L’antenne dipôle active de l’expérience CODALEMA Didier CHARRIER Subatech, Nantes, France Didier.charrier@subatech.in2p3.fr the CODALEMA collaboration http://codalema.in2p3.fr SUBATECH, Nantes Observatoire de Paris-Meudon Observatoire de Nançay LAL, Orsay ESEO, Angers LPSC, Grenoble LPCE, Orleans LAOB, Besançon (FRANCE) - Design of a low noise, wide band, active dipole antenna for a cosmic ray radiodetection with the codalema experiment Didier Charrier & Lilian Martin, Nançay, le 4 Mars 2008

Dimensionnement du radiateur d’antenne Antenna used under f0 With L=1.21m  f0=111MHz Ground effect (image antenna) H max<3/8. (zenith gain) H max=1.12m H min: given by the ground cancellation  Trade off Rloss & Q-factor : Fat dipole a=0.1m  Rloss   Antenna capacitance  2 frequencies mode Short dipole : 100k / 25MHz Linear dipole : 25M / 100MHz Antenna radiator: 2 aluminium blades L=1.21m gap=10mm a=0.1m e=10mm Towards 12 bit DAC Fully differential preamplifier H=1m -This active antenna is composed of a radiator made of two aluminium slats and a differential low noise preamplifier placed very closed to the antenna radiator -A slat antenna with a length L and a width a is equivalent to a cylinder antenna of the same length but with a diameter equals to one half the width a -This antenna is used under its first resonance frequency of one hunderd and eleven mega hertz -The sum of incident waves and waves reflected by the fround produces interferencies wich limits the maximum antenna height for a given wavelength as given by this formula. We calculate a maximum height of one point ten meters -The choice of a fat dipole allows to decrease the ohmic loss resistance and to increase the antenna capacitance -This antenna is used as a short dipole from one hunderd kilohertz to twenty five megahertz and as a linear dipole above twenty five mega hertz Ground Didier Charrier & Lilian Martin, Nançay, le 4 Mars 2008

Simulation de la directivité d’antenne Half Power Beam Width Angle Zenith Directivity Simulation software: NEC2 Equivalent cylinder antenna: L=1.21m, d=55mm 17 segments, No loss Zenith gain Free space: nearly constant gain 3-100MHz Perfect GND: LF : E x 2 (+6dB) 150MHz : destructive interference effect (-dB) Half Power Beam Width Nearly constant 3-50MHz HPBW H-plane>HPBW E-plane Half Power Beam Width Angle -The software used for the simulations is the unexpensive EZNEC -For all this simulations, the antenna has been modelised by an equivalent cylinder antenna divided in seventeen segments without any loses -All the simulations you will see includes two cases: the first one is a free space antenna always represents by a red curve, the second one is an antenna one meter above a perfect ground plane always represents by a blue curve -The figure at the top is the zenith gain versus the frequency -For the free space case you can see a nearly constant antenna gain, but a drop at one hundred and fifty megahertz for the antenna above a perfect ground plane because of the wave reflection on the ground -The second figure at the bottom represents the half power beam width angle versus the frequency -It is nearly constant from three megahertz to fifty megahertz with an angle of sixty five degree in the E-plane and ninty degree in the H-plane Didier Charrier & Lilian Martin, Nançay, le 4 Mars 2008

Simulation de l’impédance d’antenne : Za=Rrad+jXa Rrad variation, for f < 25MHz ( short dipole) Free space : Rrad  f2 Rrad=202(Lf/c)2 Perfect GND plane : Rrad  f4 ! (coupling with virtual image antenna) Xa variation, for f < 25MHz Xa = -j3826 @ 5MHz  Ca=8.5pF f0 (free space) = 111,5 MHz ; f0 (GND plane) = 117,5 MHz -This figure represents the value of the antenna radiation resistance versus the frequency -For frequencies below 25MHz, the radiation resistance of the free space antenna is proportional to the square of the frequency as given by this formula (à montrer) -But for an antenna above a perfect ground plane it is proportionnal to the the frequency to the power four -What about an antenna above a real ground plane ? -I think it should be placed between this two curves Didier Charrier & Lilian Martin, Nançay, le 4 Mars 2008

Concept de l’antenne active Capacitive loaded antenna for f < 25 MHz (fo, resonance frequency) Vout/Va=A.Ca / (Cin+Cs+Ca) Transfered Power = 0 W for f=f0 Induced current is small (capacitive input impedance) Va Cs Ca Cin Vout A 50 Cs : parasitic shunt capacitance Ca : antenna capacitance Short circuit loaded antenna for f < 25 MHz Vout/Va=-Ca/Cf Cs independent (virtual GND) Transfered Power =0 W for f=f0 Induced current is high (short circuit ) Cf Ca -There are two possibilities to design an active antenna -The first one consists in loading the antenna by a preamplifier whose input impedance is a capacitance, and then the relationship betwen the antenna induced voltage and the preamplifier input voltage is given by a capacitive divider bridge wich is independant of the frequency as given by this formula -The second possibilities consist in loading the antenna by a capacitive feedback preamplifier which is equivalent to a short circuit because of the virtual ground, so the transfer function is given by the ratio of the antenna capacitance over the feedback capacitance as given by this formula -In both cases, the transfered power is null -This is the first case wich has been chosen Va Cs Zin Vout 50 Didier Charrier & Lilian Martin, Nançay, le 4 Mars 2008

Calcule de la réponse fréquentielle de l’antenne Rrad(f) Ca Leff E Va Vin Za = Rrad+jXa Cs Cin (Rloss is neglected) Vout=A(f).Vin Zin Leff: effective length: Va=E.Leff With or H(f): transfer function : Vin =H(f).Va Vin/E=Leff.H(f) Knowledge of Za & G(,,f) for f< 25 MHz : Leff=L/2(free space), and if Ca=Cin  Vin/E=L/4 -We need to calculate the relationship between an electric field E parallel to the antenna and the induced voltage Vin at the preamplifier input -The relationship between the incident electric field and the antenna induced voltage Va is given by the antenna effective length -The effective length can be written as the integral of the distribution current along the antenna which should be equivalent to this formula (à monter) -The transfer function between Va and Vin can be written as an impedance divider bridge formula with Za the antenna impedance and Zin the preamplifier input impedance in parallel with the antenna parasitic shunt capacitance -So, to calculate the overall antenna frequency response, we need to know the antenna impedance and the antenna gain by a few simulations Didier Charrier & Lilian Martin, Nançay, le 4 Mars 2008

Fonction de transfert (Vin / E) Réponse de l’antenne au zénith chargée sur 8.7pF Calculated from the simulated values of Za and G(zenith) Free space: Constant value (0.25m) from DC to 40MHz 7.6dB difference from DC to 100MHz Antenna 1m above a perfect GND plane cancellation for long wavelength 30dB difference from 3MHz to 100MHz ! -Now, with the knowledge of the antenna impedance and the antenna gain, it becomes possible to plot the antenna frequency response Vin over E -This figure represents the preamplifier input voltage over the electic field in meters versus the frequency -For a free space antenna, the response is nearly constant up to fifty megahertz -But for an antenna above a perfect ground plane, there is a null value at one 150MHz because of the antenna gain drop and a ground cancellation for the longest wavelength because of the drop to the power four of the radiation resistance -So, what happened for an antenna above a real ground plane ? - I hope the curve is placed between this two curves Didier Charrier & Lilian Martin, Nançay, le 4 Mars 2008

Fonction de transfert global (Vout/E) Réponse globale de l’antenne (au zenith) et du LNA - This figure represents the overall antenna frequency response taking into accound the measured transfer function of the preamplifier - The top figure is the response in meter versus the frequency, whereas the bottom one is the phase versus the frequency - This curves are similar to the previous one because the preamplifier gain is very flat in this range of frequency Didier Charrier & Lilian Martin, Nançay, le 4 Mars 2008

LNA , schéma fonctionnel ASIC en technologie AMS BiCMOS 0.8 Structure totalement différentielle Impédance d’entrée: Cin=9pF Gain global (Vout/Vin) ajustable de 48 à 55dB Dynamique d’entrée maxi : 24mVc-c Consommation = 45mA sous 5.5V Sortie diff 6Vc-c sur 200  Classe A Etage d’entrée Convoyeur de courant Ampli à gain programmable In MUX Amplificateur de puissance ESD Out       gm ESD In Out Rf Sortie annexe Out Out Ampli d’entrée Gain = gm  Rf Gain=38dB 8 gains programmables par 3 bits De 10dB à 17dB Testabilité résistance de feedback externe Filtre passe-haut d’ordre 1 Polarisation DC Didier Charrier & Lilian Martin, Nançay, le 4 Mars 2008

LNA : influence du bruit sur le design La Ca Rrad Vn2 Vin2 Noiseless preamplifier Cs Cin Za In2 Zin LNA noise 2 Total LNA input noise: For f< fo /5 : In2 must be kept small when f   CMOS transistor (In2=0) For a MOS transistor: Input thermal noise  choice of a WIDE input PMOS transistor -All noisy preamplifier can be modelized by a perfect noisless preamplifier with two noise sources at its input -A parallel noise and a serial noise -The total preamplifier input noise V in squared is the sum ot the serial noise plus the product of the parallel noise times the antenna impedance in parallel with the preamplifier input capacitance as written by this formula -For the lowest frequencies, this formula can be simplified like this and we observe that the parallel noise could become dominant because of the increase of the antenna impedance with the decrease of the frequency -This is the reson why I choose a transistor without parallel noise, I mean a CMOS transistor -For this kind of transistor, the thermal input noise can be written like this -So the widest the transistor and the highest the drain current, the lowest the voltage input thermal noise -This the reason why the input transistor is a wide PMOS K: technological constant ID: drain current W: transistor Width L: transistor length Didier Charrier & Lilian Martin, Nançay, le 4 Mars 2008

LNA , étage d’entrée Paire différentielle à PMOS Very wide transistor W/L = 2*3072u / 0.8u Common mode noise Vd Paire différentielle à PMOS Courant de polarisation élevé (10mA) Bruit thermique  Linéarité  Structure cascodée par le convoyeur de courant  Produit Gain-Bande  Le bruit global est dominé par les PMOS de la paire différentielle : 75% à 50MHz gm.Vd Virtual Ground gm.Vd Vers convoyeur de courant Didier Charrier & Lilian Martin, Nançay, le 4 Mars 2008

LNA : Layout de l’ASIC - On this photo of the ASIC, you can see the wide PMOS input transistor Didier Charrier & Lilian Martin, Nançay, le 4 Mars 2008

LNA : mesure de gain Fc (-3dB) ajustable de 10kHz à 1MHZ Fc (-3dB) > 200MHZ -The preamplifier features are a bandwidth ranging from eighty kilohertz to much more than two hundert megahertz (montrer le plot), -a preamplifier input noise better than one nano volt per square root hertz above therty megahertz, - a differential input capacitance of nine pico farad - and a max input dynamic of twenty four mili volt pick to pick Didier Charrier & Lilian Martin, Nançay, le 4 Mars 2008

LNA : mesure de bruit Cette mesure de bruit est effectuée avec une capacité de 10pF connectée sur l’entrée du LNA L’idéal serait de connecter un circuit RLC équivalent à l’impédance d’antenne mais: Résistance sans bruit thermique ! Résistance variable avec la fréquence ! 1 10 100 0.1 MHz 1nV/Hz 2nV/Hz Densité de bruit en nV/Hz Didier Charrier & Lilian Martin, Nançay, le 4 Mars 2008

LNA: réponse transitoire Réponse à une impulsion Réponse à un échelon OUT Géné: tfall = trise=2ns Out: tfall = trise = 2.3ns  trise LNA = 1.2ns OUT IN IN Réponse à des ‘gerbes cosmiques’ Gerbe proche Gerbe éloignée Gerbe très éloignée OUT OUT OUT IN IN IN Didier Charrier & Lilian Martin, Nançay, le 4 Mars 2008

Le LNA dans son environnement Filtre ‘France Inter’ LNA BALUN Alim Choix Gain sortie Signal ! Entrée Différentielle Antenne Didier Charrier & Lilian Martin, Nançay, le 4 Mars 2008

Mesure de bruit du ciel avec le dipôle CODALEMA Vsky2 Za Cin LNA H(f) 4kTR Calculé Mesuré Bruit du LNA mesuré Didier Charrier & Lilian Martin, Nançay, le 4 Mars 2008

Quelques spectres LW band SW band FM band Bruit «atmosphérique » Spectre mesuré sur le site d‘Auger dans la pampa Argentine Bruit «atmosphérique » Nuit 75dB ! Spectre mesuré à Nançay de jour et de nuit Jour 0.1 1 10 100 Frequency, MHz Didier Charrier & Lilian Martin, Nançay, le 4 Mars 2008

Antenne filaire croisée active Antenne filaire active à 2 polarisations (XY) Utilise deux LNA identiques à Codalema La forme en V-shape permet la détection d’onde électromagnétique en provenance de l’horizon La longeure total de l’antenne est augmentée de 1.21m à 3.22m ce qui augmente largement la sensibilité et abaisse la fréquence de résonance de 120MHz à 55MHz Maximum galactique Minimum galactique Le bruit galactique est 5dB au dessus du bruite du LNA à 30MHz et 80MHz Cette différence atteint 17dB à 55MHz Bruit du LNA Didier Charrier & Lilian Martin, Nançay, le 4 Mars 2008

Variations du bruit galactique Didier Charrier & Lilian Martin, Nançay, le 4 Mars 2008

Comparaison mesures / simulation 1-Modélisation de l’antenne et extraction de Za avec NEC2 3-Obtention de la réponse au ciel de l’antenne et comparaison avec les mesures 2-Calcul du bruit galactique à partir de cette température de ciel Didier Charrier & Lilian Martin, Nançay, le 4 Mars 2008

FIN Didier Charrier & Lilian Martin, Nançay, le 4 Mars 2008