1174 IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-19, NO. 3, MAY 1983

R I N G OSCILLATOR EXPERIYENT USING A HWFLE C I R C U I T

N. K o t e r a , A. Asano, Y. Harada, and U. Kawabe

Cen t ra l Resea rch Labora to ry , H i t ach i , L td . Kokubunji , Tokyo 185, Japan

A b s t r a c t

A r i n g o s c i l l a t o r c i r c u i t composed of Josephson j u n c t i o n d e v i c e s h a s b e e n d e s i g n e d and t e s t e d f o r t h e f i r s t t i m e . Nine h u f f l e c i r c u i t s , e a c h o f w h i c h i n c l u d e s two J o s e p h s o n j u n c t i o n s of a n i n - l i n e g a t e t y p e , a r e combined t o form an i n v e r t e r r i n g . The r i n g o s c i l l a t o r h a s p r o d u c e d o s c i l l a t i o n s w i t h 1.8- t o 5.2-11s p e r i o d s u s i n g DC power sou rces . Inve r t e r and f l i p - f l o p l o g i c o p e r a t i o n s a r e a l s o o b s e r v e d w i t h t h e s i n g l e h u f f l e c i r c u i t . Toe o p e r a t i n g m a r g i n s were i n v e s t i g a t e d and found t o b e r e a s o n a b l e a s a result o f comparing experimental threshold curves and a t h e o r e t i c a l - p r e d i c t i o n .

I n t r o d u c t i o n

DC-powered J o s e p h s o n l o g i c c i r c u i t s h a v e an advan tage ove r AC-powered o n e s i n t h a t t h e p o w e r i n g system i s s i m p l e . I n a d d i t i o n , c i r c u i t s y s t e m d e s i g n is p a r a l l e l w i t h c o n v e n t i o n a l s e m i c o n d u c t o r l o g i c c i r c u i t s .

So f a r t h r e e r e p r e s e n t a t i v e DC-powered Josepqsgn c i r c u i t s h a v e b e e n p r o p o s e d : h u f l e c i r c u i t s ’ -, c u r r e n t - s t e e r i n g f l i p - f l o p c i r c u i t 3 , and constant, v o l t a g e d r i v e n f l i p - f l o p c i rcui ts . Among t h e s e , h u f f l e c i r c u i t s m a t c h t h e p r e s e n t f a b r i c a t i o n t e c h n o l o g y i n t e r m s o f t h e j u n c t i o n c u r r e n t d e n s i t y , a n d h a v e t h e p o t e n t i a l f o r d r i v i n g t r a n s m i s s i o n l ines w i t h t e r m i n a t i n g resistors.

Some e x p e r i m e n t s h a v e b e e n r e p o r t e d t o d a t e on s u c h h u f f l e c i r c u i t s as cur ren t -swi tched Jaws-huff le c i r c u i t s , a n d h - s t a g e c h a i n c i r c u i t s i n w h i c h c r o s s - t y p e j u n c t i o n s a r e u s e d a s t h e s w i t c h i n g d e v i c e . However, m a g n e t i c a l 1 . y s w i t c h e d d e v i c e s a r e s u p e r i o r t o c u r r e n t s w i t c h e d d e v i c e s when t h e s i g n a l i s o l a t i o n be tween i npu t s and ou tpu t s i s c o n s i d e r e d .

Tn t h i s p a p e r , a r i n g o s c i l l a t o r c i r cu i t has been i n v e s t i g a t e d f o r a DS-powered j o s e p h s o n l o g i c c i r cu i t i n which a h u f f l e c i r c u i t w i t h m a g n e t i c a l l y s w i t c h e d i n - l i n e g a t e j u n c t i o n s i s adopted . The 9 - s t a g e r i n g o s c i l l a t o r o p e r a t i o n h a s b e e n e x p e r i m e n t a l l y a f f i r m e d and the c i r c u i t o p e r a t i n g m a r g i n s t u d i e d f o r f l i p - f l o p l o g i c o p e r a t i o n . The limits t o c i rcui t hang up, and r e s i s t i v e - l o a d d r i v a b i l i t y a r e d i s c u s s e d .

s

Circui t Des ign and Fabr i ca t ion

The h u f f l e c i r c u i t is d e s i g n e d t o h a v e a p a i r o f i n - l i n e g a t e j u n c t i o n s , GI and G2. To f a c i l i t a t e f l i p - f l o p l o g i c o p e r a t i o n a s well a s s i n g l e i n p u t NOR ( i n v e r t e r ) o p e r a t i o n , two c o n t r o l l i n e s a r e p r o v i d e d i n e a c h j u n c t i o n , a s F i g . 1 shows. Tn t h e h u f f l e c i r c u i t , t h e two s e p a r a t e i n p u t l i n e s , Il and 1 can be used for t h e f l i p - f l o p o p e r a t i o n . The common?;nput l i n e , IIN , i n t h e c i r c u i t i s used for i n v e r t e r o p e r a t i o n . I n t h a t c a s e , T I and I2 y e used a s i n d e p e n d e n t S i a s l i n e s t o s h i f t t h e o p e r a t l n g p o i n t s . Use of t h e common i n p u t l i n e is advantageous for a v o i d i n g a hung up phenomenon i n w h i c h b o t h s w i t c h i n g d e v i c e s , G I and G 2 , a r e i n t h e v o l t a g e s t a t e and t h e c i r c u i t d o e s n o t r e s p o n d t o i n p u t s i g n a l s . C o n s t a n t c u r r e n t s o u r c e s w i t h d i f f e r e n t p o l a r i t y , I ( + I and Z (-), a r e c o n n e c t e d a t source resistors, R . GBipolar c u r r e n t o u t p u t , I c a n t h e n b e o b t a i n e g t h r o u g h a c i r c u i t r e s i s t o r , ‘1:’ a n d a n i n d u c t i v e o u t p u t l i n e , denoted L i n F i g . 1.

G

Manuscr ip t received November 3 0 , 1982.

‘OUT1 L

R S

F i g . 1. H u f f l e c i r c u i t c o n f i g u r a t i o n i n c l u d i n g two i n - l i n e g a t e j u n c t i o n s . P a r a m e t e r s a r e : L = 57 pH, R = O.h$L, and Rs = 6 9.

F i g . 2. P h o t o m i c r o g r a p h o f t h e f a b r i . c a t e d h u f f l e c i r c u i t . Vinimum l i n e w i d t h i s 5 pm. J u n c t i o n windows a r e 25 pm square .

The c i r c u i t i s f ab r i ca t ed by means o f conven t iona l Pb-a l loy technology, wi th a minimum l i n e w i d t h o f 5 pm. %e j u n c t i o n s a r e fsrrned with Pb-In-Au and Pb-Si m a t e r i a l s . The r e s i s t o r m a t e r i a l is Au-Tn . The junction s i z e i s 23 pm s q u a r e and t h e s u p e r g u r r e n t d e n s i t y i s 750 A / c m . As a consequence i n - l i ne ga t e j u n c t i o n s h a v e a g a i n g r e a t e r t h a n o n e . The o b t a i n e d r e s i s t o r s , R and RS, h a v e v a l u e s of 0.6 and 5 a, r e s p e c t i v e l y . A photomicrograph of the fundamenta l h u f f l e c i r c u i t i s shown i n F i g . 2.

Expe r imen ta l Resu l t s

F l i p - F l o p L0gi.c O p e r a t i o n

Two j u n c t i o n s i n t h e s i n g l e h u f f l e c i r c u i t had s i m i l a r c u r r e n t - v o l t a g e and t h r e s h o l d c h a r a c t e r i s t i c s , withLn a d i f f e r e n c e o f o n l y 5 $. Rfter s w i t c h i n g , t h e j u n c t i o n r e s i s t a n c e , w a s e s t i m a t e d t o b e 5 8. The r e s i s t a n c e of p a i r e d ::’and R was 0.54sL. To test t h e f l i p - f l o p l o g i c O p e r a t i o n , t h e i n p u t was a p p l i e d a l t e r n a t e l y a t I1 and 12. I n p u t l e v e l s w e r e set t o b e 0 and 7 mA w h e r e g a t e c u r r e n t was I (+) I I (-) = 3.5 mA. F l i p - f l o p o p e r a t i o n was @hen af9irmed by d e t e c t i n g t h e v o l t a g e s , V ( G 1 ) and V(52). a c r o s s a c o u p l e o f c i r c u i t r e s i s t o r s , R . a s shown i n F i g . 3.

M)18-9464/83/0500-1174$01.00 0 1983 IEEE

1175

F i g . 3, O s c i l l o s c o p e t r a c e s h o w i n g f l i p - f l o p l o g i c o p e r a t i o n . The v o l t a g e s , V ( G 1 ) and V ( G 2 ) , swing from 0 t o t l . 9 mV i n c o r r e s p o n d e n c e w i t h t h e i n p u t c u r r e n t s , Il and 12.

w2 b o a - IO 0 IO

INPUT 1 1 ,I2 ( m A ) c3

F i g . 4 , Measured limit o f c i r c u i t o p e r a t i o n f o r i n p u t s I1 and 12, and magn i tude o f ga t e cu r ren t . The measured th reshold curve of t h e d e v i c e i s a l s o shown. The hung-up phenomenon occurs beyond IG = 3.75 mA.

I G '

The o u t p u t c u r r e n t was e s t i m a t e d t o b e 3 . 2 mA or 90 % o f t h e g a t e c u r r e n t l e v e l . The v o l t a g e 'J(G1) was 1.9 mV .

By c h a n g i n g t h e i n p u t c u r r e n t l e v e l s i n I1 and I2 s e p a r a t e l y a t a f i x e d g a t e c u r r e n t , IG, t h e o p e r a t i n g r e g i o n f o r t h e h u f f l e c i r c u i t was c l a r i f i e d i n t h e manner shown i n F i g . 4, w h e r e t h e h a t c h e d r e g i o n s a r e s u i t a b l e t o c i r c u i t o p e r a t i o n . The measured th reshold c u r v e is a l s o shown i n t h e f i g u r e , and t h i s c u r v e a g r e e s well w i t h t h e o p e r a t i n g p o i n t limit. Beyond a g a t e c u r r e n t o f 3.75 mA, b o t h d e v i c e s , G1 and G2, b e c a m e l a t c h e d i n t h e v o l t a g e s t a t e , and t h e hung up phenomenon o c c u r r e d .

The c i r c u i t g a i n , d e f i n e d a s IOUT/IIB, c a n b e d e d u c e d t o b e 1 . 5 f r o m F i g , 4 , I f I and are set a t -6.5 and' 4.5 mA, r e spec t ive ly . ' and &e i n p u t c u r r e n t a t 1 . 5 mA, t h e o u t p u t is e x p e c t e d t o b e 2.3 mA. D i s s i p a t e d power w i t h a g a t e c u r r e n t o f 2 . 5 mA is approximate ly 79 FW, where Joule l o s s due to r e s i s t a n c e s , R and RS, h a s b e e n t a k e n i n t o c o n s i d e r a t i o n .

I n v e r t e r a n d R i n g O s c i l l a t o r O p e r a t i o n s

I n v e r t e r l o g i c o p e r a t i o n was observed when t h e i n p u t c u r r e n t was a p p l i e d t o t h e c m m o n i n p u t l i n e , IIN, and when t h e DC b i a s c u r r e n t s , I1 = -7 m A and I = 3.5 mA. were s u p p l i e d t o t h e b i a s l i n e s . The D Z g a t e c u r r e n t was 3 mA. The c i r c u i t o p e r a t i o n mode is s c h e m a t i c a l l y shown i n , F i g . 5. The i n p u t c u r r e n t a p p l i e d a t ITN is c o u p l e d w i t h t h e d e v i c e s , G1 and G 2'

z t W

u t

.L'n? I I I

W O I ' ' I I I " I I

a II O I2 (3 TOTAL I N P U T

I I

l-

F i g . 5. Operation mode f o r t h e i n v e r t e r . DC b i a s c u r r e n t s , I, and 12, are s u p p l i e d and b i p o l a r i n p u t c u r r e n t is a p p l i e d a s shown b y t h i c k a n d t h i n a r r o w .

D C POWER MON I TOR (+I ?

F i g . 6. S c h e m a t i c i n t e r c o n n e c t i o n o f 9 - s t a g e r i n g o s c i l l a t o r .

s i m u l t a n e o u s l y a n d i n t h e same d i r e c t i o n , e i t h e r p a r a l l e l or a n t i p a r a l l e l t o the g a t e c u r r e n t .

When t h e i n p u t was p a r a l l e l , a s shown b y t h i c k a r r o w s i n F i g . 5, d e v i c e s G and G were r e s p e c t i v e l y i n t h e s u p e r c o n d u c t i n g and i n Zhe v o l t a g e s t a t e . R e s u l t a n t l y , t h e o u t p u t c u r r e n t was n e g a t i v e and d i r e c t e d i n w a r d t o w a r d s t h e c i r c u i t . When t h e i n p u t was a n t i - p a r a l l e l , t h e o u t p u t was p o s i t i v e and d i r e c t e d o u t w a r d . The c i r c u i t f u n c t i o n was s i n g l e - i n p u t NOR. If t h e d i r e c t i o n o f . t h e g a t e c u r r e n t was r e v e r s e d , t h e c i r c u i t f u n c t i o n became

e x p e r i m e n t a l l y a f f i r m e d i n 4- and 5 - s t a g e c h a i n s . s i n g l e - i n p u t OR. 'Chis i n v e r t e r o p e r a t i o n w a s

F o r p e r f o r m a n c e o f t h e r i n g o s c i l l a t o r o p e r a t i o n , DC b i a s c u r r e n t s w e r e i n i t i a l l y s u p p l i e d , a n d l a t e r t h e Dc g a t e c u r r e n t was i n c r e a s e d g r a d u a l l y . The s c h e m a t i c i n t e r c o n n e c t i o n o f t h e + s t a g e r i n g o s c i l l a t o r i s shown i n F i g . 6. O s c i l l a t o r o p e r a t i o n was monitored a t a c i r c u i t r e s i s t o r i n t h e p r e d e t e r m i n e d h u f f l e c i r c u i t .

3.15 mA, t h e v o l t a g e o s c i l l a t i o n shown i n F i g . G7 was When I 1 = -5 mA, I2 3.5 mA and IG(+) = I (-) =

d e t e c t e d a t t h e m o n i t o r t e r m i n a l . The o s c i l l a t i o n p e r i o d , T , was 5 n s a t a f r equency o f 187 MHz which i m p l i e s t h a t t h e c i r c u i t d e l a y , T , was around 0.27 ns . The c i r c u i t d e l a y i s simply deduced from T = T/18 f o r t h e 9 - s t a g e r i n g o s c i l l a t o r .

By chang ing t he DC b i a s c u r r e n t s , I1 and 12, a s well as t h e DC g a t e c u r r e n t , I (+) z IG(-), t h e p e r i o d , T , was var ied f rom 1 .8 to g.2 n s a s shown i n F i g . 8. The c o r r e s p o n d i n g c i r c u i t d e l a y , T , r a n g e d from 0 . 1 t o 0 .45 ns . Under a c o n s t a n t DC power

k e p t c o n s t a n t f o r more t h a n an hour . Sys temat ic s o u r c e , o s c i l l a t i o n was s t a b l e and pe r iod cou ld be

measurement was made a t t h r e e k i n d s of DC b i a s

1176

- T I M E F i g . 7. O s c i l l o s c o p e t r a c e s h o w i n g 9 - s t a g e r i n g o s c i l l a t o r o p e r a t i o n o f h u f f l e c i r c u i t . O s c i l l a t i o n per iod is 5 n s ( a b s c i s s a : 5 ns /d iv . ) .

1 0 7 ' 1 c n fn U

0.4

0.2 > a -I

0' I I I ' I 0 2.6 3.0 3.4

G A T E C U R R E N T I G ( m A )

Fig . 8. O s c i l l a t i o n p e r i o d o f r i n g o s c i l l a t o r measured a s f u n c t i o n o f DC g a t e c u r r e n t , IG. From l o w e s t c u r v e t o h i g h e s t , DC b i a s c u r r e n t s a r e : (I,, I?) = ( -7, 3.5) , ( -7 , 3 . 0 ) . and (-5, 3.5) i n u n i t s o f mB. Deduced c i r c u i t d e l a y i s a l s o shown.

c o n d i t i o n s : ( I1 ' I ) = ( -7 , 3.5). ( I1 , 12) = ( -7 , 3 . 0 ) . and (I1, 12? =. ( - 5 , 3 . 5 ) . U n i t s h e r e a r e mA. and a g r a p h i c d e s c r l p t l o n i s provided in F ig . 8. As e x p e c t e d , t h e p e r i o d d e c r e a s e d when t h e g a t e c u r r e n t was i n c r e a s e d .

The c i r c u i t d e l a y i s b a s i c a l l y d e t e r m i n e d b y t h e r a t i o L/R. S i n c e t h e o u t p u t l i n e i n d u c t a n c e , L, is d e s i g n e d t o b e 67 pH, the expec ted Va lue O f Tis around 110 p s . The e x p e r i m e n t a l r e s u l t s a g r e e f a i r l y w e l l w i t h t h i s . I n d u c t a n c e i n t h e h u f f l e l oop composed o f t w o i n - l i n e g a t e j u n c t i o n s and two c i r c u i t r e s i s t o r s was e s t i m a t e d t o b e 10 pH d u r i n g d e s i g n . The i n d u c t a n c e d o e s n o t g r e a t l y a f f e c t c i r c u i t r e s p o n s e .

Discuss ion

The Hung Up L i m i t Theory, and I ts Correspondence with Experiment

c o n s i d e r t h e c a s e where device G i s superconduct ing I n t h e h u f f l e c i r c u i t shown i n F i g . 1. l e t ' s

b u t d e v i c e G2 is i n t h e v o l t a g e s t a t e . A prob lem tha t d e v e l o p s i s whether device G i s r e t u r n e d t o the s u p e r c o n d u c t i n g s t a t e when d e v i g e GI i s s w i t c h e d i n t o t h e v o l t a g e s t a t e . A t t h e moment, t h e i n p u t f o r G has a l r eady been r emoved , and t he i npu t fo r G1 hag newly been appl ied to . It must be assumed tha t the

1

time c o n s t a n t , L/R, i s much g r e a t e r t h a n t h e d e v i c e swi tch ing time. I f t h e d e v i c e G2 f a i l s t o r e t u r n t o t h e s u p e r c o n d u c t i n g s t a t e , b o t h d e v i c e s a r e i n t h e v o l t a g e s t a t e , and t h e c i r c u i t h a s become hung up.

A t t h e i n s t a n t when d e v i c e 'GI h a s j u s t b e e n s w i t c h e d , v o l t a g e a p p e a r s a c r o s s t h e d e v i c e . As long a s RJ+2R 2 2R where R i s t h e n o r m a l r e s i s t a n c e of the junct io!N'device, %e v o l t a g e t h a t a p p e a r s is e q u a l t o t h e g a p v o l t a g e , V (2 .8 mV i n e x p e r i m e n t s ) . I n t h e e a r l y s t a g e s o f t h e s w i t c h i n g , t h e e f f e c t o f t h e i n d u c t a n c e , L, c an be i gnored . Thus, d e v i c e G j u s t s e e s t h e l o a d r e s i s t a n c e R +2R. where device G 1

i s e q u i v a l e n t t o r e s i s t a n c e R ~ . J~ v o l t a g e p u l s e w i t 2 a magn i tude o f V /(2R/R +1) is t h e r e f o r e a p p l i e d i n s t a n t a n e o u s l y t o 8 e v i c e 6,.

I n d e v i c e G2, a n e g a t i v e v o l t a g e -Vo, i s main ta ined th roughou t , because G2 was i n t h e s t e a d y v o l t a g e s t a t e , t h e v a l u e o f which i s

G

A t t h e i n s t a n t o f GI s w i t c h i n g , t h i s v o l t a g e p u l s e is superimposed on G2. If

t h e r e is a c h a n c e t h a t d e v i c e G may be under zero a p p l i e d v o l t a g e . T h i s i n e q u a l i t y i s a n e c e s s a r y c o n d i t i o n f o r d e v i c e G2 t o r e t u r n t o t h e s u p e r c o n d u c t i n g s t a t e , i .e. t o a v o i d t h e hung up phenomenon. By s u b s t i t u t i n g ( 1 ) i n t o ( 2 ) . we g e t

2

Under a r easonab le a s sumpt ion o f

R J / R N N = 10, ( 4 )

Eq. ( 3 ) c a n e a s i l y b e s o l v e d , a n d t h e l i m i t i n g

I I I I I

x

0 0.5 I. 0

CURRENT RATIO IG / A J C F i g . 9. A n a l y t i c a l l y c a l c u l a t e d l i m i t i n g c u r v e f o r occurence o f hung-up phenomenon ( s o l i d l i n e ) . Computer s i m u l a t i o n r e s u l t s a r e a l s o shown. Crosses c o r r e s p o n d t o where hung-up phenomenon occur s . So l id circles c o r r e s p o n d t o where hang-up d o e s n o t a p p e a r .

g a t e c u r r e n t , T , c a n b e d e r i v e d a s a f u n c t i o n o f t h e c i r c u i t r e s i s t a g c e , R.

The l i m i t i n g c u r v e i.s c a l c u l a t e d n u m e r i c a l l y , a n d shown b y t h e s o l i d l i n e i n F i g . 9. I n t h e f i g u r e , R N N I G / V G ranges f rom 0 t o 0 . 6 . The a b s c i s s a c a n b e rewrr t ten by the approximate Ambegaokar-Bara tof f ' s r e l a t i o n ,

RNNAJC = 0.6VG, (5 )

where A i s t h e j u n c t i o n a r e a , and J t h e s u p e r c u r r e n t d e n s i t y . As i s shown i n F i g . 9. g h e l i m i t i n g c u r v e is wel l approximated by the s imple equat ion R I = 0.8V . R e s u l t a n t l y , t h e n e c e s s a r y c o n d i t i o n ( 3 ) For avo ig ing t he hung up phenomenon can be g iven by

RIG &0.8VG. ( 5 )

Using t he expe r imen ta l va lues R 0 .6 and V = 2.8 mV, we c a n d e t e r m i n e t h e maximum g a t e c u r r h t va lue o f 3 .7 mA. T h i s t h e o r e t i c a l p r e d i c t i o n a g r e e s well wi th t he expe r imen ta l va lue o f 3 .75 mA shown i n F i g . 4.

I n o r d e r t o c o n f i r m t h e a n a l y t i c a l e x p r e s s i o n i n ( 6 ) . computer s imula t ions were unde r t aken o f t he c i r c u i t o p e r a t i o n . The hung up phenomenon occurred beyond t h e limits o f ( 6 ) a s shown b y t h e c r o s s e s i n F ig . 9. The s o l i d c i rc les i n d i c a t e t h a t t h e c i r c u i t d o e s n o t e n t e r a hung-up s t a t e . Under t h e c o n s t r i c t i o n o f R I G = 0.5VG. it was f o u n d t h a t t h e hung up phenomenon wlll n o t o c c u r .

Cons ide ra t ions Rega rd ing t he T ime Cons tan t , L/R

Tn o r d e r f o r c i r c u i t h a n g up t o b e a v o i d e d , t h e o u t p u t l i n e i n d u c t a n c e , L. s h o u l d n o t b e t o o s m a l l . If t h e time c o n s t a n t , L/R, is s m a l l e r t h a n t h e b u i l d up time f o r t h e v o l t a g e p u l s e , t h e d e v i c e i n t h e v o l t a g e s t a t e c a n n o t h a v e a n o p p o r t u n i t y t o r e t u r n t o t h e s u p e r c o n d u c t i n g s t a t e .

The b u i l d up time can be approximated by the sum of two RC time c o n s t a n t s , ( R C +R C ) , where CJ is t h e c a p a c i t a n c e of t h e i n - l i n e g a t e J u n c t i o n , and R i s g iven by 2RRJ/(2R+R ) C 2R. In the example , R i N C J c o r r e s p o n d s t o t h e gime f o r a v o l t a g e , V G , t o a p p e a r a t d e v i c e G , , w h i l e R C i s t h e t i m e f o r a v o l t a g e p u l s e t o b e b u i l t up 'a$ d e v i c e G2., Thus , t he f o l l o w i n g e q u a t i o n i s n e c e s s a r y f o r a v o r d a n c e o f t h e hung up phenomenon,

N N J P J

, ( L / R ) / C J ( R N N + R p ) >> 1. ( 7 )

I n t h e e x p e r i m e n t , t h i s c o n d i t i o n was wel l s a t i s f i e d . T h e r e f o r e , t h e c i r c u i t d e l a y time was m o s t l y t h o u g h t t o b e d e t e r m i n e d b y t h e t i m e c o n s t a n t L/R.

C i r c u i t A p p l i c a t i o n

I n t h i s s t u d y , a s i m p l e i n - l i n e g a t e j u n c t i o n was used fo r t he swi t ch ing dev ice . However, it i s d e s i r a b l e t o u s e i n t e r f e r o m e t e r d e v i c e s f o r a p p l i c a t i o n . to i n t e g r a t e d c i r c u i t s b e c a u s e d e v i c e impedance can then be des igned to be much h i g h e r . The c i r c u i t d e l a y i s t h u s e x p e c t e d t o b e much s m a l l e r . If a two-input 1-2-1 i n t e r f e r o m e t e r is a d o p t e d , a two-input NOR c i r c u i t i s obta ined . Such a c i r c u i t f u n c t i o n i s e n o u g h f o r c o n s t r u c t i o n o f a n y of l o g i c f u n c t i o n s .

I n o r d e r t o c l a r i f y w h e t h e r t h e h u f f l e c i r c u i t c a n d r i v e a r e s i s t i v e l o a d o r n o t , we conducted computer s imula t ions by add ing a load r e s i s t o r , r , connec ted in series t o t h e i n d u c t a n c e , L. It was found t h a t t h e c h o i c e of r = R = R / 2 l e a d s t o a good r e s u l t . T h e r e f o r e , t h e h u f f l e c i r c u i t i s p r o m i s i n g f o r f u t u r e a p p l i c a t i o n s t o J o s e p h s o n l o g i c s y s t e m s .

N M

1177

Conclusion

( 1 ) Ring o s c i l l a t o r o p e r a t i o n was demonst ra ted us ing Josephson j unc t ion dev ices . To b r i n g t h i s a b o u t , a DC-powered h u f f l e c i r c u i t was des igned t o form a 9 - s t a g e i n v e r t e r r i n g . 'he o s c i l l a t i o n p e r i o d could be changed from 1 .9 t o 8 . 2 n s i n e x p e r i m e n t s . ( 2 ) F l i p - f l o p l o g i c o p e r a t i o n was observed expe r imen ta l ly , and ope ra t ing po in t limits were c l a r i f i e d . ( 3 ) T t was f o u n d t h a t t h e o p e r a t i n g m a r g i n was de te rmined by t he t h re sho ld cu rve , and t he hung up f o r m u l a t h a t was a r r i v e d a t a n a l y t i c a l l y .

References

1. A.F. Hebard, S.S. Pei. L.N. Dunkleberger, and T.A. F u l t o n , " A DC-Powered Josephson F l ip-Flop ," TEEE Trans . Magn., Vol. MAG-15, pp. 408-1 1 , January 1979.

2. T.A. F u l t o n , S.S. Pei . and L.N. Dunkleberger , "Josephson Junct ion Current-Switched Logic C i r c u i t s , " I E E E Trans. Yagn., Vol. MAG-15. pp . 1876-7'3, November 1'379.

3. A. Moser, lfLogic Gates with Shaped Josephson J u n c t i o n s , " I E E E J. o f S o l i d - S t a t e - C i r c u i t s , Vol.

4. W . Raechto ld , I r A Flip-Flop and Logic Gate with Josephson Junc t ions ,? ' I E E E ISSCC D i g e s t o f Technica l Paper , No. FAM14.2, pp. 164-55 and 227, February 1975.

SC-14, pp. 572-79, August 1979.

N o t e t h a t t h e h u f f l e c i r c u i t d e s c r i b e d h e r e is e x a c t - l y t h e same as t h e "Complementary Josephson Junction C i rcu i t " r epor t ed by W. Baechto ld , T . F o r s t e r , W. Heu- be rge r and T.O. Mohr i n E l e c t r o n i c s Letters, 2 , 2 0 3 , May 1975.