The development of quantum limited heterodyne receivers requires that you understand and are able to measure the major sources of noise in the receiver system. The major sources of noise in millimeter- and submillimeter-wave SIS heterodyne receivers are the optical front end losses, vacuum fluctuation noise associated with the RF coupling efficiency to the SIS tunnel junction and IF amplifier noise. Although there are several methods for estimating the optical losses and IF amplifier noise, the RF coupling efficiency or quantum efficiency has been much more difficult to ascertain. This paper describes in situ techniques for measuring all three noise contributions in a standard receiver used for astronomical observations. Results are presented for the receivers operating in the Owens Valley Millimeter Interferometer Array.>
The development of quantum limited heterodyne receivers requires that you understand and are able to measure the major sources of noise in the receiver system. The major sources of noise in millimeter- and submillimeter-wave SIS heterodyne receivers are the optical front end losses, vacuum fluctuation noise associated with the RF coupling efficiency to the SIS tunnel junction and IF amplifier noise. Although there are several methods for estimating the optical losses and IF amplifier noise, the RF coupling efficiency or quantum efficiency has been much more difficult to ascertain. This paper describes in situ techniques for measuring all three noise contributions in a standard receiver used for astronomical observations. Results are presented for the receivers operating in the Owens Valley Millimeter Interferometer Array.>
We present measurements on two high frequency S(uperconductor)I(nsulator) S(uperconductor) heterodyne receivers at 660 GHz and 810 GHz. Their noise temperatures are 163 K and 860 K, respectively. The contribution of the high frequency (RF) input loss to this number is measured with the commonly used method of intersecting lines. The result is compared to a more rigorous analysis using the full quantum theory of mixing. We will show that in this frequency range the method of intersecting lines in general leads to an overestimation of the RF input loss. Even at 660 GHz, with a well matched SIS junction, the input noise contribution is overestimated by about 25 percent. This is due to the summation of the contributions due to the vacuum fluctuations, which is the highest contribution, the mismatch at the intermediate frequency (IF), and the fact that the mixer output noise is not completely independent of local oscillator (LO) power. At 810 GHz, due to a much stronger loss-induced mismatch of the SIS junction, the mixer output noise varies considerably with LO-power. The large loss of approximately 8 dB directly in front of the mixer enhances this contribution to the measured RF input loss. The contribution of the IF-mismatch is likewise enhanced. Together with the contribution of the vacuum fluctuations, this can lead to an overestimation of the RF input noise by more than 100 percent, depending on the actual mismatch of the mixers. We therefore conclude that, for the severe mismatch that occurs in Nb based SIS mixers above 700 GHz, the method of intersecting lines gives incorrect results and the full analysis should be applied.
Accurate knowledge of radio-frequency (RF) losses in front of superconductor--insulator--superconductor (SIS) mixers is essential for optimizing millimeter- and terahertz heterodyne receivers. However, such losses are difficult to extract from conventional hot/cold-load measurements because the measured receiver noise combines contributions from the input optics, the mixer, and the intermediate-frequency chain. In this work, we present a numerical implementation of the straight-line, or intersecting-lines, method for estimating input-referred RF noise and optical loss from standard Y-factor measurements. Hot/cold IF spectra are acquired while sweeping the local oscillator (LO) pumping level from under-pumped to over-pumped operation, so that each IF frequency bin gives a family of linear relations between receiver output power and input load temperature. We evaluate all pairwise intersections between these lines and analyze the crossing cloud to obtain the effective RF input noise. We repeat the analysis across the IF band and multiple LO tunings which provides a frequency-resolved characterisation of the input losses over the full band. Our implementation turns the straight-line technique into a reproducible framework for receiver characterisation and relative optical-loss comparison in low-loss front-end configurations.
Observation and analysis of submillimeter-wave radiation (300GHz-3THz) in astronomy and atmospheric sciences requires increasingly performant receivers. The most sensitive receivers working in this range of electromagnetic spectrum use superconductor-insolator-superconductor (SIS) junctions. In order to increase the bandwidth and the sensitivity, we are developing a quantum-noise limited heterodyne receiver based on several parallel SIS junctions with broad (larger than 30%) fixed tuned bandwidth. These circuits can be viewed as passband filters which have been optimized by varying the spacings between junctions. We have designed such 5-junction arrays for operation in the range 480-640 GHz. Fabrication and heterodyne characterization of these devices has been done. The 1 μm2 junctions current density ranges from 4 to 13 kA/cm2, using optical lithography and Nb/Al2Nb5/Nb trilayer sputtering technology. The fabrication process and yield are presented in this paper, along with measured performances
A new type of ultra broad band SIS low noise radiometer has been developed. The SIS receiver instantaneous band of about 30% is significantly increased compared to traditional designs. We avoid a restriction related to the limited band of the Intermediate Frequency (IF) low noise amplifier by using a multifrequency heterodyne power source. In this regime the frequency mixing is performed simultaneously in a big number of the subbands and the IF signals are combined in a common amplifier. A low noise operation in this regime is possible due to the quantum nature of the frequency mixing in SIS quasi particle tunnel junctions. The SIS receiver covers 25 GHz band centered at 90 GHz with an equivalent noise temperature of about 50 K. The receiver uses a fixed tuned SIS mixer with Nb/Al Ox/Nb junctions. The critical Josephson current density is about 2.6 KA/cm/sup 2/ and the junction area is of about 1.5 /spl mu/m/sup 2/. The limits to further improvement of the radiometer and the possibilities for extension of the technique to the sub millimeter band are discussed.
The development of ultra-low-noise receivers at frequencies of >200 GHz is primarily driven by astronomical applications. The most sensitive millimeter-wave receivers that have presently been constructed make use of the strong mixing afforded by superconductor-insulator-superconductor (SIS) tunnel junctions. In a collaborative project between UKC and RAL, a 230 GHz laboratory receiver has been built using a lead alloy SIS junction as the mixing element, mounted in 4:1 reduced-height waveguide, terminated at one end with a circular cross-section, adjustable backshort. Best results indicate a double-sideband receiver noise temperature of T/sub R/(DSB) approximately=120 K, which represents state-of-the-art performance at this frequency. Initial difficulties in achieving good mixing performance prompted an investigation into the importance of the 'quality' of mixer block backshort tuning elements. An analysis of the mixer equivalent circuit is presented which allows for loss in a non-ideal mixer block, and 3-port mixer model calculations are discussed which incorporate this analysis into the Tucker quantum theory of mixing.
The SIS quasiparticle tunnel junction has been demonstrated to be the most sensitive mixing element for millimeter wave receivers [1,2]. Conversion gain greater than 1 and noise temperature approaching the quantum limit [3,4] has been observed. The required local oscillator power, which can be as small as 10−8W, leads to a low saturation threshold for these mixers [5,6]. The practical benefits, including higher saturation power, of using series arrays are sufficiently important that a substantial number of experiments using arrays have been reported [2,6,7,8,9]. It has not been clear, however, whether there has been any sacrifice in mixer conversion efficiency (gain) or noise. We present a systematic experimental investigation of mixing performance in series arrays of N junctions, where N = 1, 5, 10, 25 and 50.