengkodekan data dengan sebuah kode khusus yang diasosiasikan dengan tiap kanal yang ada dan menggunakan sifat-sifat interferensi konstruktif dari kode-kode khusus itu untuk melakukan pemultipleksan.

CDMA juga mengacu pada sistem telepon seluler digital yang menggunakan skema akses secara bersama ini,seperti yang diprakarsai oleh Qualcomm.

CDMA adalah sebuah teknologi militer yang digunakan pertama kali pada Perang Dunia II oleh sekutu Inggris untuk menggagalkan usaha Jerman mengganggu transmisi mereka. Sekutu memutuskan untuk mentransmisikan tidak hanya pada satu frekuensi, namun pada beberapa frekuensi, menyulitkan Jerman untuk menangkap sinyal yang lengkap.

Sejak itu CDMA digunakan dalam banyak sistem komunikasi, termasuk pada Global Positioning System (GPS) dan pada sistem satelit OmniTRACS untuk logistik transportasi. Sistem terakhir didesain dan dibangun oleh Qualcomm, dan menjadi cikal bakal yang membantu insinyur-insinyur Qualcomm untuk menemukan Soft Handoff dan kendali tenaga cepat, teknologi yang diperlukan untuk menjadikan CDMA praktis dan efisien untuk komunikasi seluler terrestrial.

Keuntungan CDMA

Teknologi CDMA sendiri memiliki berbagai keuntungan jika diaplikasikan dalam sistem seluler. Keuntungan-keuntungan tersebut antara lain : hanya membutuhkan satu frekuensi yang dibutuhkan untuk beberapa sektor/cell tidak membutuhkan equalizer untuk mengatasi gangguan spektrum sinyal dapat bergabung dengan metode akses lainnya, tidak membutuhkan penghitung waktu (guard time) untuk melihat rentang waktu dan penjaga pita (guard band) untuk menjaga intervensi antarkanal tidak membutuhkan alokasi dan pengelolaan frekuensi memiliki kapasitas yang halus untuk membatasi para pengguna akses memiliki proteksi dari proses penyadapan.

GSM

Global System for Mobile Communication disingkat GSM adalah sebuah teknologi komunikasi selular yang bersifat digital. Teknologi GSM banyak diterapkan pada komunikasi bergerak, khususnya telepon genggam. Teknologi ini memanfaatkan gelombang mikro dan pengiriman sinyal yang dibagi berdasarkan waktu, sehingga sinyal informasi yang dikirim akan sampai pada tujuan. GSM dijadikan standar global untuk komunikasi selular sekaligus sebagai teknologi selular yang paling banyak digunakan orang di seluruh dunia.

Sejarah dan perkembangan GSM

Teknologi komunikasi selular sebenarnya sudah berkembang dan banyak digunakan pada awal tahun 1980-an, diantaranya sistem C-NET yang dikembangkan di Jerman dan Portugal oleh Siemens, sistem RC-2000 yang dikembangkan di Prancis, sistem NMT yang dikembangkan di Belanda dan Skandinavia oleh Ericsson, serta sistem TACS yang beroperasi di Inggris. Namun teknologinya yang masih analog membuat sistem yang digunakan bersifat regional sehingga sistem antara negara satu dengan yang lain tidak saling kompatibel dan menyebabkan mobilitas pengguna terbatas pada suatu area sistem teknologi tertentu saja (tidak bisa melakukan roaming antar negara).

Teknologi analog yang berkembang, semakin tidak sesuai dengan perkembangan masyarakat Eropa yang semakin dinamis, maka untuk mengatasi keterbatasannya, negara-negara Eropa membentuk sebuah organisasi pada tahun 1982 yang bertujuan untuk menentukan standar-standar komunikasi selular yang dapat digunakan di semua negara Eropa. Organisasi ini dinamakan Group Special Mobile (GSM). Organisasi ini memelopori munculnya teknologi digital selular yang kemudian dikenal dengan nama Global System for Mobile Communication atau GSM.

GSM muncul pada pertengahan 1991 dan akhirnya dijadikan standar telekomunikasi selular untuk seluruh Eropa oleh ETSI (European Telecomunication Standard Institute). Pengoperasian GSM secara komersil baru dapat dimulai pada awal kuartal terakhir 1992 karena GSM merupakan teknologi yang kompleks dan butuh pengkajian yang mendalam untuk bisa dijadikan standar. Pada September 1992, standar type approval untuk handphone disepakati dengan mempertimbangkan dan memasukkan puluhan item pengujian dalam memproduksi GSM. Pada awal pengoperasiannya, GSM telah mengantisipasi perkembangan jumlah penggunanya yang sangat pesat dan arah pelayanan per area yang tinggi, sehingga arah perkembangan teknologi GSM adalah DCS (Digital Cellular System) pada alokasi frekuensi 1800 Mhz. Dengan frekuensi tersebut, akan dicapai kapasitas pelanggan yang semakin besar per satuan sel. Selain itu, dengan luas sel yang semakin kecil akan dapat menurunkan kekuatan daya pancar handphone, sehingga bahaya radiasi yang timbul terhadap organ kepala akan dapat di kurangi. Pemakaian GSM kemudian meluas ke Asia dan Amerika, termasuk Indonesia. Indonesia awalnya menggunakan sistem telepon selular analog yang bernama AMPS (Advances Mobile Phone System) dan NMT (Nordic Mobile Telephone). Namun dengan hadir dan dijadikannnya standar sistem komunikasi selular membuat sistem analog perlahan menghilang, tidak hanya di Indonesia, tapi juga di Eropa. Pengguna GSM pun semakin lama semakin bertambah. Pada akhir tahun 2005, pelanggan GSM di dunia telah mencapai 1,5 triliun pelanggan. Akhirnya GSM tumbuh dan berkembang sebagai sistem telekomunikasi seluler yang paling banyak digunakan di seluruh dunia. (Mela Dewinta 0606094516).

Spesifikasi teknis GSM

Di Eropa, pada awalnya GSM didesain untuk beroperasi pada frekuensi 900 Mhz. Pada frekuensi ini, frekuensi uplinks-nya digunakan frekuensi 890–915 MHz , sedangkan frekuensi downlinksnya menggunakan frekuensi 935–960 MHz. Bandwith yang digunakan adalah 25 Mhz (915–890 = 960–935 = 25 Mhz), dan lebar kanal sebesar 200 Khz. Dari keduanya, maka didapatkan 125 kanal, dimana 124 kanal digunakan untuk suara dan satu kanal untuk sinyal. Pada perkembangannya, jumlah kanal 124 semakin tidak mencukupi dalam pemenuhan kebutuhan yang disebabkan pesatnya pertambahan jumlah pengguna. Untuk memenuhi kebutuhan kanal yang lebih banyak, maka regulator GSM di Eropa mencoba menggunakan tambahan frekuensi untuk GSM pada band frekuensi di range 1800 Mhz dengan frekuensi 1710-1785 Mhz sebagai frekuensi uplinks dan frekuensi 1805-1880 Mhz sebagai frekuensi downlinks. GSM dengan frekuensinya yang baru ini kemudian dikenal dengan sebutan GSM 1800, yang menyediakan bandwidth sebesar 75 Mhz (1880-1805 = 1785–1710 = 75 Mhz). Dengan lebar kanal yang tetap sama yaitu 200 Khz sama, pada saat GSM pada frekuensi 900 Mhz, maka pada GSM 1800 ini akan tersedia sebanyak 375 kanal. Di Eropa, standar-standar GSM kemudian juga digunakan untuk komunikasi railway, yang kemudian dikenal dengan nama GSM-R.

Arsitektur jaringan GSM

Secara umum, network element dalam arsitektur jaringan GSM dapat dibagi menjadi:

Mobile Station (MS)

Base Station Sub-system (BSS)

Network Sub-system (NSS),

Operation and Support System (OSS)

Secara bersama-sama, keseluruhan network element di atas akan membentuk sebuah PLMN (Public Land Mobile Network).

Mobile Station atau MS merupakan perangkat yang digunakan oleh pelanggan untuk melakukan pembicaraan. Terdiri atas:

Mobile Equipment (ME) atau handset, merupakan perangkat GSM yang berada di sisi pengguna atau pelanggan yang berfungsi sebagai terminal transceiver (pengirim dan penerima sinyal) untuk berkomunikasi dengan perangkat GSM lainnya.

Subscriber Identity Module (SIM) atau SIM Card, merupakan kartu yang berisi seluruh informasi pelanggan dan beberapa informasi pelayanan. ME tidak akan dapat digunakan tanpa SIM didalamnya, kecuali untuk panggilan darurat. Data yang disimpan dalam SIM secara umum, adalah:

IMMSI (International Mobile Subscriber Identity), merupakan penomoran pelanggan.

MSISDN (Mobile Subscriber ISDN), nomor yang merupakan nomor panggil pelanggan.

Base Station System atau BSS, terdiri atas:

BTS Base Transceiver Station, perangkat GSM yang berhubungan langsung dengan MS dan berfungsi sebagai pengirim dan penerima sinyal.

BSC Base Station Controller, perangkat yang mengontrol kerja BTS-BTS yang berada di bawahnya dan sebagai penghubung BTS dan MSC

Network Sub System atau NSS, terdiri atas:

Mobile Switching Center atau MSC, merupakan sebuah network element central dalam sebuah jaringan GSM. MSC sebagai inti dari jaringan seluler, dimana MSC berperan untuk interkoneksi hubungan pembicaraan, baik antar selular maupun dengan jaringan kabel PSTN, ataupun dengan jaringan data.

Home Location Register atau HLR, yang berfungsi sebagai sebuah database untuk menyimpan semua data dan informasi mengenai pelanggan agar tersimpan secara permanen.

Visitor Location Register atau VLR, yang berfungsi untuk menyimpan data dan informasi pelanggan.

Authentication Center atau AuC, yang diperlukan untuk menyimpan semua data yang dibutuhkan untuk memeriksa keabsahaan pelanggan. Sehingga pembicaraan pelanggan yang tidak sah dapat dihindarkan.

Equipment Identity Registration atau EIR, yang memuat data-data pelanggan.

Operation and Support System atau OSS, merupakan sub sistem jaringan GSM yang berfungsi sebagai pusat pengendalian, diantaranya fault management, configuration management, performance management, dan inventory management.

Frekuensi pada 3 Operator Terbesar di Indonesia

Indosat : 890 – 900 Mhz (10 Mhz)

Telkomsel : 900 – 907,5 Mhz (7,5 Mhz)

Excelcomindo : 907,5 – 915 Mhz (7,5 Mhz)

Keunggulan GSM sebagai Teknologi Generasi Kedua (2G)

GSM, sebagai sistem telekomunikasi selular digital memiliki keunggulan yang jauh lebih banyak dibanding sistem analog, di antaranya:

Kapasitas sistem lebih besar, karena menggunakan teknologi digital dimana penggunaan sebuah kanal tidak hanya diperuntukkan bagi satu pengguna saja. Sehingga saat pengguna tidak mengirimkan informasi, kanal dapat digunakan oleh pengguna lain.

Sifatnya yang sebagai standar internasional memungkinkan international roaming

Dengan teknologi digital, tidak hanya mengantarkan suara, tapi memungkinkan servis lain seperti teks, gambar, dan video.

Keamanan sistem yang lebih baik

Kualitas suara lebih jernih dan peka.

Mobile (dapat dibawa kemana-mana)

Bagaimanapun, keunggulan GSM yang beragam pantas saja membuatnya menjadi sistem telekomunikasi selular terbesar penggunanya di seluruh dunia.

Pengendali mikro

Pengendali mikro (Inggris: microcontroller) adalah sistem mikroprosesor lengkap yang terkandung di dalam sebuah chip. Mikrokontroler berbeda dari mikroprosesor serba guna yang digunakan dalam sebuah PC, karena sebuah mikrokontroler umumnya telah berisi komponen pendukung sistem minimal mikroprosesor, yakni memori dan antarmuka I/O.

Rancangan embedded

Sistem komputer dewasa ini paling banyak justru terdapat di dalam peralatan lain, seperti telepon, jam, perangkat rumah tangga, kendaraan, dan bangunan. Sistem embedded biasanya mengandung syarat minimal sebuah sistem mikroprosesor yaitu memori untuk data dan program, serta sistem antarmuka input/output yang sederhana. Antarmuka semacam keyboard, tampilan, disket, atau printer yang umumnya ada pada sebuah komputer pribadi justru tidak ada pada sistem mikrokontroler. Sistem mikrokontroler lebih banyak melakukan pekerjaan-pekerjaan sederhana yang penting seperti mengendalikan motor, saklar, resistor variabel, atau perangkat elektronis lain. Seringkali satu-satunya bentuk antarmuka yang ada pada sebuah sistem mikrokontroler hanyalah sebuah LED, bahkan ini pun bisa dihilangkan jika tuntutan konsumsi daya listrik mengharuskan demikian

Integrasi yang lebih padat

Berbeda dengan CPU serba-guna, mikrokontroler tidak selalu memerlukan memori eksternal, sehingga mikrokontroler dapat dibuat lebih murah dalam kemasan yang lebih kecil dengan jumlah pin yang lebih sedikit.

Sebuah chip mikrokontroler umumnya memiliki fitur:

* central processing unit - mulai dari prosesor 4-bit yang sederhana hingga prosesor kinerja tinggi 64-bit.

* input/output antarmuka jaringan seperti port serial (UART)

* antarmuka komunikasi serial lain seperti I²C, Serial Peripheral Interface and Controller Area Network untuk sambungan sistem

* periferal seperti timer dan watchdog

* RAM untuk penyimpanan data

* ROM, EPROM, EEPROM atau Flash memory untuk menyimpan program komputer

* pembangkit clock - biasanya berupa resonator rangkaian RC

* pengubah analog-ke-digital

Sejarah mikrokontroler

Mikrokontroler populer yang pertama dibuat oleh Intel pada tahun 1976, yaitu mikrokontroler 8-bit Intel 8748. [1]Mikrokontroler tersebut adalah bagian dari keluarga mikrokontroler MCS-48. [2] Sebelumnya, Texas instruments telah memasarkan mikrokontroler 4-bit pertama yaitu TMS 1000 pada tahun 1974. TMS 1000 yang mulai dibuat sejak 1971 adalah mikrokomputer dalam sebuah chip, lengkap dengan RAM dan ROM.

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Printed circuit board

A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board (PWB) or etched wiring board. A PCB populated with electronic components is a printed circuit assembly (PCA), also known as a printed circuit board assembly (PCBA).



PCBs are inexpensive, and can be highly reliable. They require much more layout effort and higher initial cost than either wire-wrapped or point-to-point constructed circuits, but are much cheaper and faster for high-volume production. Much of the electronics industry's PCB design, assembly, and quality control needs are set by standards that are published by the IPC organization.

History


The inventor of the printed circuit was the Austrian engineer Paul Eisler (1907–1995) who, while working in England, made one circa 1936 as part of a radio set. Around 1943 the USA began to use the technology on a large scale to make rugged radios for use in World War II. After the war, in 1948, the USA released the invention for commercial use. Printed circuits did not become commonplace in consumer electronics until the mid-1950s, after the Auto-Sembly process was developed by the United States Army.

Before printed circuits (and for a while after their invention), point-to-point construction was used. For prototypes, or small production runs, wire wrap or turret board can be more efficient. Predating the printed circuit invention, and similar in spirit, was John Sargrove's 1936-1947 Electronic Circuit Making Equipment (ECME) which sprayed metal onto a Bakelite plastic board. The ECME could produce 3 radios per minute.

Originally, every electronic component had wire leads, and the PCB had holes drilled for each wire of each component. The components' leads were then passed through the holes and soldered to the PCB trace. This method of assembly is called through-hole construction. In 1949, Moe Abramson and Stanislaus F. Danko of the United States Army Signal Corps developed the Auto-Sembly process in which component leads were inserted into a copper foil interconnection pattern and dip soldered. With the development of board lamination and etching techniques, this concept evolved into the standard printed circuit board fabrication process in use today. Soldering could be done automatically by passing the board over a ripple, or wave, of molten solder in a wave-soldering machine. However, the wires and holes are wasteful since drilling holes is expensive and the protruding wires are merely cut off.

In recent years, the use of surface mount parts has gained popularity as the demand for smaller electronics packaging and greater functionality has grown.

Manufacturing

Materials

Conducting layers are typically made of thin copper foil. Insulating layers dielectric are typically laminated together with epoxy resin prepreg. The board is typically coated with a solder mask that is green in color. Other colors that are normally available are blue and red. There are quite a few different dielectrics that can be chosen to provide different insulating values depending on the requirements of the circuit. Some of these dielectrics are polytetrafluoroethylene (Teflon), FR-4, FR-1, CEM-1 or CEM-3. Well known prepreg materials used in the PCB industry are FR-2 (Phenolic cotton paper), FR-3 (Cotton paper and epoxy), FR-4 (Woven glass and epoxy), FR-5 (Woven glass and epoxy), FR-6 (Matte glass and polyester), G-10 (Woven glass and epoxy), CEM-1 (Cotton paper and epoxy), CEM-2 (Cotton paper and epoxy), CEM-3 (Woven glass and epoxy), CEM-4 (Woven glass and epoxy), CEM-5 (Woven glass and polyester). Thermal expansion is an important consideration especially with BGA and naked die technologies, and glass fiber offers the best dimensional stability.
Typical density of a raw PCB (an average amount of traces, holes, and vias, with no components) is 2.15g / cm3

Patterning (etching)

The vast majority of printed circuit boards are made by bonding a layer of copper over the entire substrate, sometimes on both sides, (creating a "blank PCB") then removing unwanted copper after applying a temporary mask (e.g. by etching), leaving only the desired copper traces. A few PCBs are made by adding traces to the bare substrate (or a substrate with a very thin layer of copper) usually by a complex process of multiple electroplating steps.

There are three common "subtractive" methods (methods that remove copper) used for the production of printed circuit boards:
Silk screen printing uses etch-resistant inks to protect the copper foil. Subsequent etching removes the unwanted copper. Alternatively, the ink may be conductive, printed on a blank (non-conductive) board. The latter technique is also used in the manufacture of hybrid circuits.
Photoengraving uses a photomask and chemical etching to remove the copper foil from the substrate. The photomask is usually prepared with a photoplotter from data produced by a technician using CAM, or computer-aided manufacturing software. Laser-printed transparencies are typically employed for phototools; however, direct laser imaging techniques are being employed to replace phototools for high-resolution requirements.
PCB milling uses a two or three-axis mechanical milling system to mill away the copper foil from the substrate. A PCB milling machine (referred to as a 'PCB Prototyper') operates in a similar way to a plotter, receiving commands from the host software that control the position of the milling head in the x, y, and (if relevant) z axis. Data to drive the Prototyper is extracted from files generated in PCB design software and stored in HPGL or Gerber file format.

"Additive" processes also exist. The most common is the "semi-additive" process. In this version, the unpatterned board has a thin layer of copper already on it. A reverse mask is then applied. (Unlike a subtractive process mask, this mask exposes those parts of the substrate that will eventually become the traces.) Additional copper is then plated onto the board in the unmasked areas; copper may be plated to any desired weight. Tin-lead or other surface platings are then applied. The mask is stripped away and a brief etching step removes the now-exposed original copper laminate from the board, isolating the individual traces. Some boards with plated thru holes but still single sided were made with a process like this. General Electric made consumer radio sets in the late 1960s using boards like these.

The additive process is commonly used for multi-layer boards as it facilitates the plating-through of the holes (to produce conductive vias) in the circuit board.

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Kondensator

Kondensator atau sering disebut sebagai kapasitor adalah suatu alat yang dapat menyimpan energi di dalam medan listrik, dengan cara mengumpulkan ketidakseimbangan internal dari muatan listrik. Kondensator memiliki satuan yang disebut Farad dari nama Michael Faraday. Kondensator juga dikenal sebagai "kapasitor", namun kata "kondensator" masih dipakai hingga saat ini. Pertama disebut oleh Alessandro Volta seorang ilmuwan Italia pada tahun 1782 (dari bahasa Itali condensatore), berkenaan dengan kemampuan alat untuk menyimpan suatu muatan listrik yang tinggi dibanding komponen lainnya. Kebanyakan bahasa dan negara yang tidak menggunakan bahasa Inggris masih mengacu pada perkataan bahasa Italia "condensatore", bahasa Perancis condensateur, Indonesia dan Jerman Kondensator atau Spanyol Condensador.
Kondensator diidentikkan mempunyai dua kaki dan dua kutub yaitu positif dan negatif serta memiliki cairan elektrolit dan biasanya berbentuk tabung.



Lambang kondensator (mempunyai kutub) pada skema elektronika.

Sedangkan jenis yang satunya lagi kebanyakan nilai kapasitasnya lebih rendah, tidak mempunyai kutub positif atau negatif pada kakinya, kebanyakan berbentuk bulat pipih berwarna coklat, merah, hijau dan lainnya seperti tablet atau kancing baju.

Lambang kapasitor (tidak mempunyai kutub) pada skema elektronika.

Namun kebiasaan dan kondisi serta artikulasi bahasa setiap negara tergantung pada masyarakat yang lebih sering menyebutkannya. Kini kebiasaan orang tersebut hanya menyebutkan salah satu nama yang paling dominan digunakan atau lebih sering didengar. Pada masa kini, kondensator sering disebut kapasitor (capacitor) ataupun sebaliknya yang pada ilmu elektronika disingkat dengan huruf (C).

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Transistor

A transistor is a semiconductor device used to amplify and switch electronic signals. It is made of a solid piece of semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Because the controlled (output) power can be much more than the controlling (input) power, the transistor provides amplification of a signal. Some transistors are packaged individually but many more are found embedded in integrated circuits.



The transistor is the fundamental building block of modern electronic devices, and its presence is ubiquitous in modern electronic systems.

History

Physicist Julius Edgar Lilienfeld filed the first patent for a transistor in Canada in 1925, describing a device similar to a Field Effect Transistor or "FET".[1] However, Lilienfeld did not publish any research articles about his devices,[citation needed] nor did his patent cite any examples of devices actually constructed. In 1934, German inventor Oskar Heil patented a similar device.[2]

In 1947, John Bardeen and Walter Brattain at AT&T's Bell Labs in the United States observed that when electrical contacts were applied to a crystal of germanium, the output power was larger than the input. Solid State Physics Group leader William Shockley saw the potential in this, and over the next few months worked to greatly expand the knowledge of semiconductors, and thus could be described as the "father of the transistor". The term was coined by John R. Pierce.[3] According to physicist/historian Robert Arns, legal papers from the Bell Labs patent show that William Shockley and Gerald Pearson had built operational versions from Lilienfeld's patents, yet they never referenced this work in any of their later research papers or historical articles.

The first silicon transistor was produced by Texas Instruments in 1954.[5] This was the work of Gordon Teal, an expert in growing crystals of high purity, who had previously worked at Bell Labs. The first MOS transistor actually built was by Kahng and Atalla at Bell Labs in 1960.

Importance

The transistor is the key active component in practically all modern electronics, and is considered by many to be one of the greatest inventions of the twentieth century.[8] Its importance in today's society rests on its ability to be mass produced using a highly automated process (semiconductor device fabrication) that achieves astonishingly low per-transistor costs.

Although several companies each produce over a billion individually-packaged (known as discrete) transistors every year,[9] the vast majority of transistors now produced are in integrated circuits (often shortened to IC, microchips or simply chips), along with diodes, resistors, capacitors and other electronic components, to produce complete electronic circuits. A logic gate consists of up to about twenty transistors whereas an advanced microprocessor, as of 2006, can use as many as 1.7 billion transistors (MOSFETs).[10] "About 60 million transistors were built this year [2002] ... for [each] man, woman, and child on Earth."[11]

The transistor's low cost, flexibility, and reliability have made it a ubiquitous device. Transistorized mechatronic circuits have replaced electromechanical devices in controlling appliances and machinery. It is often easier and cheaper to use a standard microcontroller and write a computer program to carry out a control function than to design an equivalent mechanical control function.

Simplified operation

The essential usefulness of a transistor comes from its ability to use a small signal applied between one pair of its terminals to control a much larger signal at another pair of terminals. This property is called gain. A transistor can control its output in proportion to the input signal, that is, can act as an amplifier. Or, the transistor can be used to turn current on or off in a circuit as an electrically controlled switch, where the amount of current is determined by other circuit elements.

The two types of transistors have slight differences in how they are used in a circuit. A bipolar transistor has terminals labeled base, collector, and emitter. A small current at the base terminal (that is, flowing from the base to the emitter) can control or switch a much larger current between the collector and emitter terminals. For a field-effect transistor, the terminals are labeled gate, source, and drain, and a voltage at the gate can control a current between source and drain.

The image to the right represents a typical bipolar transistor in a circuit. Charge will flow between emitter and collector terminals depending on the current in the base. Since internally the base and emitter connections behave like a semiconductor diode, a voltage drop develops between base and emitter while the base current exists. The amount of this voltage depends on the material the transistor is made from, and is referred to as VBE

Transistor as a switch

Transistors are commonly used as electronic switches, for both high power applications including switched-mode power supplies and low power applications such as logic gates.

In a grounded-emitter transistor circuit, such as the light-switch circuit shown, as the base voltage rises the base and collector current rise exponentially, and the collector voltage drops because of the collector load resistor. The relevant equations:
VRC = ICE × RC, the voltage across the load (the lamp with resistance RC)
VRC + VCE = VCC, the supply voltage shown as 6V

If VCE could fall to 0 (perfect closed switch) then Ic could go no higher than VCC / RC, even with higher base voltage and current. The transistor is then said to be saturated. Hence, values of input voltage can be chosen such that the output is either completely off,[12] or completely on. The transistor is acting as a switch, and this type of operation is common in digital circuits where only "on" and "off" values are relevant.

Transistor as an amplifier

The common-emitter amplifier is designed so that a small change in voltage in (Vin) changes the small current through the base of the transistor and the transistor's current amplification combined with the properties of the circuit mean that small swings in Vin produce large changes in Vout.

Various configurations of single transistor amplifier are possible, with some providing current gain, some voltage gain, and some both.

From mobile phones to televisions, vast numbers of products include amplifiers for sound reproduction, radio transmission, and signal processing. The first discrete transistor audio amplifiers barely supplied a few hundred milliwatts, but power and audio fidelity gradually increased as better transistors became available and amplifier architecture evolved.

Modern transistor audio amplifiers of up to a few hundred watts are common and relatively inexpensive.

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Resistor

A resistor is a two-terminal electronic component that produces a voltage across its terminals that is proportional to the electric current passing through it in accordance with Ohm's law:

V = IR

Resistors are elements of electrical networks and electronic circuits and are ubiquitous in most electronic equipment. Practical resistors can be made of various compounds and films, as well as resistance wire (wire made of a high-resistivity alloy, such as nickel/chrome).



The primary characteristics of a resistor are the resistance, the tolerance, maximum working voltage and the power rating. Other characteristics include temperature coefficient, noise, and inductance. Less well-known is critical resistance, the value below which power dissipation limits the maximum permitted current flow, and above which the limit is applied voltage. Critical resistance depends upon the materials constituting the resistor as well as its physical dimensions; it's determined by design.

Resistors can be integrated into hybrid and printed circuits, as well as integrated circuits. Size, and position of leads (or terminals) are relevant to equipment designers; resistors must be physically large enough not to overheat when dissipating their power.

Wi-Fi

The term "Wi-Fi" only applies to only the IEEE 802.11b standard. Wi-Fi (pronounced /ˈwaɪfaɪ/) is a trademark of the Wi-Fi Alliance that manufacturers may use to brand certified products that belong to a class of wireless local area network (WLAN) devices based on the IEEE 802.11 standards. Because of the close relationship with its underlying standard, the term Wi-Fi is often used as a synonym for IEEE 802.11 technology.

Internet access

A Wi-Fi enabled device such as a personal computer, video game console, mobile phone, MP3 player or personal digital assistant can connect to the Internet when within range of a wireless network connected to the Internet. The coverage of one or more (interconnected) access points — called a hotspot — can comprise an area as small as a few rooms or as large as many square miles. Coverage in the larger area may depend on a group of access points with overlapping coverage. Wi-Fi technology has been used in wireless mesh networks, for example, in London.[3]

In addition to private use in homes and offices, Wi-Fi can provide public access at Wi-Fi hotspots provided either free-of-charge or to subscribers to various commercial services. Organizations and businesses - such as those running airports, hotels and restaurants - often provide free-use hotspots to attract or assist clients. Enthusiasts or authorities who wish to provide services or even to promote business in selected areas sometimes provide free Wi-Fi access. As of 2008 more than 300 metropolitan-wide Wi-Fi (Muni-Fi) projects had started.[4] As of May 2008 the Czech Republic had 879 Wi-Fi based Wireless Internet service providers.[5][6]

Routers that incorporate a digital subscriber line modem or a cable modem and a Wi-Fi access point, often set up in homes and other premises, can provide Internet-access and internetworking to all devices connected (wirelessly or by cable) to them. One can also connect Wi-Fi devices in ad-hoc mode for client-to-client connections without a router. Wi-Fi also enables places that would traditionally not have network access to connect, for example bathrooms, kitchens and garden sheds.


The Wi-Fi Alliance, a global association of companies, promotes WLAN technology and certifies products if they conform to certain standards of interoperability. Not every IEEE 802.11-compliant device is submitted for certification to the Wi-Fi Alliance, sometimes because of costs associated with the certification process. The lack of the Wi-Fi logo does not necessarily imply a device is incompatible with Wi-Fi devices.

As of 2010 an IEEE 802.11 device is installed in many personal computers, video game consoles, smartphones, printers, and other peripherals, and virtually all laptop or palm-sized computers.

History

Wi-Fi uses both single-carrier direct-sequence spread spectrum radio-technology (part of the larger family of spread spectrum systems) and multi-carrier orthogonal frequency-division multiplexing (OFDM) radio technology. The deregulation of certain radio-frequencies[by whom?] for unlicensed spread spectrum deployment enabled the development of Wi-Fi products, Wi-Fi's onetime competitor HomeRF, Bluetooth, and many other products such as some types of cordless telephones.

In the US, the FCC first made unlicensed spread spectrum available in rules adopted on May 9, 1985.[11] Many other countries later copied or adapted these FCC regulations, enabling use of this technology in all major countries.[citation needed] The FCC action was proposed by Michael Marcus of the FCC staff in 1980 and the subsequent regulatory action took 5 more years. It was part of a broader proposal to allow civil use of spread spectrum technology and was opposed at the time by mainstream equipment manufacturers and many radio system operators.[12]

Half-size ISA 2.4 GHz WaveLAN card by AT&T

Wi-Fi technology has its origins in a 1985 ruling by the U.S. Federal Communications Commission that released several bands of the radio spectrum for unlicensed use.[13] In 1991 NCR Corporation/AT&T (later Lucent Technologies and Agere Systems) invented the precursor to the common Wi-Fi system in Nieuwegein, the Netherlands. The inventors initially intended to use the technology for cashier systems; the first wireless products were brought on the market under the name WaveLAN with speeds of 1 Mbit/s to 2 Mbit/s. Vic Hayes, who held the chair of IEEE 802.11 for 10 years and has been named[by whom?] the "father of Wi-Fi" was involved in designing standards such as IEEE 802.11b, and 802.11a.

Key portions of the IEEE 802.11 technology underlying Wi-Fi (in its a, g, and n varieties) were determined[by whom?] to be infringing on U.S. Patent 5,487,069, which was filed in 1993[14] by the CSIRO, an Australian research body. The patent became the subject of protracted and ongoing legal battles between CSIRO and major IT corporations. In 2009, the CSIRO settled with 14 companies, including Hewlett-Packard, Intel, Dell, Toshiba, ASUS, Microsoft and Nintendo, under confidential terms. The revenue arising from these settlements to October 2009 is approximately AU$200 million.[15][16][17][18][19][20]

Europe leads overall in uptake of wireless-phone technology,[citation needed] but the US leads in Wi-Fi systems partly because they lead in laptop usage. As of July 2005, there were at least 68,643 Wi-Fi locations worldwide, a majority in the US, then the UK and Germany. The US and Western Europe have about 80% of the worldwide Wi-Fi users. Plans are underway[by whom?] in areas of the US to provide public Wi-Fi coverage as a public free service. Even with these large numbers and more expansion, the extent of actual Wi-Fi usage is lower than expected. Jupiter Research found that only 15% of people have used Wi-Fi and only 6% in a public place.[21]

Dummy load

A dummy load is a device used to simulate an electrical load, usually for testing purposes.

Radio

In radio this device is also known as a dummy antenna or a radio frequency termination. It is a device used in place of an antenna to aid in testing a radio transmitter. It is substituted for the antenna while adjusting the transmitter, so that the transmitter does not interfere with other radio transmitters during the adjustments. [1] If a transmitter is tested without a load, such as an antenna or a dummy load, the transmitter could be damaged. Also, if a transmitter is adjusted without a load, it will operate differently than with the load, and the adjustments may be incorrect.

The dummy load ordinarily should be a pure resistance; the amount of resistance should be the same as the impedance of the antenna or transmission line that is used with the transmitter (usually 50 Ω or 75 Ω).[2] The radio energy that is absorbed by the dummy load is converted to heat. A dummy load must be chosen or designed to tolerate the amount of power that can be delivered by the transmitter. [3]

The ideal dummy load provides a standing wave ratio (SWR) of 1:1 at the given impedance.
See also: Antenna (radio) and Standing wave ratio

Audio

When testing audio amplifiers, it is common to replace the loudspeaker with a dummy load, so that the amplifier's handling of large power levels can be tested without actually producing intense sound. The simplest is a resistor bank to simulate the voice coil's resistance.

For loudspeaker simulation, a more complex network is more accurate, however, as actual loudspeakers are reactive and non-linear. There are many designs for loudspeaker simulators, which emphasize different characteristics of the actual speaker, such as the voice coil's inductance, mechanical suspension compliance, and cone mass.
See also: Thiele/Small and Electrical characteristics of a dynamic loudspeaker

Power supplies

There are also dummy loads for power supplies, known as load banks.

An electronic load (or e-load) is a device or assembly that simulates loading on an electronic circuit. It is used as substitute for a conventional ohmic load resistor.

As counterpart to a current source, the electronic load is a current sink. When loading a current source with a fixed resistor one can set one determined load current by the connected load resistor. The characteristic of the electronic load is that the load current can be set and varied in a defined range. The load current is regulated electronically.

The electronic load consumes electric energy and in most cases transforms it into heat energy. Fans or water-cooled elements are used as coolers. Under certain conditions, energy-recycling into the public power supply system is also possible.

Electronic loads are used in diverse applications, particularly for the test of power supplies, batteries, solar and fuel cells, generators. AC loads are used to test transformers, uninterruptible power supplies (UPS) or onboard power supplies. The equipment and power spectrum of such electronic loads begins with simplest circuits consisting in general of a potentiometer for current setting and a transistor circuit for power transforming. Further developed electronic loads supply several operating modes, in most cases constant current, voltage, power and resistance. Nowadays, the equipment may be controlled by a PLC or remotely by a PC. Settings and measured values such as input voltage and actual load current are indicated on a display.

Standing wave ratio

In telecommunications, standing wave ratio (SWR) is the ratio of the amplitude of a partial standing wave at an antinode (maximum) to the amplitude at an adjacent node (minimum), in an electrical transmission line.



The SWR is usually defined as a voltage ratio called the VSWR, for voltage standing wave ratio. For example, the VSWR value 1.2:1 denotes a maximum standing wave amplitude that is 1.2 times greater than the minimum standing wave value. It is also possible to define the SWR in terms of current, resulting in the ISWR, which has the same numerical value. The power standing wave ratio (PSWR) is defined as the square of the VSWR.

Relationship to the reflection coefficient

The voltage component of a standing wave in a uniform transmission line consists of the forward wave (with amplitude Vf) superimposed on the reflected wave (with amplitude Vr).

Reflections occur as a result of discontinuities, such as an imperfection in an otherwise uniform transmission line, or when a transmission line is terminated with other than its characteristic impedance. The reflection coefficient Γ is defined thus:

Γ is a complex number that describes both the magnitude and the phase shift of the reflection. The simplest cases, when the imaginary part of Γ is zero, are:
Γ = − 1: maximum negative reflection, when the line is short-circuited,
Γ = 0: no reflection, when the line is perfectly matched,
Γ = + 1: maximum positive reflection, when the line is open-circuited.

For the calculation of VSWR, only the magnitude of Γ, denoted by ρ, is of interest. Therefore, we define
ρ = | Γ | .

At some points along the line the two waves interfere constructively, and the resulting amplitude Vmax is the sum of their amplitudes:

At other points, the waves interfere destructively, and the resulting amplitude Vmin is the difference between their amplitudes:

The voltage standing wave ratio is then equal to:

As ρ, the magnitude of Γ, always falls in the range [0,1], the VSWR is always ≥ +1.

The SWR can also be defined as the ratio of the maximum amplitude of the electric field strength to its minimum amplitude, i.e. Emax / Emin.

Further analysis

To understand the standing wave ratio in detail, we need to calculate the voltage (or, equivalently, the electrical field strength) at any point along the transmission line at any moment in time. We can begin with the forward wave, whose voltage as a function of time t and of distance x along the transmission line is:


where A is the amplitude of the forward wave, ω is its angular frequency and k is the wave number (equal to ω divided by the speed of the wave). The voltage of the reflected wave is a similar function, but spatially reversed (the sign of x is inverted) and attenuated by the reflection coefficient ρ:


The total voltage Vt on the transmission line is given by the superposition principle, which is just a matter of adding the two waves:


Using standard trigonometric identities, this equation can be converted to the following form:


where

This form of the equation shows, if we ignore some of the details, that the maximum voltage over time Vmot at a distance x from the transmitter is the periodic function


This varies with x from a minimum of A(1 − ρ) to a maximum of A(1 + ρ), as we saw in the earlier, simplified discussion. A graph of Vmot against x, in the case when ρ = 0.5, is shown below. The maximum and minimum Vmot in a periods are Vmin and Vmax and are the values used to calculate the SWR.

Standing wave ratio for a range of ρ. In this graph, A and k are set to unity.

It is important to note that this graph does not show the instantaneous voltage profile along the transmission line. It only shows the maximum amplitude of the oscillation at each point. The instantaneous voltage is a function of both time and distance, so could only be shown fully by a three-dimensional or animated graph.

Practical implications of SWR

The most common case for measuring and examining SWR is when installing and tuning transmitting antennas. When a transmitter is connected to an antenna by a feed line, the impedance of the antenna and feed line must match exactly for maximum energy transfer from the feed line to the antenna to be possible. The impedance of the antenna varies based on many factors including: the antenna's natural resonance at the frequency being transmitted, the antenna's height above the ground, and the size of the conductors used to construct the antenna.[1]

When an antenna and feedline do not have matching impedances, some of the electrical energy cannot be transferred from the feedline to the antenna.[2] Energy not transferred to the antenna is reflected back towards the transmitter.[3] It is the interaction of these reflected waves with forward waves which causes standing wave patterns.[2] Reflected power has three main implications in radio transmitters: Radio Frequency (RF) energy losses increase, distortion on transmitter due to reflected power from load[2] and damage to the transmitter can occur.[4]

Matching the impedance of the antenna to the impedance of the feed line is typically done using an antenna tuner. The tuner can be installed between the transmitter and the feed line, or between the feed line and the antenna. Both installation methods will allow the transmitter to operate at a low SWR, however if the tuner is installed at the transmitter, the feed line between the tuner and the antenna will still operate with a high SWR, causing additional RF energy to be lost through the feedline.

Many amateur radio operators consider any impedance mismatch a serious matter.[1] However, this is not the case. Assuming the mismatch is within the operating limits of the transmitter, the radio operator needs only be concerned with the power loss in the transmission line. Power loss will increase as the SWR increases, however the increases are often less than many radio amateurs might assume. For example, a dipole antenna tuned to operate at 3.75MHz—the center of the 80 meter amateur radio band—will exhibit an SWR of about 6:1 at the edges of the band. However, if the antenna is fed with 250 feet of RG-8A coax, the loss due to standing waves is only 2.2dB.[2] Feed line loss typically increases with frequency, so VHF and above antennas must be matched closely to the feedline. The same 6:1 mismatch to 250 feet of RG-8A coax would incur 10.8dB of loss at 146MHz