Radio Frequency (RF) Theory
Radio Frequency (RF) Theory
EM Wave
When a changed particle is accelerated, such as in an anthena, it induces oscillating mutually perpendicular electric and magnetic fields. These fields form an electromagnetic (EM) wave.
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Radio Frequency (RF) Theory
RF (Radio Frequency)
RF (or radio frequencies) are the EM waves on the frequency range from 3kHz to 300GHz. They are mostly used in wireless communication and radar applications.
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Radio Frequency (RF) Theory
EM Wave
This picture describes basic properties of an EM wave (or any other wave). Amplitude is the height of the wave, wavelength is the peak to peak distance, the highest point is peak, the lowest is through.
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Radio Frequency (RF) Theory
Frequency Bands
EM waves of frequencies within certain ranges get categorized into so called bands. This picture lists the bands and their frequencies:
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Radio Frequency (RF) Theory
Frequency & Propagation Distance
Lower frequencies can propagate to longer distances and through obstances better than higher frequencies, but they can transmit data at lower rate.
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Radio Frequency (RF) Theory
Amplitude
Amplitude of an RF signal is the measure of change of the electric field of the EM wave in time. In the picture, (1) is the peak amplitude, (2) is the peak-to-peak amplitude, (3) is the RMS (root mean square) amplitude, and (4) is the wavelength.
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Radio Frequency (RF) Theory
Phase
The phase of an RF wave is the offset of where the wave starts expressed in radians. A wave that is offset by 2 PI (180 degrees) has the opposite waveform compared to the original wave with 0 phase.
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Radio Frequency (RF) Theory
Modulation
In order for EM waves to transmit information, they have to modify the amplitude, frequency and phase characteristics of the wave to encode the information in it. This process is called moduleation and is done by mixing a modulating signal, which contains the information to be transmitted, into a periodic carrier waveform (CW), which propagates the signal through the environment.
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Radio Frequency (RF) Theory
Amplitude Modulation
Amplitude modulation (AM) is an analog modulation technique, where the amplitude (signal strength) of the carrier wave is varied in proportion to that of the message signal, such as an audio signal. It's the earliest modulation method.
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Radio Frequency (RF) Theory
Frequency Modulation
Frequency modulation (FM) is the analog encoding of information in a carrier wave by varying the instantaneous frequency of the wave. Phase modulation is one of the two principal forms of angle modulation, together with phase modulation.
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Radio Frequency (RF) Theory
Phase Modulation
Phase modulation (PM) is the analog encoding of information in a signal as variations in the instantaneous phase of a carrier wave. Phase modulation is one of the two principal forms of angle modulation, together with frequency modulation.
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Radio Frequency (RF) Theory
Digital Modulation
Digital modulation was invented as a way to transmit digital information (0s and 1s) over a carrier wave. To achieve this, discrete RF energy states are used to represent some quantity of the digital information. These states (usually 0 or 1 bits) are transmitted in the form of waveforms that represent the bits. These waveforms are usually referred to as "symbols". In the image below, there are 10 bits transmitted and they are carried in the 10 symbols.
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Radio Frequency (RF) Theory
Symbol Rate & Bit Rate
Symbol rate: Rsym=1/Tsym is the rate at which bits are transmitted, where Tsym is the time for one symbol to transmit. Similarly, bit rate is Rb=1/Tb, where Tb is the time to transmit one bit. In the image below, Tsym=Tb. If N is the number of bits per symbol, then Rb=N*Rsym. If M is the number of different available symbols (2 (0 and 1) in the example image below), then N=log2M.
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Radio Frequency (RF) Theory
Shift Keying
In binary digital modulation, there are two symbols and each carries 1 bit of information. Just as we can vary amplitude, frequency or phase of a high-frequency carrier in accordance with an analog information (message) waveform, we can do the same with digital waveform. Since bits vary between 0 and 1, digital modulation techniques that vary the carrier's amplitude, frequency or phase are referred to as "shift keying".
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Radio Frequency (RF) Theory
Frequency-Shift Keying (FSK)
Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes (shifts) of a carrier wave. The simplest form of FSK is binary FSK (BFSK), in which carrier wave is shifted to low or high frequency to transmit 0s or 1s.
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Radio Frequency (RF) Theory
Amplitude Shift Keying (ASK)
Amplitude shift keying is a form of amplitude modulation that represents digital data as shifts in the amplitude of the carrier wave. For example, a small amplitude represents 0, a large amplitude represents 1.
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Radio Frequency (RF) Theory
On-Off Keying (OOK)
The simplest ASK modulation scheme is On-Off Keying (OOK), in which a carrier wave is transmitted for 1 and nothing is transmitted for 0.
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Radio Frequency (RF) Theory
Binary Phase Shift Keying (BPSK)
Binary phase shift keying (PBSK) is a phase modulation scheme where digital information (0s and 1s) is represented by shifting the carrier wave phase between two allowed values.
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Radio Frequency (RF) Theory
Comparison of Digital Modulation Schemes
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Radio Frequency (RF) Theory
M-ary Digital Modulation
M-ary digital modulation schemes are a set of modulation schemes that use more than one symbol (0 or 1 typical for binary modulation schemes). This means that for each symbol, more than one bit can be trasmitted at a time.
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Radio Frequency (RF) Theory
Whitening Transformation of Data
A whitening transformation or sphering transformation is a linear transformation that transforms a vector of random variables with a known covariance matrix into a set of new variables whose covariance is the identity matrix, meaning that they are uncorrelated and each have variance 1. Data whitening is a process of reducing data to white noise vector.
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Radio Frequency (RF) Theory
Frequency-Hopping Spread Spectrum (FHSS)
Frequency-hopping spread spectrum (FHSS) is a method of transmitting radio signals by rapidly changing the carrier frequency among many distinct frequencies occupying a large spectral band. The changes are controlled by a code known to both transmitter and receiver. FHSS is used to avoid interference, to prevent eavesdropping, and to enable code-division multiple access (CDMA) communications.
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Radio Frequency (RF) Theory
ISM Band
The ISM radio bands are portions of the radio spectrum reserved internationally for industrial, scientific and medical (ISM) purposes other than telecommunications. Examples of applications for the use of radio frequency (RF) energy in these bands include radio-frequency process heating, microwave ovens, and medical diathermy machines. The powerful emissions of these devices can create electromagnetic interference and disrupt radio communication using the same frequency, so these devices are limited to certain bands of frequencies. In general, communications equipment operating in these bands must tolerate any interference generated by ISM applications, and users have no regulatory protection from ISM device operation.
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