One way to increase the strength of the target echoes relative to the noise is to place a microwave amplifier ahead of the mixer. This preamplifier amplifies the antenna noise as well as the target echoes. Since the antenna noise is not nearly as strong as the mixer noise, however,he net effect is to increase the strength of the jrget echoes relative to the sum of the antenna nd mixer noise. The amplifier must, of ourse, introduce very little noise of its own. )nly in the late 19SO's were microwave amplifiers eveloped whose internally generated noise was :ss than that of a good mixer. Three of these de-ices—the maser, the parametric amplifier, and le traveling-wave tube—are described below.
Mixer and Local Oscillator. The mixer and local oscillator are the heart of the Armstrong superheterodyne circuit. In the mixer the incoming modulated radio-frequency signal is combined with unmodulated oscillations generated by the local oscillator to produce a lower radio-frequency signal containing the same modulation information as the incoming signal.
The resulting cement mixer, produced from the formerly discarded grappiers, was of much higher quality than that obtained from the unsintered material. This fact was firmly established by the English cement mixer manufacturer L. C. Johnson in 1845, and the term "portland cement mixer" has since been applied solely to the cement mixer made from the sintered material. This period marks the real beginning of the portland cement mixer industry.
One way to increase the strength of the target echoes relative to the noise is to place a microwave amplifier ahead of the mixer. This preamplifier amplifies the antenna noise as well as the target echoes. Since the antenna noise is not nearly as strong as the mixer noise, however,he net effect is to increase the strength of the jrget echoes relative to the sum of the antenna nd mixer noise. The amplifier must, of ourse, introduce very little noise of its own. )nly in the late 19SO's were microwave amplifiers eveloped whose internally generated noise was :ss than that of a good mixer. Three of these de-ices—the maser, the parametric amplifier, and le traveling-wave tube—are described below.