9.7
Réponse en fréquence
Le tracé de Bode est un outil essentiel dans l'analyse des systèmes de contrôle, car il cartographie la réponse en fréquence d'un système via un tracé…
Les électrodes tissulaires des électrocardiogrammes (E-C-G) établissent une voie conductrice pour les courants électriques entre les tissus et les électrodes de mesure, permettant l’observation de l’activité cardiaque.
La dynamique de l’interface électrode-tissu a un modèle de circuit englobant la résistance de l’électrode, la capacité à l’interface électrode-tissu et la résistance tissulaire. La différence de potentiel représente la différence de tension entre l’électrode et le tissu.
Ici, l’impédance d’entrée est égale à la résistance du tissu. L’impédance de sortie est l’addition de la résistance tissulaire à la combinaison parallèle de résistance et de capacité à l’interface électrode-tissu.
Le rapport entre le phaseur de sortie et le phaseur d’entrée, calculé à l’aide des valeurs de résistance et de capacité connues, est la fonction de transfert.
Cela peut être approximé sur trois gammes de fréquences distinctes.
Le diagramme de magnitude de Bode sur un graphique semi-logarithmique représente le gain logarithmique calculé en décibels en fonction de la fréquence en radians par seconde.
Les asymptotes basse et haute fréquence sont des lignes horizontales à gain constant. Dans la gamme de fréquences intermédiaires, le graphique de magnitude asymptotique est linéaire avec une pente de 20 décibels par décennie.
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Q1: What is a Bode plot and why is it used in frequency response analysis?
A Bode plot is an essential tool in control system analysis that maps the frequency response of a system through a magnitude plot and a phase plot, both against a logarithmic frequency axis. It enables engineers to visualize how a system responds across different frequencies, making it invaluable for designing and analyzing circuits and control systems.
Q2: How do zeros and poles affect the construction of a Bode plot?
Zeros and poles are critical components of the transfer function that determine the Bode plot's shape. A zero at the origin contributes a positive slope starting from the origin, while each pole introduces a breakpoint or corner frequency where the magnitude plot's slope decreases by 20 decibels per decade. Superposing the individual contributions of all zeros and poles creates the overall Bode plot.
Q3: What are corner frequencies and how do they influence the magnitude plot?
Corner frequencies, also called breakpoints, are specific frequencies where poles or zeros cause changes in the magnitude plot's slope. At each corner frequency, the slope of the magnitude plot decreases by 20 decibels per decade for poles. These frequencies mark transitions between different frequency ranges and are essential for constructing accurate asymptotic Bode plots.
Q4: How does the magnitude plot behave across low, intermediate, and high frequency ranges?
At low frequencies, the magnitude plot maintains a flat response with constant gain until reaching the first corner frequency. In the intermediate frequency range, the asymptotic magnitude plot becomes linear with a 20-decibel-per-decade slope. At high frequencies, the plot again becomes flat with a constant gain determined by the cumulative effect of all poles and zeros.
Q5: What role does the phase plot play in a complete Bode plot representation?
The phase plot complements the magnitude plot by showing how the system's phase response varies with frequency on a logarithmic scale. A zero at the origin causes the phase plot to start at 90 degrees, while poles cause the phase to decrease. The phase plot bends downward at corner frequencies, approaching -90 degrees at frequencies much higher than the highest corner frequency.
Q6: How is the asymptotic Bode plot refined to approximate actual frequency response?
The asymptotic Bode plot consists of straight lines connecting the contributions of each term in the transfer function. To approximate the actual frequency response more closely, smooth curves are added that intersect the asymptotic plot at each corner frequency. This refinement typically results in slight overshoot near corner frequencies, known as peaking.
Q7: How does the electrode-tissue interface model relate to Bode plot construction in biomedical applications?
In electrocardiogram electrodes, the electrode-tissue interface has a circuit model with resistance, capacitance, and tissue resistance components. The transfer function derived from this model can be approximated across three frequency ranges and plotted as a Bode magnitude plot on a semilog graph, showing logarithmic gain in decibels against frequency in radians per second.