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Presented at ISEC-2005

1

Development of the Flux-Flow Oscillators for Submm Integrated Receiver
Valery P. Koshelets, Pavel N. Dmitriev, Andrey. B. Ermakov, Lyudmila V. Filippenko, Andrey V. Khudchenko, Oleg V. Koryukin, Alexander S. Sobolev, Mikhail Yu. Torgashin.
locking. For example, only about 50 % of the FFO power can be phase-locked by the present PLL system at the free-running FFO linewidth of 7 MHz. Low spectral ratio results in considerable error in resolving the complicated line shape [6]. Thus sufficiently small free-running FFO linewidth is vitally important for realization of the phase-locked SIR for TELIS. II. D
EPENDENCE OF THE

Abstract -- A comprehensive experimental study of the Flux Flow Oscillator (FFO) linewidth on its parameters has been carried out in order to obtain free-running FFO linewidth well below 10 MHz and ensure phase-locked operation of a Superconducting Integrated Receiver (SIR). Essential dependence of the FFO linewidth on its width and idle region dimension has been found. A free-running linewidth between 10 and 2 MHz has been achieved in the frequency range 500 ­ 700 GHz. As a result the spectral ratio of the phased-locked FFO varies from 30 to 90 % correspondingly. First measurements of the free-running FFO linewidth for the FFOs of different length and comparison of the obtained results with existing theoretical models demonstrate significant dependence of the fitting parameter on the FFO length.

FFO L

INEWIDTH ON ITS

W

IDTH

HE Superconducting Integrated Receiver (SIR) [1], [2] comprises in one chip a planar antenna integrated with an SIS mixer, a superconducting Flux Flow Oscillator (FFO) [3] acting as Local Oscillator (LO) and a second SIS harmonic mixer (HM) for FFO phase locking. In order to obtain frequency resolution required for practical application of a heterodyne spectrometer (of at least one part per million) an integrated local oscillator (LO) must be phase-locked to an external reference. Successful tests of the phase-locked FFO [2], [4] enabled the development of a 550-650 GHz integrated receiver for the Terahertz Limb Sounder (TELIS) [5] intended for measuring a variety of atmospheric constituents and scheduled to fly on a balloon in 2006. Even for ultra wideband PLL system an effective regulation bandwidth is limited mainly by the cable length (about 7 MHz for typical PL loop length of two meters). It means that freerunning FFO linewidth has to be well below 10 MHz to ensure stable FFO phase locking with reasonably good spectral ratio (SR) - the ratio between the carrier and total power emitted by FFO, determines quality of FFO phase
V.P. Koshelets, P.N. Dmitriev, A.B. Ermakov, L.V. Filippenko, O.V. Koryukin, A.V. Khudchenko, A.S. Sobolev, and M.Yu. Torgashin are with the Institute of Radio Engineering and Electronics, Russian Academy of Science, Mokhovaya 11, 125009, Moscow, Russia; V.P. Koshelets, A.B. Ermakov, O.V. Koryukin, and M.Yu. Torgashin are also partially with the Space Research Organization of the Netherlands (SRON), P.O. Box 800, 9700 AV Groningen, the Netherlands, (telephone: 7-095-2032784, e-mail: valery@hitech.cplire.ru) The work was supported in parts by the RFBR projects 03-02-16748, INTAS project 01-0367, ISTC projects # 2445, 3174 and the President Grant for the Leading Scientific School 1344.2003.2.

T

I. I

NTRODUCTION

Previous measurements have demonstrated very prominent dependence of the FFO linewidth on FFO voltage (frequency) and considerable increase of the FFO linewidth with FFO current density [7]. Recently it was shown [8] that for all frequencies of interest the linewidth considerably decreases with increasing FFO width, W, (consequently, the spectral ratio is getting much higher). We have further explored this approach by increasing the FFO width up to 28 µm (that corresponds to more than 5 times Josephson penetration depth J). A number of FFOs with the same electrodes layout, but different width of FFO junction (W = 4, 8, 12, 16, 20 and 28 µm) were fabricated using the same technological procedure with similar parameters (RnS =30 *µm2). The results of the LW measurements of these circuits at three FFO frequencies are presented in Fig. 1. Even for the largest tested width (W = 28 µm) there is no evidence of deterioration of the FFO behavior. Furthermore, power delivered to mixer is getting higher and linewidth lower at all frequencies. Decreasing of the FFO linewidth with increasing FFO width is quite explainable according to existing theoretical models. The differential resistance on bias current, RdB, decreases more or less inversely proportional to bias current, Ib (in contrast to the case of changing of the FFO current density). Since the FFO linewidth is proportional to Rd2*Ib, it has to go down linearly with an increase of the FFO width. Of course, one can expect saturation of the linewidth decrease and deterioration of the FFO behavior at further width increase (for example, due to appearance of transversal modes). Since there is no reliable theory the optimal value of the FFO width has to be determined experimentally. Note that for a wide FFO there is a shift of the FFO centerline from the edge of control line that results in considerable decrease of the noise coming via magnetic field. Furthermore, wider FFO provides, presumably, more uniform bias current distribution [8] ­ due to much smaller inductance of the overlapping electrodes.


Presented at ISEC-2005

2 FFO length for tested FFOs. Dependence of the fitting parameter K on the FFO length is shown in Fig. 2. Data presented by the open symbols obtained if for fitting in (1) the following expression for Si(0) [11] is used: Si(0) = (e/2) {Iqp coth(v) +2 Is coth(2v)}, v = (eVdc)/(2 kB T). Here superconducting and quasiparticle parts of bias current are accounted separately, while the solid symbols are for the case if bias current treated entirely as normal. Note that K value appears to be below unity at L < 150 µm for "traditional" approach [11]. Additional study is required to clarify which expression is more adequate for long Josephson junctions.
2,0

30 25 FFO Linewidth (MHz) 20 15 10 5 0 4 6 8
, , , fFFO = 526 GHz fFFO = 616 GHz fFFO = 706 GHz

90 75 60 45 30 15 0 10 12 14 16 18 20 22 24 26 28 30 Spectral Ratio of the PLL FFO (%)

FFO Width, W (µm)
Fig. 1. Linewidth of free-running FFOs (left axis) and according Spectral Ratio for the phase-locked FFO (right axis) measured at different FFO frequencies as a function of FFO width. All circuits are fabricated using the same technological procedure with similar parameters (RnS =30 *µm2)..

Fitting Parameter K

1,5

1,0
" T r a d ition al" ap pr oa c h ( Is a nd Iqp) Bia s cur r ent c o ns id e r ed as Iq p

III. D

EPENDENCE OF THE

FFO L

INEWIDTH ON

LENGTH

Detailed measurements of the FFO linewidth [4], [9] demonstrate Lorentzian shape of the FFO line in a wide frequency range up to 700 GHz, both at higher voltages on the flux flow step (FFS) and at lower voltages in the resonant regime on the Fiske steps (FS's). It means that the freerunning ("natural") FFO linewidth in all operational regimes is determined by the wideband thermal fluctuations and the shot noise (this is different from most traditional microwave oscillators) To explain the experimentally measured dependence of the free-running FFO linewidth, f, an additional term proportional to differential resistance on control line current, RdCL, (control line produces magnetic field for FFO operation) has been introduced to the noise model of FFO [4], [9]. Actually at fixed bias current RdCL and RdB are unambiguously related RdCL = A*RdB [7] (A is ranged from 1 to 3 and depends only on bias current and FFO design). It means that experimental dependencies of the FFO linewidth can be well described by expression:

0,5

0

50

100

150

200

250

300

350

400

450

FF O L e ng th (µ m)

Fig. 2. Dependence of the fitting parameter K in formula (1) on FFO length for 2 different expressions for Si(0) - the power density of low frequency current fluctuations.. Open symbols - both superconducting and quasiparticle parts of bias current are accounted separately [11]; solid symbols ­ all bias current is treated as quasiparticle.

REFERENCES
V. P. Koshelets and S. V. Shitov, "Integrated Superconducting Receivers," Superconductor Science and Technology, vol. 13, pp. R53R69, 2000. [2] S. V. Shitov et al., "An Integrated Receiver with Phase-Locked Superconducting Oscillator," IEEE Trans. on Appl. Supercond.y, vol. 13, no. 2, pp. 684-687, June 2003. [3] T. Nagatsuma, K. Enpuku, F. Irie, and K. Yoshida, "Flux-flow type Josephson oscillator for mm and submm wave region," J. Appl. Phys., vol. 54, p. 3302, 1983, see also Pt. II: J. Appl. Phys. vol. 56, p. 3284, 1984; Pt. III, J. Appl. [4] V. P. Koshelets et al, "Towards a Phase-Locked Superconducting Integrated Receiver: Prospects and Limitations," Physica C, vol. 367, pp. 249-255, 2002. [5] P. Yagoubov et al, "550-650 GHz spectrometer development for TELIS" Abstract for the 16-th International Symposium on Space Terahertz Technology, p. 156, Sweden, May 2005. [6] V. P. Koshelets et al., "Superconducting Integrated Receiver for TELIS", IEEE Trans. on Appl. Supercond.y, vol. 15, no. 2, June 2005 [7] V.P. Koshelets, et al., "Superconducting Phase-Locked Local Oscillator for Submm Integrated Receiver", Superconducting Science and Technology, v. 17, pp. $127-$131, 2004. [8] V. P. Koshelets et al., "Optimization of the Phase-Locked Flux-Flow Oscillator for the Submm Integrated Receiver", IEEE Trans. on Appl. Supercond.y, vol. 15, no. 2, June 2005. [9] V. P. Koshelets et al., "Radiation Linewidth of Flux-Flow Oscillators", Supercond. Sci. Technol., vol. 14, p. 1040 - 1043, 2001. [10] V. P. Koshelets, et al., ""Self-Pumping Effects and Radiation Linewidth of Josephson Flux Flow Oscillators", Phys Rev B, vol.56, p. 5572, 1997. [11] A.J. Dahm, A. Denenstein, D.N. Langenberg, W.H., Parker, D. Rogovin, D.J. Scalapino, Phys. Rev. Lett. 22, p. 1416., 1969) [1]

f = (2/02) (K*RdB)2 Si(0);

(1)

where Si(0) is the power density of low frequency current fluctuations. According to existing theoretical models there should not be any significant dependence of the FFO linewidth on its length, L. We have measured linewidth in the frequency range 300 ­ 700 GHz for FFOs which are identical except for their length that is varied in the range 100 ­ 450 µm. Our measurements demonstrated that for short FFO even at V > VJSC [10] (VJSC = 950 µV for Nb-AlOx-Nb junctions) Fiske steps considerably modified the FFO IV-curve; as a result dependence of the FFO linewidth on FFO voltage became non-monotonic. Nevertheless, dependence of the FFO linewidth on differential resistance RdB is smooth, but the fitting parameter K in formula (1) varies considerably with