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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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AD8341ACPZ-WP1 Fiches technique(PDF) 10 Page - Analog Devices |
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AD8341ACPZ-WP1 Fiches technique(HTML) 10 Page - Analog Devices |
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10 / 20 page  AD8341 Rev. 0 | Page 10 of 20 THEORY OF OPERATION The AD8341 is a linear RF vector modulator with Cartesian baseband controls. In the simplified block diagram given in Figure 26, the RF signal propagates from the left to the right while baseband controls are placed above and below. The RF input is first split into in-phase (I) and quadrature (Q) compo- nents. The variable attenuators independently scale the I and Q components of the RF input. The attenuator outputs are then summed and buffered to the output. By controlling the relative amounts of I and Q components that are summed, continuous magnitude and phase control of the gain is possible. Consider the vector gain representation of the AD8341 expressed in polar form in Figure 27. The attenuation factors for the I and Q signal components are represented on the x- and y-axis, respectively, by the baseband inputs, VBBI and VBBQ. The resultant of their vector sum represents the vector gain, which can also be expressed as a magnitude and phase. By applying different combinations of baseband inputs, any vector gain within the unit circle can be programmed. A change in sign of VBBI or VBBQ can be viewed as a change in sign of the gain or as a 180° phase change. The outermost circle represents the maximum gain magnitude of unity. The circle origin implies, in theory, a gain of 0. In practice, circuit mismatches and unavoidable signal feedthrough limit the minimum gain to approximately −34.5 dB. The phase angle between the resultant gain vector and the positive x-axis is de- fined as the phase shift. Note that there is a nominal, systematic insertion phase through the AD8341 to which the phase shift is added. In the following discussions, the systematic insertion phase is normalized to 0°. The correspondence between the desired gain and phase set- points, GainSP and PhaseSP, and the Cartesian inputs, VBBI and VBBQ, is given by simple trigonometric identities () () []2 2 / / O BBQ O BBI SP V V V V Gain + = ( ) BBI BBQ SP V V Phase / arctan = where: VO is the baseband scaling constant (500 mV). VBBI and VBBQ are the differential I and Q baseband voltages, respectively. Note that when evaluating the arctangent function, the proper phase quadrant must be selected. For example, if the principal value of the arctangent (known as the Arctangent(x)) is used, quadrants 2 and 3 could be interpreted mistakenly as quadrants 4 and 1, respectively. In general, both VBBI and VBBQ are needed in concert to modulate the gain and the phase. Pure amplitude modulation is represented by radial movement of the gain vector tip at a fixed angle, while pure phase modula- tion is represented by rotation of the tip around the circle at a fixed radius. Unlike traditional I-Q modulators, the AD8341 is designed to have a linear RF signal path from input to output. Traditional I-Q modulators provide a limited LO carrier path through which any amplitude information is removed. LINEAR ATTENUATOR LINEAR ATTENUATOR V-I V-I 0 °/90° I-V VBBQ Q CHANNEL INPUT SINGLE-ENDED OR DIFFERENTIAL 50 Ω INPUT Z VBBI I CHANNEL INPUT OUTPUT DISABLE SINGLE-ENDED OR DIFFERENTIAL 50 Ω OUTPUT Figure 26. Simplified Architecture of the AD8341 |A| θ A +0.5 –0.5 +0.5 –0.5 Vi Vq MIN GAIN MAX GAIN Figure 27. Vector Gain Representation RF QUADRATURE GENERATOR The RF input is directly coupled differentially or single-ended to the quadrature generator, which consists of a multistage RC polyphase network tuned over the operating frequency range of 1.5 GHz to 2.4 GHz. The recycling nature of the polyphase net- work generates two replicas of the input signal, which are in precise quadrature, i.e., 90°, to each other. Since the passive network is perfectly linear, the amplitude and phase information contained in the RF input is transmitted faithfully to both chan- nels. The quadrature outputs are then separately buffered to drive the respective attenuators. The characteristic impedance of the polyphase network is used to set the input impedance of the AD8341. |
Numéro de pièce similaire - AD8341ACPZ-WP1 |
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Description similaire - AD8341ACPZ-WP1 |
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