The first step in transmitting radio-frequency signals through space is to create a pure carrier at the transmitter. This electrical wave must be of stable, unvarying amplitude, frequency and phase. A quartz crystal oscillator frequently serves the purpose. Information can be conveyed when the carrier is modulated. At the receiver, the carrier is demodulated, separating out an audio or another signal.
It is also possible, using the common super-heterodyne principle, to alter the carrier frequency. The idea uses frequency mixing to convert a received signal to a fixed intermediate frequency (IF) which can be more conveniently processed than the original radio carrier frequency. For each frequency that is broadcast, a different beat frequency is required if a consistent IF is to result.
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To this end, in the early years of radio, broadcasters sent a heterodyne frequency that they generated alongside the primary broadcast carrier. Soon it was found to be more efficient to generate the beat frequency locally, i.e. at the receiver, and that is how it is done currently. That is why the traditional variable capacitor was two-gang, with twin sets of plates on the same shaft. As different stations were tuned in, the receiver created the appropriate beat frequency. Today essentially the same process takes place in an electronic tuner.
In various modulation schemes, any readable alteration of the carrier’s signal qualities can suffice to carry information. The usual parameters are amplitude, frequency and phase. These can be used singly or in combination, and they are applicable to both analog and digital modulation. A previous article discussed analog modulation. Now we’ll take a closer look at digital modulation.
Digital modulation modifies an analog carrier signal with another discrete signal. Digital modulation methods can be considered digital-to-analog conversion and the corresponding demodulation or detection analog-to-digital conversion. Digital transmission has significant advantages over its analog counterpart. For one thing, it is relatively noise free. If noise is introduced, it originates outside of the digital transmission proper, that is in the pre-modulation and post-demodulation zones. Furthermore, digital modulation makes better use of bandwidth. As there is increasing competition for limited spectrum, this characteristic assumes ever-greater importance.
The standard method for representing digital modulation is by means of a polar diagram. This display represents amplitude and phase in a single graphic. The length of a vector arrow corresponds to the magnitude and the angle of that line with respect to a horizontal line connecting the origin to the zero-degree point on the circle represents the phase angle that is invoked in the modulation process. Amplitude and phase modulation are used in concert to convey the digital information. Any instantaneous value of the digital signal can be represented by a point within the polar diagram.
Digital modulation is generally represented by making reference to I and Q. The I axis is the zero-degree horizontal reference line on the polar diagram. The Q axis is the vertical line that represents a 90° polarization angle.
The vector in the polar diagram represents an RF carrier with an output power represented by the length of the vector and a certain phase angle represented by the angle with the horizontal axis. If the RF carrier has a constant output power, it could be represented on the polar diagram as a vector with a constant length (amplitude) which follows the trajectory of a circle.
Phase and magnitude fluctuations taken together convey the digital information. The result is expressed in terms of I and Q where the signal vector’s projection onto the I axis lies on the zero degrees reference (thus called the In-phase component) and the projection onto the Q axis lies on the 90° shifted phase reference (Quadrature component). The phase and amplitude information of the signal, often called S(t), with a carrier frequency w is then expressed in terms of I and Q by the equation:
S(t) = I(t)cos(w)+Q(t)sin(w)
Recall from algebra that the sine and cosine are 90° out of phase with each other. That relationship leads to the basic topology of a digital modulator or demodulator.
For a transmitter, I and Q data are applied at the inputs of two different mixers driven by a local oscillator (or RF carrier) frequency of w. The local oscillator is shifted by 90° before it drives the mixer for the Q data. The mixers form the multiplication of the terms given in the equation.
Nearly every digital modulator or demodulator uses this principle. Data to be transmitted gets coded into I/Q pairs before being fed to an I/Q modulator. The necessary circuitry can be built with digital logic or programmed in a DSP. Moreover, at a cost of greater complexity, the I-Q modulation scheme is more bandwidth-efficient than analog modulation.
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