Experiments have shown that there is a relationship between noise output and tube gas pressure. Lowering the pressure of the tubes has produced noticeable changes in the noise levels. At the present, the vacuums utilized are not much better than torr. The improved vacuum of space might reduce the overall spurious noise output. Since the application is narrowband, the background noise bandwidth may be reduced by narrowing the passband of the tube. A bandpass filter may also be used to suppress the background noise. Bandpass filters have been attached to the output of magnetrons to reduce the harmonic output. The information obtained on noise reduction will be described because the magnetron is similar to the amplitron and utilizes the same means of converting dc into microwave power. The measurements of the noise generated by the magnetron with a cold cathode are therefore similar to those which will be obtained from the amplitron. The noise measurements to be described are on a pulsed tube with a hot cathode, and it would be expected that the noise characteristics of a cw tube, such as the type to be used in the MPTS, would be better than those of a pulsed tube. The standard production model RK6344 magnetron has a second harmonic power output typically about 50 dB below the main signal power level. The third harmonic is approximately 45 dB down. The background noise is 80 to 90 dB below the main signal level as measured over a 3 MHz band. This signal-to-noise ratio is therefore 85 to 95 dB/MHz. A waffle-iron waveguide filter was attached to the output of an RK6344 magnetron. With the filter on the output of the tube, the second harmonic was 90 dB below the carrier level, which represented a reduction of 40 dB as compared with the standard tube. The third harmonic level was 80 dB below the carrier, indicating a reduction of 35 dB. From the background noise measurements of the RK6344 magnetron, it is concluded that the cw amplitron to be used for the MPTS would have a signal-to- noise ratio of 85 dB/MHz. For this single frequency application, filters may be coupled to the tube. The output signal of the amplitron, therefore, would pass through a filter before reaching the tube’s waveguide output system. This will
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