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Towards Ultimate Performance of THz Heterodyne Receivers: SIS Frequency Multiplier and Wideband Passive Components

Hawal Rashid

Published Jan 1, 2016• 0 citations
OpenAlex

Abstract

This thesis focuses on the development of advanced components for the future wideband side band separating and / or multipixel receivers for radio-astronomy with ultimate sensitivity.\nIn this thesis, a novel frequency multiplier based on distributed superconducting tunnel (SIS) junctions has been demonstrated for the first time, taking advantage of the extreme inherent nonlinearity of such device. This practical demonstration is the result of an extensive theoretical exploration and modelling, showing the importance of the reactive quasi-particle tunnel current\nand its impact on the SIS tunnel junction frequency multiplier. In addition, a novel technique\nwas suggested that allows for the first time direct measurement of the reactive quasi-particle tunnel current.\nThis work comprehends the analysis, design and characterisation of the first frequency multiplier using distributed SIS junctions. The measured output power generated by the distributed SIS junction at the second harmonic of the input frequency were in good agreement\nwith the model. Furthermore, the distributed SIS junction as a frequency multiplier were for the first time able to pump an SIS mixer. An efficiency of the distributed SIS junction multiplier of 15-30 % for a fractional bandwidth of 10% with excellent spectral line purity were\ndemonstrated. The -3 dB line width of the multiplied signal is better than 1 Hz, which were the lowest resolution bandwidth of the spectrum analyser. Consequently, the results attained in this work show that the distributed SIS junction frequency multiplier has considerable future potential, and could possibly be used in LO source in single-ended and multi-pixel SIS mixer\nlayouts.\nFurthermore, in order to improve the sideband rejection ratio of cryogenically cooled sideband separating receivers for radio astronomy receivers to more than 15 dB over a wide RF and IF bandwidth, novel wideband RF and IF hybrid couplers have been developed and demonstrated\nwith very low imbalances. More specifically, a 4-8 GHz IF assembly comprising a compact 90° hybrid chip, two bias-T circuits and two transmission line circuits was designed and characterised. The compact size of the hybrid chip allows it to be integrated into virtually any sideband separating (2SB) mixer operating at cryogenic temperatures and is furthermore especially advantageous for multi-pixel 2SB receivers or low noise balanced amplifier layouts.\nThe characterisation of the hybrid assembly was performed at a cryogenic temperature of 4 K and sophisticated calibration and fixture de-embedding procedures were used. To further improve the sideband rejection ratio, a novel design for a RF 90° waveguide hybrid with very\nlow amplitude imbalance over a fractional bandwidth of 30% (159-216 GHz) was demonstrated. Both Hybrids are currently implemented in the ALMA Band 5 receivers, which\nrepresent the state-of-the-art for ultra-low noise sideband separated receivers.\nIn order to allow complete characterisation of the ALMA Band 5 cartridge a highly versatile measurement setup has been built. The measurement setup includes all necessary hardware and achieves automatic measurements for the system noise and sideband rejection with in-built\noptimisation procedures. The measurement setup additionally comprises measurements of the receiver saturation, phase and amplitude stability, as well as optical beam characterisation.

Related papers

Mixer, Multiplier and Passive Components for Low Noise THz Receivers

Bhushan Billade

This thesis presents the work done in the field of development of low noise instruments for radio astronomy applications, and addresses some of the key components in the receiver. Part of this thesis describes the design and development of a Superconductor-Insulator-Superconductor (SIS) junction mixer covering RF frequencies from 163 to 211 GHz, reaching the quantum limited noise performance. The mixer uses an integrated approach, where an LO coupler and a resistive LO termination are fabricated on the same mixer chip.Another part of the thesis describes the work on the design and development of the Band 5 receiver cartridge for the Atacama Large Millimeter Array (ALMA) project. Details on the development of Band 5 receiver cartridge, sideband separation mixer, and a superconducting IF hybrid is presented.Additionally, this thesis presents first ever experimental demonstration of an SIS device as frequency multiplier, and presents successful off-chip detection of the multiplied signal. The experimental frequency multiplier was designed for the 182 to 192 GHz output frequency band, and uses an array of SIS junctions to boost the total output power. Dependence of multiplier output power on the DC bias voltage and pumping power are investigated. The measurements successfully confirm x2 multiplication, where extremely sharp multiplied signal was detected with no observable spurious signals. The SIS multiplier shows no deterioration of the linewidth of the original pumping signal.

•Jan 1, 2013•0 citations

Frequency Multiplier Based on Distributed Superconducting Tunnel Junctions: Theory, Design, and Characterization

Hawal Rashid, Sascha Krause, Denis Meledin +3 more

In this paper, we present the analysis, design, and characterization of the first frequency multiplier using distributed superconductor-insulator-superconductor (SIS) junctions. We derived analytical expressions describing the properties of the distributed SIS junction as a frequency multiplier. The modeling of the distributed SIS junctions shows that high conversion efficiency can be achieved when used as the multiplier. The measured output power generated by such multiplier employing the distributed SIS junction at the second harmonic of the input frequency is in good agreement with the model. Furthermore, the frequency multiplier based on the distributed SIS junction for the first time was able to pump an SIS mixer. The multiplication efficiency of the distributed SIS junction is 15-30% for a fractional bandwidth of 10% with excellent spectral line purity. The -3 dB line width of the multiplied signal is 1 Hz, which was limited by the resolution bandwidth of the spectrum analyzer. The results attained in this paper show that the distributed SIS junction frequency multiplier has considerable future potential, and could possibly be used in LO source in single-end and multipixel SIS mixer receivers.

IEEE Transactions on Terahertz Science and Technology•Jan 1, 2016•12 citations

Low-noise THz Niobium SIS Mixers

Mei Bin

This thesis describes the development of low-noise heterodyne receivers at THz frequencies for submillimeter astronomy using Nb-based superconductor-insulator-superconductor (SIS) tunneling junctions. The mixers utilize a quasi-optical configuration which consists of a planar twin-slot antenna and antisymmetrically-fed two-junctions on an antireflection-coated silicon hyperhemispherical lens. On-chip integrated tuning circuits, in the form of microstrip lines, are used to obtain maximum coupling efficiency in the designed frequency band. To reduce the rf losses in the integrated tuning circuits above the superconducting Nb gap frequency (~ 700 GHz), normal-metal Al is used to replace Nb as the tuning circuits. To account the rf losses in the micros trip lines, we calculated the surface impedance of the AI films using the nonlocal anomalous skin effect for finite thickness films. Nb films were calculated using the Mattis-Bardeen theory in the extreme anomalous limit. Our calculations show that the losses of the Al and Nb microstrip lines are about equal at 830 GHz. For Al-wiring and Nb-wiring mixers both optimized at 1050 GHz, the RF coupling efficiency of Al-wiring mixer is higher than that of Nb-wiring one by almost 50%. We have designed both Nb-wiring and Al-wiring mixers below and above the gap frequency. A Fourier transform spectrometer (FTS) has been constructed especially for the study of the frequency response of SIS receivers. This FTS features large aperture size (10 inch) and high frequency resolution (114 MHz). The FTS spectra, obtained using the SIS receivers as direct detectors on the FTS, agree quite well with our theoretical simulations. We have also, for the first time, measured the FTS heterodyne response of an SIS mixer at sufficiently high resolution to resolve the LO and the sidebands. Heterodyne measurements of our SIS receivers with Nb-wiring or Al-wiring have yielded results which arc among the best reported to date for broadband heterodyne receivers. The Nb-wiring mixers, covering 400 - 850 GHz band with four separate fixed-tuned mixers, have uncorrected DSB receiver noise temperature around 5hv/kb to 700 GHz, and better than 540 K at 808 GHz. An Al-wiring mixer designed for 1050 GHz band has an uncorrected DSB receiver noise temperature 840 K at 1042 GHz and 2.5 K bath temperature. Mixer performance analysis shows that Nb junctions can work well up to twice the gap frequency and the major cause of loss above the gap frequency is the rf losses in the microstrip tuning structures. Further advances in THz SIS mixers may be possible using circuits fabricated with higher-gap superconductors such as NbN. However, this will require high-quality films with low RF surface resistance at THz frequencies.

•Sep 1, 1997•8 citations

Modular 2SB SIS Receiver for 600–720 GHz: Performance and Characterization Methods

Andrey V. Khudchenko, Ronald Hesper, Andrey M. Baryshev +2 more

A modular sideband-separating (2SB) receiver for 600-720 GHz has been built and tested. The used modular design allows to characterize all the parts separately, including testing of the superconductor-insulator-superconductor (SIS) junctions individually in a double-sideband mode before building them in to the 2SB assembly. The developed 2SB mixer has a single sideband noise temperature below 380 K in the entire operating band and reaches a level of 200 K in the best point. At the same time, the image rejection ratio (IRR) was demonstrated to be better than 11.5 dB over the entire band. However, we have found a discrepancy between the observed and expected performance. To investigate this problem, we have developed and applied methods to characterize RF and intermediate frequency imbalances of a fully assembled 2SB mixer using the SIS junction properties. As result, the IRR of our mixer was found to be limited by the RF imbalance, which is caused by complex standing waves created by reflections from the SIS junctions, the RF hybrid and the RF absorption load.

IEEE Transactions on Terahertz Science and Technology•Dec 28, 2016•10 citations

Design and Performance of a Sideband Separating SIS Mixer for 800–950 GHz

Andrey V. Khudchenko, M. Yu. Fominsky, Christopher Heiter +11 more

We present the design and results of characterization of a new sideband separating (2SB) mixer for 800-950 GHz, based on superconductor-insulator-superconductor (SIS) junctions. This is the first waveguide 2SB SIS mixer demonstrated at such a high frequency. The design is following the classical quadrature hybrid architecture, meanwhile additional attention was put on the reduction of reflections in the RF structure in order to minimize the RF imbalance, to achieve a high image rejection ratio (IRR). The RF waveguide block was manufactured by micromilling and populated by single-ended SIS mixers developed earlier for upgrade of the CHAMP+ high-band array on the APEX telescope. These SIS mixers have double-sideband (DSB) noise temperatures from 210 to 400 K. The assembled 2SB mixer yields a SSB noise temperature from 450 to 900 K, with an IRR above 15 dB in 95% of the band. Comparing the DSB and the SSB sensitivities, we find that the waveguide losses are as low as expected and do not exceed 0.6 dB. The presented mixer is a prototype for use in a 2SB dual polarization receiver planned for deployment on the APEX telescope.

IEEE Transactions on Terahertz Science and Technology•Sep 2, 2019•21 citations

Development of Balanced SIS Mixers for ALMA Band-10

S. V. Shitov, Oleg V. Koryukin, Y. Uzawa +3 more

down to only 2-3 p,W for a two polarized SIS receiver, if submicron SIS junctions are used in the balanced mixer configuration. There are a few options for waveguide balanced and sideband separating mixers employing the straightforward connection of the DSB mixers using hybrid couplers [2]. However, the low-loss operation of the waveguide-based connections and the required balancing of the mixers both are of a great concern in the THz frequency range. This is a good reason to look for the more compact design of the RF circuit for waveguide or/and consider an integrated quasioptical SIS mixer design. It is known that the conversion gain of a SIS junction is reducing essentially above the gap frequency, and it turns negligibly low above twice the gap frequency. The ALMA Band-10 DSB receiver specification for TRx is 230 K within 80% of the band 787-950 GHz and 345 K at any frequency of the band. The IF range of 4-12 GHz is required for the DSB receiver with ripples below 4 dB within each 2 GHz segment. All these make the accurate design of the mixer's RF and IF circuits very important. Since the Q-factor of a SIS junction at submillimeter wavelength is high (Q>>1), the twin-SIS junction [3]-[5] is a good solution providing about twice RF bandwidth of a single junction. However, it is known that two junctions in the twin-SIS are unequally coupled at RF across the band, thus providing less gain than one can expect from the equivalent junction of the same R. This effect is difficult to estimate experimentally, since two junctions are connected in parallel at DC. The growing importance of such accurate analysis in THz range can be motivated with concern of the essential drop of conversion gain of a Nb-based SIS mixer closer to 1 THz. To analyze the effect of unequal pump and dynamic resistance for a two-junction SIS mixer, the Tucker's 3-port model can be used yet being expanded to multi-junction mixing arrays. Abstract-A few concepts of a wide-band balanced SIS mixer employing submicron-sized SIS junctions are under development for 787-950 GHz frequency range. A quasioptical DSB balanced mixer with integrated cross-slot antenna is considered as the less laborious and cheaper option. The silicon lens-antenna beam efficiency is expected above 80 % across the whole band with first-order sidelobe below -16 dB. To use the conservative horn antenna solution, a singlechamber waveguide DSB balanced mixer is developed. Two equal probe-type SIS chips are inserted into a full-height waveguide through its opposite broad walls; these two mixers are driven by the signal waveguide in series. The LO current is transferred to the mixers in parallel via a capacitive probe inserted through the narrow wall of the signal waveguide from the neighboring LO waveguide. The HFSS model demonstrated the LO power coupling efficiency above -3 dB, almost perfect signal transfer and the LO cross talk below 30 dB that take into account misalignment (misbalance) of the chips. It is demonstrated numerically using Tucker's 3-port model that unequal pump of junctions of a twin-SIS mixer can lead, in spite of the perfect signal coupling, to degradation of the gain performance up to -3 dB, especially at the top of the ALMA Band-10.

•May 1, 2006•2 citations
This thesis focuses on the development of advanced components for the future wideband side band separating and / or multipixel receivers for radio-astronomy with ultimate sensitivity.\nIn this thesis, a novel frequency multiplier based on distributed superconducting tunnel (SIS) junctions has been demonstrated for the first time, taking advantage of the extreme inherent nonlinearity of such device. This practical demonstration is the result of an extensive theoretical exploration and modelling, showing the importance of the reactive quasi-particle tunnel current\nand its impact on the SIS tunnel junction frequency multiplier. In addition, a novel technique\nwas suggested that allows for the first time direct measurement of the reactive quasi-particle tunnel current.\nThis work comprehends the analysis, design and characterisation of the first frequency multiplier using distributed SIS junctions. The measured output power generated by the distributed SIS junction at the second harmonic of the input frequency were in good agreement\nwith the model. Furthermore, the distributed SIS junction as a frequency multiplier were for the first time able to pump an SIS mixer. An efficiency of the distributed SIS junction multiplier of 15-30 % for a fractional bandwidth of 10% with excellent spectral line purity were\ndemonstrated. The -3 dB line width of the multiplied signal is better than 1 Hz, which were the lowest resolution bandwidth of the spectrum analyser. Consequently, the results attained in this work show that the distributed SIS junction frequency multiplier has considerable future potential, and could possibly be used in LO source in single-ended and multi-pixel SIS mixer\nlayouts.\nFurthermore, in order to improve the sideband rejection ratio of cryogenically cooled sideband separating receivers for radio astronomy receivers to more than 15 dB over a wide RF and IF bandwidth, novel wideband RF and IF hybrid couplers have been developed and demonstrated\nwith very low imbalances. More specifically, a 4-8 GHz IF assembly comprising a compact 90° hybrid chip, two bias-T circuits and two transmission line circuits was designed and characterised. The compact size of the hybrid chip allows it to be integrated into virtually any sideband separating (2SB) mixer operating at cryogenic temperatures and is furthermore especially advantageous for multi-pixel 2SB receivers or low noise balanced amplifier layouts.\nThe characterisation of the hybrid assembly was performed at a cryogenic temperature of 4 K and sophisticated calibration and fixture de-embedding procedures were used. To further improve the sideband rejection ratio, a novel design for a RF 90° waveguide hybrid with very\nlow amplitude imbalance over a fractional bandwidth of 30% (159-216 GHz) was demonstrated. Both Hybrids are currently implemented in the ALMA Band 5 receivers, which\nrepresent the state-of-the-art for ultra-low noise sideband separated receivers.\nIn order to allow complete characterisation of the ALMA Band 5 cartridge a highly versatile measurement setup has been built. The measurement setup includes all necessary hardware and achieves automatic measurements for the system noise and sideband rejection with in-built\noptimisation procedures. The measurement setup additionally comprises measurements of the receiver saturation, phase and amplitude stability, as well as optical beam characterisation.
This thesis presents the work done in the field of development of low noise instruments for radio astronomy applications, and addresses some of the key components in the receiver. Part of this thesis describes the design and development of a Superconductor-Insulator-Superconductor (SIS) junction mixer covering RF frequencies from 163 to 211 GHz, reaching the quantum limited noise performance. The mixer uses an integrated approach, where an LO coupler and a resistive LO termination are fabricated on the same mixer chip.Another part of the thesis describes the work on the design and development of the Band 5 receiver cartridge for the Atacama Large Millimeter Array (ALMA) project. Details on the development of Band 5 receiver cartridge, sideband separation mixer, and a superconducting IF hybrid is presented.Additionally, this thesis presents first ever experimental demonstration of an SIS device as frequency multiplier, and presents successful off-chip detection of the multiplied signal. The experimental frequency multiplier was designed for the 182 to 192 GHz output frequency band, and uses an array of SIS junctions to boost the total output power. Dependence of multiplier output power on the DC bias voltage and pumping power are investigated. The measurements successfully confirm x2 multiplication, where extremely sharp multiplied signal was detected with no observable spurious signals. The SIS multiplier shows no deterioration of the linewidth of the original pumping signal.
In this paper, we present the analysis, design, and characterization of the first frequency multiplier using distributed superconductor-insulator-superconductor (SIS) junctions. We derived analytical expressions describing the properties of the distributed SIS junction as a frequency multiplier. The modeling of the distributed SIS junctions shows that high conversion efficiency can be achieved when used as the multiplier. The measured output power generated by such multiplier employing the distributed SIS junction at the second harmonic of the input frequency is in good agreement with the model. Furthermore, the frequency multiplier based on the distributed SIS junction for the first time was able to pump an SIS mixer. The multiplication efficiency of the distributed SIS junction is 15-30% for a fractional bandwidth of 10% with excellent spectral line purity. The -3 dB line width of the multiplied signal is 1 Hz, which was limited by the resolution bandwidth of the spectrum analyzer. The results attained in this paper show that the distributed SIS junction frequency multiplier has considerable future potential, and could possibly be used in LO source in single-end and multipixel SIS mixer receivers.
This thesis describes the development of low-noise heterodyne receivers at THz frequencies for submillimeter astronomy using Nb-based superconductor-insulator-superconductor (SIS) tunneling junctions. The mixers utilize a quasi-optical configuration which consists of a planar twin-slot antenna and antisymmetrically-fed two-junctions on an antireflection-coated silicon hyperhemispherical lens. On-chip integrated tuning circuits, in the form of microstrip lines, are used to obtain maximum coupling efficiency in the designed frequency band. To reduce the rf losses in the integrated tuning circuits above the superconducting Nb gap frequency (~ 700 GHz), normal-metal Al is used to replace Nb as the tuning circuits. To account the rf losses in the micros trip lines, we calculated the surface impedance of the AI films using the nonlocal anomalous skin effect for finite thickness films. Nb films were calculated using the Mattis-Bardeen theory in the extreme anomalous limit. Our calculations show that the losses of the Al and Nb microstrip lines are about equal at 830 GHz. For Al-wiring and Nb-wiring mixers both optimized at 1050 GHz, the RF coupling efficiency of Al-wiring mixer is higher than that of Nb-wiring one by almost 50%. We have designed both Nb-wiring and Al-wiring mixers below and above the gap frequency. A Fourier transform spectrometer (FTS) has been constructed especially for the study of the frequency response of SIS receivers. This FTS features large aperture size (10 inch) and high frequency resolution (114 MHz). The FTS spectra, obtained using the SIS receivers as direct detectors on the FTS, agree quite well with our theoretical simulations. We have also, for the first time, measured the FTS heterodyne response of an SIS mixer at sufficiently high resolution to resolve the LO and the sidebands. Heterodyne measurements of our SIS receivers with Nb-wiring or Al-wiring have yielded results which arc among the best reported to date for broadband heterodyne receivers. The Nb-wiring mixers, covering 400 - 850 GHz band with four separate fixed-tuned mixers, have uncorrected DSB receiver noise temperature around 5hv/kb to 700 GHz, and better than 540 K at 808 GHz. An Al-wiring mixer designed for 1050 GHz band has an uncorrected DSB receiver noise temperature 840 K at 1042 GHz and 2.5 K bath temperature. Mixer performance analysis shows that Nb junctions can work well up to twice the gap frequency and the major cause of loss above the gap frequency is the rf losses in the microstrip tuning structures. Further advances in THz SIS mixers may be possible using circuits fabricated with higher-gap superconductors such as NbN. However, this will require high-quality films with low RF surface resistance at THz frequencies.
A modular sideband-separating (2SB) receiver for 600-720 GHz has been built and tested. The used modular design allows to characterize all the parts separately, including testing of the superconductor-insulator-superconductor (SIS) junctions individually in a double-sideband mode before building them in to the 2SB assembly. The developed 2SB mixer has a single sideband noise temperature below 380 K in the entire operating band and reaches a level of 200 K in the best point. At the same time, the image rejection ratio (IRR) was demonstrated to be better than 11.5 dB over the entire band. However, we have found a discrepancy between the observed and expected performance. To investigate this problem, we have developed and applied methods to characterize RF and intermediate frequency imbalances of a fully assembled 2SB mixer using the SIS junction properties. As result, the IRR of our mixer was found to be limited by the RF imbalance, which is caused by complex standing waves created by reflections from the SIS junctions, the RF hybrid and the RF absorption load.
We present the design and results of characterization of a new sideband separating (2SB) mixer for 800-950 GHz, based on superconductor-insulator-superconductor (SIS) junctions. This is the first waveguide 2SB SIS mixer demonstrated at such a high frequency. The design is following the classical quadrature hybrid architecture, meanwhile additional attention was put on the reduction of reflections in the RF structure in order to minimize the RF imbalance, to achieve a high image rejection ratio (IRR). The RF waveguide block was manufactured by micromilling and populated by single-ended SIS mixers developed earlier for upgrade of the CHAMP+ high-band array on the APEX telescope. These SIS mixers have double-sideband (DSB) noise temperatures from 210 to 400 K. The assembled 2SB mixer yields a SSB noise temperature from 450 to 900 K, with an IRR above 15 dB in 95% of the band. Comparing the DSB and the SSB sensitivities, we find that the waveguide losses are as low as expected and do not exceed 0.6 dB. The presented mixer is a prototype for use in a 2SB dual polarization receiver planned for deployment on the APEX telescope.
down to only 2-3 p,W for a two polarized SIS receiver, if submicron SIS junctions are used in the balanced mixer configuration. There are a few options for waveguide balanced and sideband separating mixers employing the straightforward connection of the DSB mixers using hybrid couplers [2]. However, the low-loss operation of the waveguide-based connections and the required balancing of the mixers both are of a great concern in the THz frequency range. This is a good reason to look for the more compact design of the RF circuit for waveguide or/and consider an integrated quasioptical SIS mixer design. It is known that the conversion gain of a SIS junction is reducing essentially above the gap frequency, and it turns negligibly low above twice the gap frequency. The ALMA Band-10 DSB receiver specification for TRx is 230 K within 80% of the band 787-950 GHz and 345 K at any frequency of the band. The IF range of 4-12 GHz is required for the DSB receiver with ripples below 4 dB within each 2 GHz segment. All these make the accurate design of the mixer's RF and IF circuits very important. Since the Q-factor of a SIS junction at submillimeter wavelength is high (Q>>1), the twin-SIS junction [3]-[5] is a good solution providing about twice RF bandwidth of a single junction. However, it is known that two junctions in the twin-SIS are unequally coupled at RF across the band, thus providing less gain than one can expect from the equivalent junction of the same R. This effect is difficult to estimate experimentally, since two junctions are connected in parallel at DC. The growing importance of such accurate analysis in THz range can be motivated with concern of the essential drop of conversion gain of a Nb-based SIS mixer closer to 1 THz. To analyze the effect of unequal pump and dynamic resistance for a two-junction SIS mixer, the Tucker's 3-port model can be used yet being expanded to multi-junction mixing arrays. Abstract-A few concepts of a wide-band balanced SIS mixer employing submicron-sized SIS junctions are under development for 787-950 GHz frequency range. A quasioptical DSB balanced mixer with integrated cross-slot antenna is considered as the less laborious and cheaper option. The silicon lens-antenna beam efficiency is expected above 80 % across the whole band with first-order sidelobe below -16 dB. To use the conservative horn antenna solution, a singlechamber waveguide DSB balanced mixer is developed. Two equal probe-type SIS chips are inserted into a full-height waveguide through its opposite broad walls; these two mixers are driven by the signal waveguide in series. The LO current is transferred to the mixers in parallel via a capacitive probe inserted through the narrow wall of the signal waveguide from the neighboring LO waveguide. The HFSS model demonstrated the LO power coupling efficiency above -3 dB, almost perfect signal transfer and the LO cross talk below 30 dB that take into account misalignment (misbalance) of the chips. It is demonstrated numerically using Tucker's 3-port model that unequal pump of junctions of a twin-SIS mixer can lead, in spite of the perfect signal coupling, to degradation of the gain performance up to -3 dB, especially at the top of the ALMA Band-10.