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RF Basics ITN 220 This class This is the short compilation of a number of topics. No math (only hints at it) Some physics (mostly pictures) Stop me when it starts to hurt Peroid, Amplitude, & Frequency The speed of light (or radio) 299,792,458 metres per second 1,079,252,848.8 km/h

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Rf basics l.jpg

RF Basics

ITN 220


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This class

  • This is the short compilation of a number of topics.

  • No math (only hints at it)

  • Some physics (mostly pictures)

  • Stop me when it starts to hurt



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The speed of light (or radio)

  • 299,792,458 metres per second

  • 1,079,252,848.8 km/h

  • 186,282.397 mps

  • 670,616,629.384 mph

  • ~983,571,056 fps

  • ~1 one foot per nanosecond

  • 1 nanosecond = 1GHz = 1 foot antenna

  • 2.4 Ghz = ~.416 nanoseconds = .409' = 4.9”


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Phase

  • A comparison of two identical waves which denotes the amount of delay between them

  • Expressed in degrees


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Propagation, Transmission and Radiation

  • Propagation: The spread of energy into an area or space

  • Transmission: The process of emitting a signal

  • Radiation (noun): Energy that is radiated or transmitted

  • Radiation (verb): The act of propagation


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Reflection

  • The act of a propagating wave being thrown back from a surface. Involves a radiated pattern hitting a surface of another medium or object.


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Refraction

  • Refraction – The act of “bending” a signal as it passes from one medium into another.


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Diffraction

  • The act of a wave pattern “bending around” an object or surface.

    • Can create shadows

    • Affected by angle

    • Output doesn't necessarily contain an entire copy of the input (prism effect == loss of content)


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Polarization

  • The angle at which radio waves propogate. Can be expressed as an angle. The receiver must have the same polarization as the transmitter.


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Scattering, Absorption, and Polarization

  • Scattering - The haphazard separation of a wave into components with differing frequences (can involve diffraction, reflection, refraction, etc.)

  • Absorption – the retention of radiated energy by an object (heat)


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Noise

  • Noise – the “background” electrical signals (within a specific bandwidth) generated by natural and synthetic sources (lightning, sun spots, other electronic equipment, radioactive decay, diffraction/reflection/refraction of other sources, your mom, etc.)

  • Noise Floor – The minimum level where a signal can be differentiated from noise

  • Signal-to-Noise Ratio (SNR) – A measurement of signal “quality”, expressed as a number


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Pro's/Cons

  • Lower frequency signals tend to travel greater distances (for a given power)

  • Lower frequency signals tend to travel through differing media (materials) better

  • Higher frequency signals can “carry” more data (bandwidth)


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Gain

  • The ratio of an output signal to an input signal. Expressed in decibels (dB)

    • dBi - a relative measurement of gain as compared to an isotropic antenna. Normally used to describe the gain of antennas used to receive frequencies above 1 GHz.

    • dBd - a relative measurement of gain as compaired to a half-wave dipole antenna

    • dBm - a relative measurement of power, expressed in decibels above 1 milli-Watt

    • dBm and dBd are used most often


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FHSS

  • A spread-spectrum method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver.


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Direct Sequence Spread Spectrum (DSSS)

  • A system for generating spread-spectrum transmissions. It generates a redundant pattern for each bit to be transmitted, called a chipping code. Using this, the signal is split over several frequencies, and the different parts are sent concurrently. This method is faster than FHSS, but is more sensitive to environmental factors.


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Modulation and Demodulation

  • Modulation – The process of manipulating the frequency or amplitute of a “carrier” signal so that it can “carry” “information”

  • Demodulation – The process of extracting information from a modulated signal

  • Two Categories of Modulation – Analog and Digital


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Analog Modulation

  • Amplitude Modulation

  • Frequency Modulation

  • Phase Modulation

  • Quadrature Amplitude Modulation (QAM)


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Amplitude Modulation (AM)

  • Where a carrier signal has its amplitude modulated to carry “information”


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Frequency Modulation (FM)

  • Where a carrier has its period (frequency) modified to carry “information”


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Phase Modulation (PM)

  • Where a carrier has its phase modulated to carry “information”

  • Similar to FM except the frequency of the carrier signal does not change

  • Better suited for digital information


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Quadrature Amplitude Modulation (QAM)

  • Where the amplitude of two carriers, 90 degrees out of phase with each other, is modulated to carry information

  • Digital version of QAM combines AM and PM


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Pulse Amplitude Modulation

  • The amplitude of a series of pulses “carries” a signal


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Digital Modulation

  • Amplitude Shift Keying (ASK)

  • Phase Shift Keying (PSK)

    • Binary PSK (BPSK)

    • Quadrature PSK (QPSK)

  • Frequency Shift Keying (FSK)

  • Orthogonal frequency-division multiplexing (OFDM)


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Amplitude Shift Keying (ASK)

  • An AM signal where amplitude denotes a bit

  • Signal/no signal or two different levels of signal


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Phase Shift Keying

  • Where a carrier has its phase modulated to carry information

  • Binary Phase Shift Keying (BPSK)


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Quadrature Phase Shift Keying (QPSK or 4PSK)

  • Four phases

  • Twice the through put as BPSK for the same carrier

  • Break the stream into chunks (symbols) and “indicate” them by phase

  • 4QPSK (2 carriers) used in Time Division Multiple Access (TDMA) 2G phones


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Constellation Diagrams

  • Used to map symbols

  • Used to describe a combination of digital modulations (for streams)

  • Pro - more tolerant of noise as detector can be configured to choose the closest point

  • Con – Higher orders more difficult to detect - 8PSK is normally the highest you will see


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Frequency Shift Keying (FSK)

  • A (usually small) shift in carrier frequency denoting a binary state

  • Used in the GSM mobile phone standard


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Digital QAM

  • AM and PSK

  • Pro - Even more throughput

  • Con – More sensitive equipment ($$)

  • Less range

  • Example: here


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Digital QAM

  • Most common forms:

    • 16-QAM

    • 64-QAM

    • 128-QAM

    • 256-QAM


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Orthogonal Frequency Division Multiplexing

  • For Wi-Fi, QAM is used as a modulation scheme for sub-carriers

    • higher throughput for the same bandwidth (b vs. g)

    • can be done with one radio

  • Tolerant of multi-path and narrow-band interference

  • Used in ADSL, 802.11g, WiMAX, MBWA, PLC


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Radios

  • 802.11b

    • CCK (a form of QPSK) – 1, 2, 5.5, 11 Mbps

  • 802.11a

    • OFDM – 6, 9, 12, 18, 24, 36, 48, 54 Mbps

  • 802.11g

    • CCK – 1, 2, 5.5, 11 Mbps

    • OFDM – 6, 9, 12, 18, 24, 36, 48, 54 Mbps

  • 802.16 (Redline, Proxim)

    • 64 QAM, 16 QAM, QPSK, BPSK


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Antenna

  • An arrangement of conductors that generate a radiating electromagnetic field in response to an applied signal or can be placed in an electromagnetic field so that the field will induce a signal between its terminals.


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Resonant Frequency

  • Each antenna has a resonant frequency which is related to its electrical length.

  • Antennas can be resonant on harmonic frequencies (1/2, 1/4, 1/8 - wave)

  • Some antenna designs have multiple resonant frequencies and can be used over a wide range of frequencies

  • Note: resonant frequency is not limited to antennas (e.g., trees)


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Antenna Bandwidth

  • The range of frequencies over which an antenna is effective

  • High gain antennas have narrow bandwidth reception properties


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Radiation Pattern

  • The radiation pattern is a graphical depiction of the relative field strength transmitted from or received by the antenna.


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Polarization

  • The orientation of the electric field of the radio wave with respect to the Earth's surface.

  • Determined by the physical structure of the antenna and by its orientation.

  • A simple straight wire antenna will have one polarization when mounted vertically, and a different polarization when mounted horizontally.

  • Can be expressed as an angle

  • Transmitting and receiving ends must have the same polarization to work


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Basic Antenna Models

  • Isotropic - A purely theoretical antenna that radiates equally in all directions.


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Dipole Antenna

  • The most simple practical (real world) antenna. Comprised of two wires pointed in opposite directions arranged either horizontally or vertically, with one end of each wire connected to the radio and the other end hanging free in space. Alternate form: the whip antenna.

  • Also used as reference model for other antennas; gain with respect to a dipole is labeled as dBd.


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The Electrically Short Antenna

  • An open-end wire far less than 1/4 wavelength in length - in other words only one end of the antenna is connected to the radio, and the other end is hanging free in space. Electrically short antennas are typically used where operating wavelength is large and space is limited, e.g. for mobile transceivers operating at long wavelengths.

  • Low gain but easy


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Loop Antenna

  • Loop antennas have a continuous conducting path leading from one conductor of a two-wire transmission line to the other conductor


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Parabolic Antenna

  • The parabolic antenna is a special antenna where a reflector dish is used to focus the signal from a directional antenna feeder.


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Fresnel Zone

  • (Augustin-Jean Fresnel, France, 1788-1827) - the optimal area between a transmitter and a receiver that must remain clear for a “true” signal to be received (no diffraction, reflection, diffraction, etc.). Note: the width of the zone is dependent on distance.


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