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Superconducting miniature microstrip resonators, operating at X band, have been constructed using vacuum-deposited lead on low-loss sintered alumina substrate. Unloaded Q's as high as 200 000 and 500 000 have been measured at 14.3 GHz at 4.2 and 1.8 K, respectively.

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Correspondence 329 can be obtained. For n stages, the over-all voltage transfer function is D The parameters of the kth section may be found by solving from the expressions for center frequency 1 d C k L k Wk = = (4) and fractional 3-dB bandwidthThe fundamental principle underlying the design of constant-resistance networks similar to those discussed is the dimensional equationBy utilizing this relation to constrain certain element values in a network, it is also possible to design constant-resistance low-pass, high-pass, band-pass, and band-stop filters of II, T , bridged-T, and lattice configurations.A 16-dB gross improvement in the minimum detectable signal of an infrared detector using a Xenon gas laser preamplifier with 17-dB gain was recently reported by Bridges and Picus [l].We have independently investigated the use of a Helium-Neon laser preamplifier which had substantially higher gain. I t utilized the very high gain transition a t X = 3.39p, thereby permitting operation without regenerative reflectors and thus yielding amplification over the full Doppler linewidth. Measured improvements in minimum detectable signal of 45 dB (gross) and 32 dB (net) were obtained relative to a room temperature PbS detector.' S o amplifier noise was observed in our experiments and none should have been detectable as the following analysis indicates. The S S R for a system consisting of a laser amplifier followed by a square-law envelope detector, for a coherent input signal under matched conditions and with single-mode operation, can be represented by where P, = signal hvB/q. =amplifier input quantum noise power P I h = Planck's constant v=infrared signal frequency B =amplifier effective instantaneous G =amplifier gain q,, =amplifier population inversion Af=post-detection bandwidth d=quantum efficiency of envelope detector NEP=Soise Equivalent Power of detector (referenced to Af =lc/s) square bandwidth efficiency= 1 -(nr/gl)/(nu/gu) A general theoretical treatment has been presented by Steinberg [3]. The three terms in the denominator represent [4]added fluctuation noise, shot noise and detector NEP, respectively. Additional contributions due to 290°K background are neglected in the wavelength region considered here. Also, we take G -1 =G.For threshold signals, neglecting the relatively small shot-noise contribution and Af<>hvd2B/qa. Thus, for infrared wavelengths, e.g., X > lp, substantial improvements are obtainable, since as wavelength is increased, quantum noise decreases and detectors have progressively higher NEP's. For example, for X =3.39p, and taking B = 100 Mc/s, Af = 1 cps, q. =0.64 and quantum noise limited conditions (large G) a P,,,=l.2X10-'6 watt/cps1'2 is calculated. In our experiments and those of [l], detectors with NEP's in the range 10-10 watt/ cps"* were utilized. Using this value of NEP, and G = 3 2 dB, q.=O.64, and B=l00 Mc/s [5], the last term in (2)...