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A dual-clock MASH (multi-stage noise shaping) deltasigma modulator (DSM) is proposed for high performance analog-digital converter. This employs a DSM using a frequency modulated intermediate signal (FMDSM) for the last stage. The sampling clock frequency for the last stage can be increased due to the features of the FMDSM. It is shown that this can increase the SNR beyond the conventional MASH DSMs.

4 solid disc 102 inches in diameter and 7 or 8 inches thick would be incomparably more easily and better annealed, and would be more rigid, than one of 13 inches thickness.The 101-inch mirror of the Hooker reflector, Mt Wilson Observatory, is shown in Fig. I hanging on edge on its polishing machine in the Hooker Optical Laboratory. The photograph shows the innumerable bubbles in the interior of the disc. opt. Soc.xx1x 13 198 G. W. RitcheyThe polishing machine is so designed that the mirror can quickly be turned from the horikontal position, in which it lies while being ground, polished, and figured, to the vertical position here shown, in which it hangs while being tested optically. This is done without removing the mirror from its cushioned turntable. T h e fullsize polishing-tool or plate is shown, hanging on edge on its trunnions, on the righthand side of the picture. T h e optical work on the mirror and its accessory mirrors, which included making an opticallyplane mirror 60 inches in diameter for the optical tests in the laboratory, required the greater part of the time and intensive care of myself and two assistants for five years.3. Thick, solid glass discs, even when highly homogeneous, are subject to injurious changes of optical figure when in use at night, due to temperature changes. These changes of figure vary with the thickness of the disc and, of course, with the composition of the glass used. They become of serious moment in the case of large, solid, thick mirrors of ordinary glass. 4.In my opinion metallic mirrors made of cast metal alloys such as speculummetal and "stellite" cannot properly be regarded as optical mirrors in the strict, modern sense of that term. Optically polished surfaces of these alloys, when examined with even a low-power microscope, show a granular or crystalline structure, and even castings of moderate size invariably have porous areas which are extremely troublesome in optical work. 5.On the other hand, however, optically polished surfaces of vitreous materials such as glass, pyrex, and fused quartz have a liquid-like quality; their structure is so fine that it cannot be detected even with the highest-power microscopes. When silvered by either the chemical or the cathode method, the silver film, deposited molecule by molecule on the polished vitreous surface, is so fine in structure that it takes the liquid-like quality of that surface. Further, the silver film is so thin, about 0~000004 inch, that it retains perfectly the optical figure (form) of the vitreous surface. Still further, its structure is so fine and dense that it is nearly opaque to the brightest light. These qualities, taken together, account for the very high reflective power of silvered vitreous surfaces when kept in fine condition. They account also for the fact that optically good mirrors with these surfaces permit the use of the full theoretical magnifying powers due to their apertures ; these surfaces can therefore be regarded as real optical surfaces. CELLULAR DIscs FOR OPTICAL MIRRORS 6 . Cellular disc...

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A resonant tunneling logic gate MOBILE (Monostable-Blstable transition Logic Element) is an ultrahigh-speed functional logic gate,. which is based on the monostable-bistable transition caused by the clock in the series-connected RTD circuit [I]. The MOBILE switches at the rising edge of the clock and the output remains in one of the two stable states while the clock is high. This indicates that the MOBlLE works as an ideal 1 -bit quantizer for high-speed analog-to-digital converters (ADCs). Recently, we proposed a simple AZ ADC using MOBILES [2]. The AZ ADC has a significant advantage compared to other types of ADCs; higher resolution can be easily obtained by increasing the sampling rate. It does not require high-accuracy analog components to achieve high resolution. However, the operation frequency and the resolution of our previous proposal was still restricted by the negative feedback loop. In this paper, we will propose a novel AE ADC having no feedback loop, which can make the best use of high-frequency performance of the RTDs. Figure 1 shows the block diagram of the AZ m C . The input analog signal is first converted to the pulse density modulation signal, then it is converted to the Nyquist rate high-resolution (multi-bit) digital signal by a decimation filter. The key component of this is the AX modulator, which converts the analog signal to the pulse density digital signal. Here, we employ a novel AE modulation technique suitable to MOBILEs using a voltage controlled oscillator (VCO) [3]. The configuration of the novel AE modulator is shown in Fig. 2 together with the conventional one. This implementation is based on the fact that the phase of the frequency modulation (FM) signal from VCO, O(t), is the integral of the input signal x(t) as hete,fc represents the output frequency of the VCO when input is zero, and k the frequency sensitivity.Therefore the integrator in the conventional configuration can be removed. Moreover, the negative feedback is inherently embedded in the VCO, because the phase returns to zero when it reaches 2~. Owing to these features the feedback loop can be removed. Using the I-bit quantizer and the XOR the ideal pulse density signal can be obtained. However, it requires extremely high performance quantizer, having ultra short aperture time and high sensitivity at high frequency, for fully developing this concept, so that this technique has been not widely used. Employing the MOBILE, this problem can be solved, and extremely high performance should be expected.We tested this hc modulator concept using the MOBILE fabricated on the InP substrate. Figure 3 and 4 shows the experimental setup and the chip photomicrograph of the MOBILE used in the experiment. The HEMT has the gate length of 0.7 pm and a threshold voltage of about 0 V. The peak voltage, the peak current density and the peak-to-valley current ratio of the RTD are 0.35 V, 6.0~10' Ncm' and 10 at room temperature, respectively. The frequency modulation signal was synthesized by the signal generator and it was fed to the MOBI...