Minimum Noise Temperature Research Articles

Double-pumped low-noise parametric down-convertor

It is shown how a low-noise parametric down-convertor can be realised by using two incommensurable pumping generators. The properties of the system, such as gain-bandwidth product and minimum-noise temperature, are discussed.

The Ultimate S-Band Monopulse Focal-Point Feed

A newly developed high-efficiency S-band tracking feed is described. It is a focal-point feed designed primarily for use with parabolic reflectors in the 15 to 25 foot diameter class. A five-element aperture geometry was developed to obtain improved performance in sum and difference pattern efficiency over that normally obtained with four-element, hybrid-comparator feeds. Two major e mprovements were obtained: a. Elimination of the conventional hybrid comparator resulted in the "ultimate" sum-channel efficiency, hence minimum noise temperature degradation due to feed losses. b. Independent control of the difference-pattern illumination produced "near-optimum" gainslope performance simultaneously with optimized sum-channel illumination. Design theory for this feed geometry is discussed, and measured impedance and radiation pattern data obtained from the finished feed system are presented.

Noise limitation in helium-cooled parametric amplifiers

The minimum noise temperature of a parametric amplifier depends on the varactor junction capacitance, spreading resistance and its thermal temperature. The time-dependent junction capacitance remains nearly constant by cooling. But the spreading resistance remains constant with cooling only if the varactor is degenerately doped and has an abrupt p-n junction. Actually the junction region is slightly graded and therefore not degenerate. Thus the spreading resistance increases two or three times by cooling from room temperature to 4°K. The higher the thermal resistance of the varactor the more the spreading resistance is heated by the pump power. The measured thermal resistance of some diffused varactors is nearly the same at room and helium temperatures, while the thermal resistance of an epitaxial varactor increases sharply at very low temperatures. So the optimum pump frequency for minimum noise may be much lower than calculated from previous theories which neglect the pump heating. This is because the pump heating decreases with decreasing pump frequency and is of greater importance at low ambient temperatures. Considering the increase and the heating of the spreading resistance the minimum diode noise temperature of a helium cooled parametric amplifier at 4 Gc/s is calculated to be 3°–4°K. This agrees with measured data.

Quantum Noise in a Parametric Amplifier with Lossy Modes

We derive the fundamental noise properties of a parametric amplifier (paramp) which is driven by a monochromatic pump but which has many lossy signal and idler modes. We use a quantum-mechanical approach similar to that employed by Louisell and co-workers in their treatment of paramps and lasers with lossless modes. Our results are not directly deducible from the lossless mode analysis, but they do give the same limiting noise temperature as that analysis. This minimum noise temperature is the same as that of an ideal laser. In fact, we find that one can always design a laser whose field correlation functions would be the same as those of any given paramp. Many results for laser noise in various configurations can be carried over directly and applied to parametric amplifiers by using correspondence substitutions which we set down. For example, in analogy to Pound's description of the maser (in terms of a Nyquist theorem extended to negative temperatures), the calculation of paramp noise is found to be equivalent to applying Nyquist's theorem to both the active and passive elements in the (classically computed) equivalent circuit of a signal mode of interest. From this one obtains the spectra of effective noise generators which, when amplified in the circuit, duplicate the "quantum" as well as thermal noise that is present in the output. The Nyquist theorem is extended to apply to each active element by simply taking the element temperatures to be minus the real temperature of the corresponding idler mode (whose parametric coupling into the signal circuit gives rise to that element) multiplied by the ratio of the pump minus the idler to the idler frequencies.

Noise reduction schemes in transverse modulation tubes

The paper is concerned with a general analysis of noise in d.c. pumped amplifiers based on transverse waves. It is shown that any amplifier involving either the cyclotron waves or the synchornous waves has the same minimum noise temperature determined by the physical conditions at the cathode. The noise temperature can be reduced to an arbitrarily small value by increasing the magnetic field at the cathode or by reducing the cathode diameter. It is also shown that the minimum noise temperature of the transverse waves is reduced by the smoothing effect of the potential minimum near the cathode. The minimum noise temperature can be realized by inserting a suitable noise transformer between the cathode and the amplifier. In general, structures with electric or magnetic fields having rotational symmetry such as used in normal electron guns can perform the required transformation. Some specific examples of noise transformers are discussed.

The Minimum Noise of Varactor Amplifiers

The series resistance of commercially available varactors provides a high-frequency limit on varactor parametric amplifiers. The values of pump frequency for which gain can be achieved and the resulting noise temperature are restricted. For a given signal frequency and a given varactor there is a minimum noise temperature that is achieved by correctly selecting the pump frequency. This minimum noise temperature can be related to measurable varactor parameters and to the power necessary for pumping the varactor. A plot of achievable noise temperature vs frequency indicates areas in which varactor amplifiers are competitive for low-noise receivers.

A microwave Adler tube

A description is given of an Adler tube operating at 2700 Mc/s. This is a sealed-off version of a previously described demountable experiment at 4000 Mc/s. Gain of 24 dB can be obtained and the double channel effective input noise temperature is 96°K (noise factor 1·24 dB), as measured at the input terminal of the tube. By correcting for circuit losses the effective beam temperature is given as 62°K. A band-width of 10 Mc/s is obtained at points where the minimum noise temperature is doubled. Improvements which may reduce the noise temperature even further are described.

The physical aspects of low noise electronics

Quantum effects are beginning to play a role in the newer forms of low noise amplifiers. The ultimate noise performance of these amplifiers has been variously treated on the bases of spontaneous emission and zero-point energy to give a lower limit of noise temperature T n = (1/ln 2) (hv/k). It can be shown very generally, though, that the uncertainty principle of quantum mechanics can be used to prove that every linear amplifier adds some noise. Moreover, the minimum noise temperature of any linear amplifier can be shown to be T n= In 2-1/G 1-1/G −1 hv K The proper evaluation of this result requires a quantum theory of communication which does not yet exist.

Minimum noise temperature of DC-pumped transverse-wave electron beam amplifiers

The paper is concerned with a general analysis of noise in dc-pumped amplifiers based on transverse waves. It is shown that any amplifier involving either the cyclotron waves or the synchronous waves has the same minimum noise temperature determined by the conditions at the cathode. The noise temperature can be reduced to an arbitrarily small value by increasing the magnetic field at the cathode or by reducing the cathode diameter. It is also shown that the minimum noise temperature is reduced by the smoothing effect of the potential minimum near the cathode. The minimum noise temperature can be realized by inserting a suitable noise transformer between the cathode and the amplifier. In general, structures with electric or magnetic fields having rotational symmetry such as used in normal electron guns can perform the required transformation. Some specific examples of noise transformers are discussed.

Optimum performance of parametric diodes at S-band

It is shown that a Fourier expansion of the inverse of the barrier capacitance describes the potential noise performance of a semiconductor parametric diode better than the commonly used expansion of the capacitance itself. The Fourier coefficients are evaluated for point-contact and graded-junction diodes. Experiments carried out on diodes in two different amplifiers have given results in good agreement with the theoretical analysis over a wide range of pump power. Germanium, silicon and gallium-arsenide diodes all showed clearly defined pumping conditions for optimum noiseperformance. The theoretical analysis suggests a definition of a simple static figure of merit (an extension of Uhlir's cut-off frequency) which is sasily measurable at microwave frequencies. The noise performance expected from this figure of merit agrees well with the minimum noise temperatures measured for germanium and gallium-arsenide diodes in a degenerate amplifier at 2.4Gc/s. For silicon diodes the measured noise was higher than predicted by the simple theory, owing to the eerious influence of an anomalous reverse current.

Optimum Noise and Gain-Bandwidth Performance for a Practical One-Port Parametric Amplifier

A practical one-port parametric amplifier is analyzed to determine the conditions under which minimum effective noise temperature and maximum gain-bandwidth product can be obtained. The analysis considers the effects of resistive loss and stray parasitic reactance in the junction-diode that provides the essential nonlinear reactance. It is shown that the conditions necessary for minimum noise temperature are compatible with those necessary for maximum gain-bandwidth product only if the diode has a high self-resonant frequency (the frequency at which the average diode capacitance resonates with the diode lead inductance). It is also shown that minimum noise temperature is always achieved when the diode loss alone is used as the idler circuit loading (regardless of the temperature of any additional idler loading), that there is a characteristic figure of merit for the diode, and that there is an optimum pump frequency. Based on the equations derived, universal curves have been drawn that enable the design of an optimum amplifier when the signal frequency and diode characteristics are specified. In conclusion, it is shown that if junction-diode parametric amplifiers operated at room temperature are to seriously challenge the low-noise performance of the maser at microwave frequencies, a substantial improvement in the diode figure of merit is required.