The Crystalizer


Posted on Feb 7, 2014

A classic amateur radio transmitter topology, the oscillator-amplifier (sometimes called master oscillator, power amplifier, or MOPA), used one tube for frequency control and another for RF power amplification, and in quartz-crystal-controlled form commonly allowed operation on two or more bands with one crystal. Frequency multiplication ”generat


The Crystalizer
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ion and selection of harmonics (multiples) of the crystal oscillator`s fundamental frequency ”was the basis for this. In the earlier days of frequency multiplication, separate stages performed the roles of frequency generator and harmonic multiplier because only triode vacuum tubes were available. The advent of the screen-grid tube (tetrodes, pentodes, and beyond), the embedded screen grid/control grid/cathode triode in which could serve as a triode oscillator at the crystal fundamental while harmonic output was obtained by suitably tuning the plate, made two-tube, three- and four-band oscillator-amplifier transmitters possible by 1933 CE. Nearly all screen-grid oscillators capable of this class of operation are a form of tri-tet, although non-tri-tet variations, such as Frank Jones`s harmonic Pierce oscillator, were possible as well. Seeking to approach the utility of the tri-tet multiband oscillator with transistors, and having achieved excellent results in synthesizing a tetrode with cascode-connected vacuum-tube triodes for regenerative-detector service, I recalled the cascoded-BJTs VCO buffer amplifier described by David Stockton, GM4ZNX, in the Oscillators and Frequency Synthesizers chapter he wrote for the 1995 CE ARRL Handbook for Radio Amateurs and wondered if an oscillator based on cascoded transistors might approach the multiband utility of the tri-tet. Through private correspondence, Wes Hayward, W7ZOI, pointed...




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