Définition:
La fréquence de coupure à – 3 dB est la fréquence pour laquelle on a T = Us/Ue = Tmax/sqrt(2) ou G = Gmax – 3 dB.
D’où la méthode générale : déterminer la transmittance maximale puis rechercher la fréquence pour laquelle cette valeur maximale est divisée par racine de 2 ou bien déterminer le gain maximal puis rechercher la fréquence pour laquelle cette valeur maximale est diminuée de 3dB.
Remarques:
· Travailler en régime sinusoïdal. Répéter la méthode s’il existe 2 fréquences de coupure.
· Visualiser simultanément les tensions ue et us afin de vérifier que le fonctionnement est linéaire.
· Pour plus de rapidité, les réglages sont effectués de telle sorte que les calculs soient réduits (même calibres, valeurs « simples » et adoption de 1/sqrt(2) » 0,7).
· S’assurer que la bande passante des appareils de mesure soit compatible avec les fréquences de travail.
· Faire un « balayage » en fréquence et observer les signaux (amplitudes et gamme de fréquence). En particulier, vérifier si l’amplitude de la tension d’entrée varie avec la fréquence. Dans ce cas on pourra établir un tableau [ f ; T ou G ] autour de la fréquence pour laquelle T ou G sont maximaux.
1/ Mesure à l’aide de l’oscilloscope :
Pour une tension d’entrée d’amplitude donnée, faire varier la fréquence f jusqu’à ce que l’amplitude de us soit maximale.
Régler l’amplitude de ue de sorte que la valeur crête de us mesure 4 divisions sur l’écran.
Régler la fréquence f jusqu’à ce que la valeur crête de us mesure 2,8 divisions sur l’écran.
Alors cette fréquence est la fréquence de coupure à -3dB
2/ Mesure à l’aide d’un voltmètre numérique efficace ou d’un oscilloscope numérique :
Pour une tension d’entrée d’amplitude donnée, faire varier la fréquence f jusqu’à ce que la valeur efficace de us soit maximale.
Régler l’amplitude de ue de sorte que la valeur efficace de us s’exprime par une valeur « arrondie ».
Rechercher la fréquence f jusqu’à ce que la valeur efficace de us soit égale à 0,7.(Us)max .
Alors cette fréquence est la fréquence de coupure à -3dB.
3/ Mesure à l’aide du décibelmètre :
On rappelle que G = Gs – Ge où Gs et Ge sont les valeurs données par le dBmètre lorsqu’il est connecté à la sortie puis à l’entrée du quadripôle.
· Connecter le dBmètre à la sortie du quadripôle, régler la fréquence de sorte que la valeur de Gs soit maximale et régler la tension d’entrée pour que cette valeur soit simple.
· Connecter le dBmètre à l’entrée du quadripôle, mesurer le gain et en déduire le gain maximal Gmax du quadripôle.
· Rechercher la fréquence f jusqu’à ce que le gain G soit diminué de 3dB.
Alors cette fréquence est la fréquence de coupure à -3dB.
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