This paper reports on a theoretical treatment of the noise figure for iterative travelingwave parametric amplifiers and gives experimental data from noise figure measurements made on such an amplifier which operated at 1.7 kmc. Using information about the diodes supplied by the manufacturer (Microwave Associates) we calculated from the theory a double-channel noise figure of 1.3 db. The measured noise figure was 1.2 db,

The resistive loading which an electron beam produces in an adjacent structure can be made to exhibit a very low noise temperature. This is achieved by coupling to the fast cyclotron wave in a large magnetic field; the noise temperature at a given signal frequency is shown to be equal to the cathode temperature times the ratio of signal frequency to cyclotron frequency. An experiment is described in which this ratio is 19. The coupling structure interacts with the fast cyclotron wave but rejects the slow cyclotron wave. A noise temperature of 186°K is measured. In conclusion, it is shown that the large magnetic field required for beam cooling need not extend throughout the tube.

The variable parameter (or parametric) principle of amplification is characterized by a typical arrangement in which a variable energy storage element, such as an inductor or a capacitor, is suitably coupled to two resonant circuits. If the value of the energy storage element is made to vary in the proper way, energy is fed from the source which drives the element (that is, the pump) to the fields of both the resonant circuits. This paper describes the behavior relative to gain, band width, and noise of this type of amplifier. Specifically, it is shown that to increase gain, the Q of one of the resonant circuits, the one commonly called the idling circuit, must be increased or the variation in the variable reactance must be increased. The band width is inversely proportional to this Q and to the voltage gain. Hence, for high gain, the amplifier is normally a narrow band device. One of the most important sources of noise is the thermal noise originating in the idling circuit. However, in principle this source can be reduced indefinitely by making the idling frequency approach the pumping frequency or by artificially cooling the idling circuit. In this fashion very low noise figures should be possible. The parametric principle can also be applied to producing frequency conversion with large conversion gain. The appendix presents the expressions for gain, band width, and noise figure for this application. The behavior of the converter relative to gain, band width, and noise is quite similar to that of the amplifier.

From theoretical considerations of the fluctuations in an electron beam, an expression for the noise figure of a transverse-field amplifier is derived. There are found to be three statistically uncorrelated sources of noise in the beam of such a tube—(1) that arising from the fluctuations in the transverse emission velocity of electrons in a direction normal to the surface of the beam, (2) that arising from fluctuations in the transverse emission velocity of electrons in a direction parallel to the surface of the beam, and (3) that arising from fluctuations in the mean position of the beam and depending on its thickness. The analysis shows that the use of a collimator is required to give low noise figures. This is illustrated by the following example: for a tube designed to operate at 1000 mc with a collimator whose width is 0.004 in., the theoretical noise figure is 2 db. Without the collimator, the noise figure would be 11 db.