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Journal of Applied Physics

Low-noise properties of unbiased even-harmonic Josephson mixers

K. Fujisawa, S. Kita, Y. Ohmae, A. Yamaguchi

Published Mar 15, 1985• 3 citations
Publisher

Abstract

Noise properties of Josephson harmonic mixers were investigated both by simulation studies and by experiments. It was clarified that the unbiased mixing operation peculiar to even-harmonic Josephson mixers has a lower noise temperature than the biased operation. A minimum noise temperature of 121 K was obained for an unbiased second-harmonic Josephson mixer at 70 GHz. The cause of this low-noise property of the unbiased mixing was attributed mainly to the nonexistence of noise down-conversions by the odd-harmonics of LO and the Josephson oscillation.

DOI
10.1063/1.334402

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Japanese Journal of Applied Physics•Apr 1, 1981•8 citations
Noise properties of Josephson harmonic mixers were investigated both by simulation studies and by experiments. It was clarified that the unbiased mixing operation peculiar to even-harmonic Josephson mixers has a lower noise temperature than the biased operation. A minimum noise temperature of 121 K was obained for an unbiased second-harmonic Josephson mixer at 70 GHz. The cause of this low-noise property of the unbiased mixing was attributed mainly to the nonexistence of noise down-conversions by the odd-harmonics of LO and the Josephson oscillation.
We have studied the JJ harmonic mixing with an analog simulator devised by Magerlein. For even-harmonic mixing, there are two operation modes: zero-bias operation and finite-bias operation. The former is peculiar to even-harmonic mixing, while the latter is common to both even- and odd-harmonic mixing. Zero-bias operation is possible with a supply of LO current that suppresses the zero-voltage step completely, and it has the advantage of eliminating the dc bias supply and its accompanying heating effect and shot noise. For relatively low LO frequency (\varOmegaLO≃0.6), zero-bias operation has a higher conversion efficiency than does finite-bias operation. 2nd harmonic mixing under zero-bias operation has a conversion efficiency nearly equal to the fundamental mixing in the range \varOmegaS\lesssim0.5.
Josephson junction (JJ) harmonic mixers with even-harmonic numbers can operate at zero bias voltage. Among them, low harmonic ones are expected to have a good conversion efficiency. Hence, we have done experiments on the 2nd-harmonic mixer at 70 GHz. It is found that mixing behaviors are classified into three types: zero-bias, finite-bias and intermediate operation. It is also found that the conversion efficiency of the 2nd-harmonic mixer is not so much less than that of the fundamental mixer. We obtained a conversion gain of +1.5 dB with the 2nd-harmonic mixer, and +2.0 dB with the fundamental mixer.
We have carried out a series of experiments at 70.03 GHz on Josephson junction harmonic mixers with harmonic numbers 1–8, using an identical Josephson junction. It was found that Josephson junction harmonic mixers operate differently depending on whether their harmonic numbers are odd or even. With odd-harmonic mixers, only finite-bias operation is possible, while with even-harmonic mixers, finite-bias and zero-bias operations are both possible. It is noted that a 2nd-harmonic mixer has a conversion efficiency equal to or higher than that of a fundamental mixer. The dependence of the conversion efficiency (η) on the harmonic number (N) was also studied, and the following approximate relations were determined: η∼N -1.6 for finite-bias operation and η∼N -1.4 for zero-bias operation.
We report on experimental studies of the noise performance and conversion efficiency of high-Te Josephson mixers at frequencies between 90 GHz and 550 GHz. The heterodyne mixing experiments have been performed by using YBa2Cu307_8 step-edge junctions and bicrystal junctions on MgO (E--9.6) substrates. We studied the Josephson mixer performance for the case of internal pumping (selfpumping) and external pumping. Receiver noise measurements in a waveg,uide setup for the case of external pumping at 90-94 GHz gave formally calculated double-side-band (DSB) mixer noise temperatures (Tm) of about 2000 K at physical temperatures of 10 K. Similar measurements in a quasioptical setup at 4.2 K and at operation frequencies of 430 GHz and 546 GHz yielded mixer noise temperatures of 1200 K and 1100 K, respectively. Using the internal pumped mode, i.e. mixing without external local oscillator signal, we also obtained clear IF response in hot/cold-measurements. The formally calculated Tm were between 580 K and 2300 K.
Even-harmonic JJ mixers allow unbiased operation, while odd-harmonic ones do not. Phenomenological explanations are presented for this phenomenon. Based on the voltage source model, a simple analysis is given that explains this phenomenon clearly. Furthermore, based on the current source model, this is studied by using a JJ simulator. In the odd-harmonic mixing, the waveform of supplied current has an odd symmetry, therefore, there is no mean junction voltage and no IF output. In the even-harmonic mixing, there is no such symmetry in the waveform of supplied current, therefore, there is a finite mean junction voltage and IF output.