Dueholm, Benny

Finnegan, T

Hansen, J

Lindelof, P.

Mygind, Jesper

Pedersen, Niels Falsig

Sørensen, O H

Three different schemes for coupling to low impedance Josephson devices have been investigated. They all employ superconducting thin-film microstrip circuit techniques. The schemes are: (i) a quarterwave stepped impedance transformer, (ii) a microstrip resonator, (iii) an adjustable impedance transformer in inverted microstrip. Using single microbridges to probe the performance we found that the most primising scheme in terms of coupling efficiency and useful bandwidth was the adjustable inverted microstrip transformer

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Downloaded from orbit.dtu.dk on: Dec 17, 2017Microstrip coupling techniques applied to thin-film Josephson junctions at microwavefrequenciesSørensen, O H; Pedersen, Niels Falsig; Mygind, Jesper; Dueholm, Benny; Finnegan, T; Hansen, J;Lindelof, P.Published in:I E E E Transactions on MagneticsLink to article, DOI:10.1109/TMAG.1981.1060965Publication date:1981Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Sørensen, O. H., Pedersen, N. F., Mygind, J., Dueholm, B., Finnegan, T., Hansen, J., & Lindelof, P. (1981).Microstrip coupling techniques applied to thin-film Josephson junctions at microwave frequencies. I E E ETransactions on Magnetics, 17(1), 107-110. DOI: 10.1109/TMAG.1981.1060965IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-17, NO. 1, JANUARY 1981 107 MICROSTRIP  COUPLING  TECHNIQUES  APPLIED TO THIN-FILM JOSEPHSON  JUNCTIONS  AT  MICROWAVE  FREQUENCIES O.H. Soerensen,  N.F. Pedersen,  J.  Mygind, B. Dueholm The  Technical  University of Denmark,  DK-2800  Lyngby T.F. Finnegan , J.  Bindslev Hansen, P.E. Lindelof H.C. 0rsted  Institute,  University of Copenhagen,  DK-2100 * Summary Three  different  schemes  for  coupling  to  low impedance  Josephson  devices  have  been  investi- gated.  They  all  employ  superconducting  thin- film  microstrip  circuit  techniques.  The  sche- mes  are:  (i) a  quarterwave  stepped  impedance transformer,  (ii)  a  microstrip  resonator,  (iii) an  adjustable  impedance  transformer  in  inver- ted  microstrip.  Using  single  microbridges  to probe  the  performance we  found  that  the  most primising  scheme in  terms of coupling  effici- ency  and  useful  bandwidth was the  adjustable inverted  microstrip  transformer. Introduction The  succesful  application  of  Josephson  junc- tions  in  high  frequency  systems  depends  upon how  efficiently  the  devicesare  coupled toexternal  microwave  lines.  The  Josephson  ele- ments  are  often  very  low  impedance  devices, e.g., microbridges or high  current-density tunnel  junctions.  This  accentuates  the  demand for  coupling  schemes of low  characteristic impedance  at  high  frequencies. In applications such  as  the  fluxon  oscillator' a  single, long junction  must  be  matched.  Also  in  other  Jo- sephson  microwave  oscillators  with  single junctions2 or coherent  arrays3, it is  advan- tageous  to  operate at low  impedance  levels  as shown  by  the  following  estimate.  If a N-junc- tion  array  is  matched  to  an  external  load, RL = NRJ,  the  maximum  available  power  in  the high  frequency  limit (Vdc > >  R  I ) becomes PN = N(R I ) /8RJ.  Since  RJIc  is a  constant, the  available  power  increases  inversely  pro- portional  to RJ (and  proportional  to  N) . The junctions cannot  be infinitely  closely  packed due  to  mutual heating, and  hence  the  number of  junctions  which  can be accomodated  at  equi- valent  positions  is  limited,  and  the  resulting rf-impedance,  NRJ,  may  remain  low. Finally,  in  applications  such  as  the  parame- tric  reflection  amplifier'  optimum  performance is  achieved  if  the  signal  feed  line  has  cha- racteristic  impedance  less  than  the  normal state  resistance  of  the  parametric  device. M a consequence, it is natural  to look for  mi- crowave  circuits  of  low  impedance, i.e., at the 0.1R to 10 level. Here, we report on three  different  methods  to approach  the  problem.  They  all  use  supercon- ducting  thin-film  microwave  integrated circuit techniques because:  the  thin-fih  Circuits are pro- duced in the same processes which are  used  to  fabricate the junctions,  and  because  superconducting  thin-film circuits  have low loss, even  at  high  frequencies.  The 2 J c  J c  *Present  address:  Physikalisch-Technische Bundesanstalt,  Institut  Berlin,  Abbestrasse 2-12, 1000 Berlin 10, BRD. three  schemes  are:  (i)  a  quarterwave  high/low impedance  transformer,  (ii)  a  strip-resonator with  the  Josephson  device  mounted at the  end or in the center, (iii) a novel  approach  in- troducing  adjustable  impedance  in  microstrip design. The  scheme  employs  an  inverted  micro- strip  transformer  coupling. In all  three  cases single  thin-film  microbridges of normal  state resistances  in  the  range  0.02R  to  0.15R were used  to  probe  the  performance of the  circuits. I.  Quarterwave  high/low  transformer The thin-film  circuit  pattern  is  shown i Fig. 1 (a). The  location of the  microbridge is indi- cated  by  the"x".  The  circuit was  made on a 25.4~25.4~1 mm soda-glass  substrate. The impe- dance  transformer  consists  of  two  2-section X/4 transformers  separated  by a X/2 section. The  first  2-step  transformer  is  b'roadband  with moderate  impedance  ratio.  The 50R input  line is  transformed  into 19R. In order to  reach  the desired  low  output  impedance,  the  second 2- step  transformer  operates  with  a  very  large impedance  discontinuity  (122R)  such  that  the overall  impedance  ratio  becomes 60:l. The circuit  was  designed  to  have a center  frequ- ency of 10 GHz.  The  performance of the  circuit is  illustrated in Fig.  2(a)  to  (e).  In  (a) the  measured  power  reflection vs.  frequency is  shown  without  microbridge. In (b)  a  simi- lar recording,  made  after  a 70 mil bridge was introduced in the  circuit,  is  shown. A compa- rison  shows  that  the cut and  the  bridge  match- es  the  transformer  structure  in  a '1.300 MHz bandwidth at 10.4 GHz.  Fig.  2(c)  shows  the measured  input  signal  necessary  to  maintain a  constant  suppression of the  supercurrent as  the  input  frequency  is  varied.  Maximum attenuation  means  maximum  power  transmitted to  the  bridge  detector. In Fig.  2  (d)  the bridge  is  biased  at a fixed  voltage  and  the receiver  frequency  is  varied.  Each  trace inthe set of curves  in  (d)  shows  the  measured Josephson  radiation  vs.  frequency.  The  fre- quency  dependence of the  individual  traces is  a  convolution  of  the  Josephson  line-width r t Fig. 1. Thin-film  circuit  patterns. (a)  4-section X/4 stepped  impedance  transfor- mer.  (b)  Strip  resonator  with  end  connected bias  leads  and  side  coupled  rf-input  line. The  junction  is  located at the X .  0018-9464/81/0100-0107$00.75 0 1981 IEEE 108 with  the  transformer  and the  receiver  response functions.  The  envelope  curve  reproduces  the transformer/receiver  pass-band. The peak  posi- tions  in  (c)  and  (d)  coincides. Finally, (e) shows  the  calculated  power  reflection  coeffi- cient  of  the  properly  terminated  transformer structure  assuming  that  all  sections  are A/4 at 10 GHz.  The  calculated  and  the  measured widths of the  reflection  minimum  are  in  good agreement.  The  small  disagreement  between  the calculated  and  the  measured  center  frequencies are  not  significant  perse  since  the  microstrip design  formulae  are  accurate  only  to  within  a few pct. It is  not  obvious  why  the  70 mQ brid- ge behaves  as  a  matched  termination  when  the theoretical  matching  impedance  is 0.951, or why the  transmission-peak  falls  below  the  reflec- tion-dip. Tentatively,  we  may understand  these results  by  invoking  the  well-established  facts that  microbridges  are  accompanied  by  a  series inductance2  and  that  the  effective  lengths ofsections  of  microstrip  are  influenced  by  step- in-width  discontinuities5. In a  model calcula- tion we  have  been  able  to  reproduce  the  rela- tive  frequency shift  between  reflection  and transmission  by  taking  these  stray  reactances into  account. The  maximum  integral  power  detected  from  a microbridge was 2.4 pW in  a 250 MHz linewidth (power  density 0.01 pW/MHz). 11. Microstrip  resonator A  different  approach to, low-impedance coupling  is  to  employ a  resonator  as  impedance transformer. The  resonator  circuit  is . hown in Fig. l ( b ) .  The  high-impedance  lines  leading  to the  ends of the  resonator  section  are  filtered bias  leads.  An  rf-line  is  loosely  sidecoupled to the resonator.  The  microbridge was located either  in  the  center  (Fig.  lb)  or at the end of  the  resonator.  Both  conventional  and  inver- ted  microstrip  versions were studied.  The  re- sonator  length was X/2 at 10 GHz  in  conventio- nal  microstrip.  With  the  bridge at the  end,the measured  resonance  frequencies  were 9.5 GHz in conventional  and 12.5 GHz in  inverted microstrip  respectively.  With  the  bridge  in the  center,the  corresponding  resonance  frequ- encies  were 13 GHz  and 17 GHz. In all  cases the  quality  factor was Q = 200  as  inferred from  power  reflection  measurements. The dif- ference  in  resonance  frequencies  between conventional  and  inverted  microstrip  simply reflects  the  different  effective  dielectric constants. The  frequency  shift  caused by  the change  in  bridge  position  is  not  as  obvious. It may  be  understood  only  if  the  presence of the  bridge  (and  the 0.5 pm wide cut across the  resonator)  changes  the  effective  length of the  resonator.  This  is  consistent  with  the observation  already  made  that  the  microbridge is  accompanied  by  a  non-negligible  series  in- ductance. Fig. 3 illustrates  the  performance of the  con- ventional  microstrip  resonator  with  a  single center-located  microbridge.  The  current-volta- ge  curve  (IVC) of the  bridge  and  the  emitted Josephson  radiation  are  shown. A cavity  indu- ced  step  is  clearly  visible  in  the  IVC at 27 pV corresponding  to  a  resonance  center  fre- quency of 13.1  GHz.  With  the  microwave  recei- ver  centered at resonance,  a  sharp  peak  in the  Josephson  radiation  is  measured  as  the After cut E bridge I 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1  9.0 9.5 10.0 10.5 11.0 11.5 I +++ 8 + +++ + + + + + + 2 Y a2 h ++ A *  0 2 - 1.01 ( e) 3 -* 3 0.8- 0 **+ i3 y - d I I I I I I I I ~ ~ ~ ~ + ~ I I I I I I I I I  -. *++ ,**- **** * ii 0.6- n 0.L - ++ ++ +*++ +++ +++ 0 + + + +  + +  .c - 9.0 9.5 10.0 10.5 11.0 -I I I I I I I I IFrequency (GHtI Fig. 2. Performance of multistep  transformer. (a)  Measured  power  reflection  vs.  frequemcy before scratching bridge. (b) Same as (a) with bridge present. (c) Measured attennuator set- ting in  input  line  to  maintain  constant  super- current  suppression.  (d)  Measured  power  output from  bridge.  Each  trace  corresponds  to a fixed dc-bias  voltage as indicated.  (e)  Calculated power  reflection  coefficient. 109 1000 750 - 4 H 500 LT LT 3 0 250 0 Indium - R4 ( RN O.l'2) T 3.29 K T,= 3.41 K ~m43.1 GHz I I I I I 0 10 20 30 LO VOLTAGE 5 [pVl Fig .  3 ,  IV-curve  for  microbr idge  in  s t r ip  resonator  and  measured  spec t ra l  dens i ty  wi th  a f ixed  rece iver  f requency  ( reso lu t ion  bandwidth  5 MHz). The i n s e t  shows the  IF  v ideo  ou tpu t  VS. LO-frequency. The two rece ive r  s idebands  a re  r e so lved .  b i a s  v o l t a g e  is swep t  pas t  t he  cav i ty  s t ep .  The measured i n t e g r a l  power is 50 pW and t h e  l inewidth  % 20  MHz a s  s e e n  i n  t h e  i n s e t ,  which shows a spectrum analyzer  f requency display of  the  de tec ted  Josephson l ine  ( the  double  peak  is an  a r t i f ac t  o f  t he  supe rhe te rodyne  de tec -  t ion scheme).  The maximum i n t e g r a l  power mea- sured  was 80 pW ( 4  pW/MHz). The measured peak i n  t h e  rf-power vs.  voltage curve i s  skewed due t o  p a r a m e t r i c  e x c i t a t i o n  o f  t h e  r e s o n a t o r  a t  1 3 . 1  GHz a t  b i a s  v o l t a g e s  on t h e  low vol-  t age  s ide  o f  t he  r e sona to r  s t ep .  Peaks  a re  a l s o  s e e n  a t  v o l t a g e s  where higher harmonics of  the Josephson frequency coincide with the r e c e i v e r  and resonator  f requency.  A s i m i l a r  "skewness" of these harmonic peaks is n o t  observed. 111. Adjus t ab le  inve r t ed  mic ros t r ip  t ransformer The t h i r d  a p p r o a c h  t o  t h e  d e s i g n  o f  low  impe- dance  coupl ing  c i rcu i t s  for  use  wi th  Joseph-  son junct ions employs inverted microstr ip .  The i n v e r t e d  m i c r o s t r i p  h a s  s w e r a l  a t t r a c t i v e  f e a t u r e s ,  (i) t h e  c h a r a c t e r i s t i c  impedance is i n v e r s e l y  p r o p o r t i o n a l  t o  t h e  s t r i p - t o  g r o u n d -  p lane  spac ing ,  (ii) t h e  dielectr ic  medium i s  vacuum o r  helium which  impl i e s  t ha t  t he  s t r ip -  to  -groundplane  spac ing  , and hence the impe- dance , may be   var ied   and   tha t   the   d ie lec-  tric l o s s e s  a r e  s m a l l ,  (iii) the   suppor t ing  s u b s t r a t e  p l a y s  n o  e s s e n t i a l  r o l e  i n  t h e  wave propagat ion  and  inexpens ive  subs t ra tes  may be used. W e  have designed a m i c r o s t r i p  holder  wi th  nominal  cen ter  f requency  a t  35 GHz.  The b a s i c  i d e a  is i l l u s t r a t e d  i n  F i g .  4. The pro to type  uses  a s i n g l e  X/4 s e c t i o n  of i n v e r t e d  m i c r o s t r i p  w i t h  a subs t ra te - to-gro-  undplane spacing which may be varied by ben- d i n g  t h e  s u b s t r a t e  by  means of  a remotely c o n t r o l l e d  d i f f e r e n t i a l  screw and eccent r ic  l e v e r  mechanism. With a s t r ip-width of  2 mm and an a i r  gap of 5 um t o  6 0  ym w e  achieve t ransformer output  impedances in  the range 0.1Q t o  5Q. The impedance transformation i s  r e a l i z e d   v i a  a 50 um s t e p   i n   t h e  ground- p l a n e  a t  a d i s t ance  X/4 from the Josephson device.  The c i r c u i t  p a t t e r n  is shown i n  t h e  upper  par t  o f  F ig .  4 .  The br idge  is loca ted  a t  t h e  x and i s  followed by a sequence of A/4 sections which transform the open ended l i n e  t o  a low impedance termination a t  t h e  br idge .  The s i d e  arms a re  dc -b ia s  l eads  and  the  inpu t  l i ne  is dc-ground. The sub- s t r a t e  i s  an inexpensive microscope cover- s l i d e   2 4 ~ 2 4 ~ 0 . 1 3  mm which i s  e a s i l y  b e n t .  The performance may be descr ibed as  fol-  lows.  With a 35 GHz s i g n a l  a p p l i e d  t o  t h e  i n p u t ,  t h e  microwave power t r a n s m i t t e d  t o  the  b r idge  was measured vs .  substrate- to-  groundplane spacing. The measured fract io-  n a l  power t r a n s f e r  was found proport ional  t o  t h e  o u t p u t  impedance  of tHe t ransformer ,  c a l c u l a t e d  a s  a func t ion  o f  subs t r a t e  pos i -  t i o n  .6 In  F ig .  5 w e  show the  measured  inc ident  power (closed symbols)  necessary to  induce a 1 0 %  suppress ion  of  the  supercur ren t  as  the  fre- quency i s  va r i ed  wi th  f ixed  subs t r a t e .  Also shown i s  t h e  i n p u t  power c o r r e c t e d  f o r  t h e  measured  re f lec ted  power (open symbols). A broadband coupling has been achieved except f o r  a s t rong  resonant  absorp t ion  peak  a t  33.5 GHz. Also t h e  microwave generation by the ac-Joseph- s o n  e f f e c t  was measured VS. s u b s t r a t e  p o s i t i o n  and frequency. These measurements were i n  p e r -  f e c t  agreement  with the resul ts  descr ibed abo-  ve.  A t  optimum a s  much a s  1 2 0  pW was e x t r a c t e d  from a 40  rnQ b r idge  and abou t  ha l f  o f  t ha t  110 24x 24x 0.13 mm Substrate I I I Eccentric. I Dielectr .   soacer  wRaz:B 50 pm transf. step tinuities  in  stripline  width.  Such  large  jumps in  width  require  that  corrections  are  made  for fringing  fields at the edges.(Design  equations for  microstrip  which  include  these  corrections do not  apply t;o the  present  extreme case.) The  resonator  coupling  works  well  in  a  narrow bandwidth  and  may  be  useful  in  fixed  frequency applications.  Considerable  power was  extracted from  a  single  microbridge.  Compared  to  the transformer-coupled  case,the  emitted  linewidth was  narrowed  by  the  regenerative  interaction between  bridge  and  resonator. The  best  performance so far was achieved  with the  adjustable  inverted  microstrip  transformer coupling. The  narrow  Josephson  line  was  tunab- le  over  a 10% bandwidth  without  significant change  in  output power. The  measured  power output  from  a  single  microbridge  was  the  lar- gest  ever  reported. Acknowledgement This  work  was  supported  in  part by  the  Danish Natural  Science  Research  Council. O.Eg is acknowledged  for  the  sample  preparation. References Fig. 4. Upper  part:  thin-film  pattern.  Lower 1. S . N .  Ern; and R.D. Parmentier,  "Micro- part: schematic cross section of the micro- wave radiation from long Josephson tunnel strip holder showing the principle of ope- junctions". (This conference) . ration. 2. N.F. Pedersen et al.. "Microwave  aenera- from  a 20 mR  bridge. The  linewidth  was 10-15 MHz (10 pW/MHz) . tion  and  complex  microwave  responsivity measurements on small-Dayem  bridges". IEEE  Trans. on MAG-13, 248 (1977) 4~ IV.  Conclusion 3 .  J.  Bindslev  Hansen et al., "Coherent We  have  found  superconducting  thin-film  micro- arrays of thin  film  Josephson  microbrid- strip  techniques  well  suited  for  Coupling  to ges".  (This  conference) . low  impedance  Josephson  devices. A . K .  Jain et al., "Large  arrays  of  Joseph- The  approach  using  a  high/low h/4 impedance son  microbridges  as  tunable  sources  of microwave  radiation".  (This  conference) . transformer  was  moderately  succesful.  In  or- der  to  achieve  very  low  Output  impedance  levels, 4. M.T. Levinsen  et al., "Externally  pumped we  have  to  operate  with  rather  extreme  discon-  millimeter  wave  Josephson  junction  para- metric  amplifier" . IEEE Trans. on Electr. Devices,  Special issue  on  Josephson  junction  devices. Oct.  1980. 80 5. K.C. Gupta,  Ramesh  Garg, I.J. Bahl, Microstrip  lines  and  slotlines,  Dedham: Artech  House  Inc.,  1979, p .  136. 60 6. T. F. Finnegan and 0. H. Soerensen, "Adjustable mi- in millimeter  wave  integrated  circuit^'^, to pub- - CrOStrip transfom for use with Josephson  devicea lished. 4 -40 20 0 33 34 35  36 WGHz) Fig. 5. Measured  incident  power  nenessary  to maintain a constant  supercurrent  suppression (closed  circles).  Input  power  corrected  for reflecti.on  (open  circles). 

Microstrip coupling techniques applied to thin-film Josephson junctions at microwave frequencies

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Microstrip coupling techniques applied to thin-film Josephson junctions at microwave frequencies

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