Radio Frequency - Signal Modulation

Modulation is necessary to allow radio wave carriers to carry information. The simplest modulation is CW (continuous wave) modulation used in early morse tranmissions: when the radio user presses the key, the transmitter start transmitting and when key is not pressed thereis no transmission. This a simplest form of digital modulation.A radio wave used to transmit audio signals is a complex signal that contains the carrier frequency of the broadcast station and the audio signal to transmit (usually from the microphone or audio amplifier source). There are various ways to combine the carrier frequency and the audio signal together. This process is called modulation. The most commonly used modulation methods are amplitude modulation (AM), frequency modulation (FM), single sideband modulation (SSB) and phase modulation (PM).Common abreviations for different modulation methods used for radio communications:

• AM (amplitude modulation): The amplitude of carrier is chaged according the modulating signal. The amplitude of the output is a function of the input signal (usually audio or video signal). In AM, the carrier itself does not fluctuate in amplitude. Instead, the modulating data appears in the form of signal components at frequencies slightly higher and lower than that of the carrier. These components are called sidebands. The lower sideband (LSB) appears at frequencies below the carrier frequency; the upper sideband (USB) appears at frequencies above the carrier frequency. The actual information is transmitted in the sidebands, rather than the carrier; both sidebands carry the same information.
• CW (continuous wave): The carrier frequency is constantly on when transmitter is activated. Continuous wave transmission is used primarily for radiotelegraphy. This is the transmission of short or long pulses of RF energy to form dots and dashes that will correspond to some code such as the Morse Code, sometimes referred to as interrupted continuous wave (ICW).
• DSB (dual sideband): This is basically an AM modulation where the main carrier freuquency is suppressed (only sidebands are left).
• FM (frequency modulation): The frequency of carrier is chaged according the modulating signal. It means that the RF-frequency will change acording to the input audio signal. A FM demodulator produces an output voltage that is proportional to the instantaneous frequency of the input. In frequency modulation (FM), the frequency of the carrier wave is varied in such a way that the change in frequency at any instant is proportional to another signal that varies with time. FM offers increased noise immunity and decreased distortion over the AM transmissions at the expense of greatly increased bandwidth. Frequency modulation uses the instantaneous frequency of a modulating signal (voice, music, data, etc.) to directly vary the frequency of a carrier signal. Modulation index, b, is used to describe the ratio of maximum frequency deviation of the carrier to the maximum frequency deviation of the modulating signal.
• ICF (interrupted contunuous wave): This is the transmission of short or long pulses of RF energy to form dots and dashes as used in Morse code.
• WFM (wide-FM): This modulation used in normal FM radio broadcasts. The FM band has become the choice of music listeners because of its low-noise, wide-bandwidth qualities; it is also used for the audio portion of a television broadcast. Normal FM radio uses +- 75 kHz deviation. TV sound used +- 25 kHz bandwidth. This FM system offers increased noise immunity and decreased distortion over the AM transmissions at the expense of greatly increased bandwidth.
• NFM (narrow-FM): A FM modulation with only few kHz of modulation deviation. Narrowband FM is defined as the condition where modulation index is small enough to make all terms after the first two in the series expansion of the FM equation negligible. In narrowband FM, commonly used in two-way wireless communications, the instantaneous carrier frequency varies by up to 5 kilohertz above and below the frequency of the carrier with no modulation.
• NBFM (narrow-band-FM): A FM modulation with only few kHz of modulation deviation. Narrowband FM is defined as the condition where modulation index is small enough to make all terms after the first two in the series expansion of the FM equation negligible. In narrowband FM, commonly used in two-way wireless communications, the instantaneous carrier frequency varies by up to 5 kilohertz above and below the frequency of the carrier with no modulation.
• PM (phase modulatio): In phase modulation charried signal phase is chaged according the modulating signal. Phase modulation, like frequency modulation, is a form of angle modulation (so called because the angle of the sinewave carrier is changed by the modulating wave). The two methods are very similar in the sense that any attempt to shift the frequency or phase is accomplished by a change in the other. The converse also holds: When the instantaneous phase is varied, the instantaneous frequency changes. But FM and PM are not exactly equivalent, especially in analog applications. When an FM receiver is used to demodulate a PM signal, or when an FM signal is intercepted by a receiver designed for PM, the audio is distorted. This is because the relationship between frequency and phase variations is not linear; that is, frequency and phase do not vary in direct proportion.
• USB (upper sideband): Single side band transmission which uses upper side band from AM modulation. This means that the signal is above reference carrier frequency. Because LSB and USB are essentially mirror images of each other, one can be discarded.
• LSB (lower sideband): Single side band transmission which uses lower side band from AM modulation. This means that the signal is below reference carrier frequency. Because LSB and USB are essentially mirror images of each other, one can be discarded.
• SSB (single sideband): Single sideband is an AM signal where one everythign else than one of the sidebands (upper or lower) is removed. In this transmission there is only one sideband AM modulation products, no base carrier or other sideband.
• VSB (vestigial sideband): Vestigial sideband is an AM signal with most of one (redundant) sideband filtered out to save bandwidth. This is used in analogu TV broadcasting. VSB transmission is similar to single-sideband (SSB) transmission, in which one of the sidebands is completely removed. In VSB transmission, however, the second sideband is not completely removed, but is filtered to remove all but the desired range of frequencies.
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• MCW (Modulated Carrier Wave) consists of keying the modulator with a fixed AF tone. Active tone described one state and no tone the other. This can be applied to for example to an AM transmitter for radio transmission.
• FSK (frequency shift keying) may be applied to an AM transmitter so that, for instance, binary 0 = 1 KHz and binary 1 = 2 KHz modulation. Anything from Morse to RS 232 serial computer data may be sent by this means.
• FM modulation: The radio carrier itself may be "frequency shifted" based on the modulating signal.FM offers increased noise immunity and decreased distortion over the AM transmissions at the expense of greatly increased bandwidth. In digital FM, the carrier frequency shifts abruptly, rather than varying continuously. The number of possible carrier frequency states is usually a power of 2. If there are only two possible frequency states, the mode is called frequency-shift keying (FSK). In more complex modes, there can be four, eight, or more different frequency states. Each specific carrier frequency represents a specific digital input data state.
• ASK (Amplitude Shift Key): The carrier amplitude is changed based on incoming data. Generally logic 1 indicated higher transmitting level and logic 0 means lower transmitter transmitting level. ASK modulation allows for the carrier to be "on" for both the transmission of a "0" and a "1". The carrier, during the transmission of a "0", is reduced in amplitude.
• OOK (on-foo keying): OOK modulation (On/Off Key) is the special case of ASK (Amplitude Shift Key) modulation where no carrier is present during the transmission of a zero. OOK modulation is a very popular modulation used in control applications. This is in part due to its simplicity and low implementation costs. costs. OOK modulation has the advantage of allowing the transmitter to idle during the transmission of a "zero", therefore conserving power.
• PSK/PM (phase shift keying / phase modulation): The phase of the radio signal is changed based on the modulating signal
• OFDM (orthogonal frequency-division multiplexing): Orthogonal frequency-division multiplexing (OFDM) is a method of digital modulation in which a signal is split into several narrowband channels at different frequencies. In some respects, OFDM is similar to conventional frequency-division multiplexing (FDM). The difference lies in the way in which the signals are modulated and demodulated. Priority is given to minimizing the interference, or crosstalk, among the channels and symbols comprising the data stream. Less importance is placed on perfecting individual channels. OFDM is used in European digital audio and TV broadcast services.
• MCM (multi-carrier modulation): Multi-carrier modulation (MCM) is a method of transmitting data by splitting it into several components, and sending each of these components over separate carrier signals. The individual carriers have narrow bandwidth, but the composite signal can have broad bandwidth. The advantages of MCM include relative immunity to fading caused by transmission over more than one path at a time (multipath fading), less susceptibility than single-carrier systems to interference caused by impulse noise, and enhanced immunity to inter-symbol interference. Limitations include difficulty in synchronizing the carriers under marginal conditions, and a relatively strict requirement that amplification be linear. The technology lends itself to digital television, and is used as a method of obtaining high data speeds in asymmetric digital subscriber line (ADSL) systems. MCM is also used in wireless local area networks (WLANs).
• COFDM (Coded Orthogonal Frequency Division Multiplexing): Coded Orthogonal Frequency Division Multiplexing (COFDM) [1, 2] has been specified for digital broadcasting systems for both audio (Digital Audio Broadcasting (DAB)) and (terrestrial) television (Digital Video Broadcasting (DVB-T)). COFDM is particularly well matched to these applications, since it is very tolerant of the effects of multipath (provided a suitable guard interval is used). Indeed, it is not limited to 'natural' multipath as it can also be used in so-called Single-Frequency Networks (SFNs) in which all transmitters radiate the same signal on the same frequency. A receiver may thus receive signals from several transmitters, normally with different delays and thus forming a kind of 'unnatural' additional multipath. Provided the range of delays of the multipath (natural or 'unnatural') does not exceed the designed tolerance of the system (slightly greater than the guard interval) all the received-signal components contribute usefully. COFDM is a modulation scheme which is especially tailored to work well with selective channels and isolated CW (or analogue TV) interferers. COFDM is an OFDM system where signal is split into several narrowband channels at different frequencies. With other rectangular-constellation modulation systems, such as 16-QAM or 64-QAM, each axis carries more than one bit, often with Gray coding. At the receiver, a soft decision can be made separately for each received bit.
• 8-VSB (Eight-level VSB): Eight-level VSB (8-VSB) was developed by Zenith for inclusion in the Advanced Television Systems Committee (ATSC set of digital television (DTV) standards used in USA.

Both OOK and ASK receivers genrally require an adaptable threshold or an automatic gain control (AGC) in order toensure an optimal threshold setting. The FSK modulation does not usuallyrequire this because it incorporates a limiter that keeps the signal envelopeamplitude constant over the useful dynamic range.

Radio modulation related link

 

• Explore inside of a Radio - This page investigates the inside of a cheap beach radio.I will show you different compontents and explain what they do. There are lot of stuff you can re-use from such radio.
• OOK, ASK and FSK Modulation in the Presence of an Interfering signal - This paper discusses three popular modulation schemes in the presence of an interfering signal. This paper will review the three modulation types and develop a mathematical model for the prediction of error due to interference.
• Quadrature FM Detectors - FM stands for Frequency Modulation. It means that the RF-frequency will change acording to the input audio signal. A FM demodulator produces an output voltage that is proportional to the instantaneous frequency of the input. There are three general categories of FM demodulator circuit: Phase-locked loop (PLL) demodulator, Slope detection/FM discriminator, Quadrature detector