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Submitted on 1 Jan 1978
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NARROW-BAND ULTRA-LOW CURRENT MEASUREMENTS WITH A RF SQUID
F. Bordoni, P. Carelli, I. Modena, G. Romani
To cite this version:
F. Bordoni, P. Carelli, I. Modena, G. Romani. NARROW-BAND ULTRA-LOW CURRENT MEA-
SUREMENTS WITH A RF SQUID. Journal de Physique Colloques, 1978, 39 (C6), pp.C6-1213-C6-
1214. �10.1051/jphyscol:19786536�. �jpa-00218023�
NARROW-BAND ULTRA-LOW CURRENT MEASUREMENTS WITH A RF SQUID
F. Bordoni, P. Carelli , I. Modena and G.L. Roman!
Laboratovio Plasma Spasio - C.N.R. - C.P. 27-Fraseati . Italy
*Labovatovio Elettvonioa Stato Solido - C.N.R. Via Cineto Romano 42 - Rome - Italy
Abstract.- The use of a ferromagnetic transformer coupled to a SQUID, as previously described, per- mits to measure very low currents. A low loss ceramic core for the transformer has yielded to further
increase in the sensitivity of about two orders of magnitude in a narrow-band around a fixed frequen- cy. At present the achieved sensitivity of about 8 x 10-1* pA in 1 Hz bandwidth at 150 Hz.
The use of a superconducting magnetometer as a high sensitivity current detector has been propo- sed by some authors a few years ago /1,2,3/. More recently, by coupling a transformer with a ferroma- gnetic core to a rf SQUID / 4 / , we have enhanced the overall current*sensitivity up to very interesting figures /5/. Due to the particular ferromagnetic material employed /6/ the useful bandwidth of the device was constrained between dc and a corner fre- quency dictated by the strong frequency dependence of the relative permeability of the used core. In order to enhance the current sensitivity at a fixed frequency we inserted a tuning capacitor in the input circuit. However, due to the high value of the electrical conductivity of the core a large amount of eddy-current losses was present, limiting the Q of the urned circuit to not interesting va- lues. This drawback has been overcome by means of a suitable choice of a ceramic core for. the trans- former. We have tested a few commercially available ferrites in order to select those which preserve an acceptable value of the relative permeability at li- quid helium temperature. The chosen material has been used to make up a superconducting transformer coupled to the SUID as shown in figure 1. The cur- rent I (v) is supplied by a current generator I to the input circuit which consists of a capacitance C paralleled to the primary L (N turns) of the
transformer. The secondary winding L (N turns) is connected with the inductance L to obtain as usual a flux transformer. All winding are made of super- conducting wire. M ™ = K_p/L L is the mutual in- ductance between L and L , while M = K/L L is the
mutual inductance between L and the inductance of the SQUID, L.
A straightforward analysis /5/ of the circuit of figure 1 shows that, once the matching condition for the flux transformer is satisfied, i.e. L„ y L„
"in situ", the current sensitivity of the device is enhanced by a factor % - ^ — - — below the resonance frequency. At resonance a further gain is obtained, given by the factor of merit Q of the input circuit.
A typical experimental result obtained with this technique is shown in figure 2, where the in- put current i"(v), which causes the SQUID to slip one flux quantum, is reported versus frequency. Da- ta are taken with the following set of experimental parameters : C : 0.312 yF, NT = 8360, N = 6, KT p = 0.95, Lg = 10.5 liH.
All superconducting coils have been wound using a 50 ym formwar insulated Nb wire. One Fig. 1 : Schematic of the experimental arrangement used for narrow-band ultra-low current measurements.
The room temperature current generator I feeds the
"tuned" input circuit C,LT : Lj and Lp are wound over the ferromagnetic core. All inductances L^,. Lp
and Lg are made up with superconducting wire. For other symbols meaning see text.
JOURNAL DE PHYSIQUE
Colloque C6, supplément au n° 8, Tome 39, août 1978, page C6-1213
Résumé.- L'utilisation d'un transformateur à noyau ferromagnétique couplé à un SQUID permet de mesu- rer de très faibles courants, comme nous l'avons précédemment montré. L'utilisation d'un noyau céra- mique à faible pertes pour le transformateur a permis d'augmenter ultérieurement la sensibilité de presque deux ordres de grandeur dans une bande étroite autour d'une fréquence donnée. Actuellement on obtient une sensibilité de presque 8 x ÎO-1* pA dans une largeur de bande de J Hz à 150 Hz.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19786536
One P h i l l i p s " p o t c o r e " Ferroxcube 3B2 g r a d e h a s been used f o r t h e t r a n s f o r m e r : g e o m e t r i c a l dimen- s i o n s o f t h e c o r e o b v i o u s l y c o n s t r a i n e d t h e maximum number of t u r n s f o r t h e p r i m a r y winding L
T'
I
, c , , a , , ,,
, , , c , c 1 ,,
, a , , , a ,1 10 100
12
' " "FREQUENCY (Hz)
F i g . 2 : (*) : e x p e r i m e n t a l b e h a v i o r iomS v e r s u s f r e q u e n c y ; i o i s t h e c u r r e n t which, f l o w i n g i n t h e i n p u t "tuned" c i r c u i t c a u s e s t h e SQUID t o s l i p one f l u x quantum. E x p e r i m e n t a l v a l u e s of t h e r e l e v a n t p a r a m e t e r s a r e : NT = 8360, Np = 6, L = 10.5 pH, L = 4.2 x lo-'' H, KTp = 0.95, M = 5.75 x lo-' H and C = 0.312 pF.
( A ) : measured root-mean s q u a r e f l u x n o i s e v a l u e s
i n u n i t s of
@,,
p e r H Z ' ~ v e r s u s f r e q u e n c y .A two-hole symmetric SQUID (L = 4.2 x 10-lo H) has been used t o o b t a i n a v a l u e f o r M a s l a r g e a s pos- s i b l e . With t h e v a l u e of t h e i n d u c t a n c e LS mentio- ned above we a c h i e v e d e x p e r i m e n t a l v a l u e f o r t h e mutual i n d u c t a n c e M = 5.75 x lo-' H. The measured v a l u e of i 0
2
22 PA/@ a t f r e q u e n c i e s below r e s o - 0 n a n c s w e l l a g r e e s w i t h t h e c a l c u l a t e d v a l u e i0c 't
- - "
Np a 20 PA/@,- It i s e v i d e n t from d a t a f i - K~~ NT %g u r e 2 t h a t an improvement i n c u r r e n t s e n s i t i v i t y of a b o u t two o r d e r s o f magnitude a t a f i x e d frequen- c y i s o b t a i n e d by means of r e s o n a n c e .
A s t h e u l t i m a t e performances of t h e system a r e de- termined by t h e s i g n a l t o n o i s e r a t i o , i t i s impor- t a n t t o know t h e o u p u t n o i s e spectrum. I n f i g u r e 2 we a l s o r e p o r t t h e measured o u t p u t n o i s e e x p r e s s e d
i n f l u x quantum u n i t s p e r v e r s u s f r e q u e n c y . Noise measurements have been performed s e n d i n g t h e
c e s s i v e l y s q u a r e d and i n t e g r a t e d . Data show t h a t t h e o u t p u t n o i s e i s m o d e r a t e l y dependent on t h e f r e q u e n c y : i t s v a l u e , a t r e s o n a n c e , i s o n l y a b o u t one o r d e r of magnitude l a r g e r t h a n t h e v a l u e o b t a i - n a b l e w i t h t h e SQUID n o t coupled t o a t r a n s f o r m e r . T h i s means t h a t , a t t h e r e s o n a n c e f r e q u e n c y , t h e minimum d e t e c t a b l e c u r r e n t , i n 1 Hz bandwidth, i s
i a 0 . 8 x A.
min 'L
The s e n s i t i v i t y of t h e d e v i c e i s l i m i t e d by t h e r e l a t i v e l y low v a l u e o f t h e f a c t o r o f m e r i t of t h e i n p u t c i r c u i t (Q
%
8 5 ) . T h i s drawback d o e s n ' t seem due t o l o s s e s i n t h e f e r r i t e c o r e . The proper- t i e s of t h e c e r a m i c c o r e a s g i v e n by f a c t o r y ' s spe- c i f i c a t i o n s s h o u l d y i e l d t o a Q s e v e r a l o r d e r of magnitude l a r g e r t h a n t h e measured one. Moreover t h e s m a l l Q-value c a n n o t b e due t o l o s s e s i n t h e c a p a c i t a n c e , b e c a u s e of t h e above mentioned beha- v i o r of t h e n o i s e v e r s u s f r e q u e n c y . We b e l i e v e t h a t t h e l i m i t a t i o n s t o t h e r e s o n a n t g a i n i n t h e i n p u t c i r c u i t may be j u s t i f i e d a s due t o an i n p u t d i s s i - p a t i v e term o f t h e SQUID, which s h o u l d b e r e p o r t e d i n t o t h e i n p u t c i r c u i t , i n c r e a s e d by a f a c t o r%
N ~ . The a u t h o r s a r e i n d e b t e d t o P r o f . R.P. G i f f a r d f o r u s e f u l d i s c u s s i o n s .R e f e r e n c e s
/ I / C l a r k e , J . , Tennant, W.E. and Woody, D . : J.
Appl. Phys.
41
C1971) 3859./ 2 / Davidson, A . , Newsbower, R.S. and B e a s l e y , M.R.
Rev. S c i . I n s t r u m .
45
(1974) 838./ 3 / C l a a s s e n , J . H . , J . Appl. Phys.
5
(1975) 2268 / 4 / C a r e l l i , P., Modena, I. and Romani, G.L., Ad-vance i n Cryogenics E n g i n e e r i n g
2
(1978) 512./ 5 / Barbanera, S., C a r e l l i , P., Modena, I. and Romani, G.L., J . Appl. Phys. (1978) i n p r e s s . 161 Cryoperm 10 s u p p l i e d by Vacuumschmeltz G.M.B.H.
Hanau, W. Germany.
" l i n e a r i z e d " SQUID o u t p u t t o a l o c k - i n used a s a narrow-band a m p l i f i e r , t h e o u t p u t of which i s suc-
