Abstract :
The analogy between a traveling-wave tube and a cascade of stages of amplification suggests the possibility of extending certain low-frequency techniques to the microwave region. Two resulting devices, a microwave logarithmic amplifier and a microwave limiter of large dynamic range, are described. Both these devices operate on an rf to video basis although, in principle, the method can be applied to rf to IF or straight rf operation. A microwave analog of a conventional successive detection logarithmic amplifier has been constructed by placing several helical output couplers along the active length of an S-band traveling-wave tube. With a signal applied to the input coupler, the output of each coupler was detected and applied to a video adding circuit. For a particular choice of the adding ratios, the total video output power was a linear function of the logarithm of the input rf power to within 0.4 db over a 55-db range of input level. The input saturation power of the tube was increased more than 40 db in this manner; the small signal gain was 27 db.
