By Smith D. (ed.)
QEX is the ARRL ''Forum for Communications Experimenters.'' released bimonthly, it beneficial properties technical articles, columns, and different goods of curiosity to radio amateurs and communications execs. The undertaking of QEX is to:1. supply a medium for the alternate of principles and knowledge between beginner Radio experimenters,2. rfile complicated technical paintings within the beginner Radio box, and3. help efforts to enhance the country of the beginner Radio art.Each factor grants as much as 4 instances the technical fabric present in every one QST. Our sixty four pages frequently accommodate seven to 10 articles, plus letters and different attention-grabbing goods. we try to keep up a stability among theoretical and useful content material. Articles variety from basic development tasks to growth in radio concept. nearly every little thing of the journal is dedicated to invaluable concepts for the technically savvy. you're invited to obtain examples of remarkable QEX articles less than, and spot for your self.
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Extra resources for QEX 2004-11
Sample text
What a delightful turn of events! It means that stabilizing the loop will be easy. The equivalent series resistances of the filter capacitors limit the ultimate phase shift to considerably less than 360°, so we could close the loop with a simple linear gain if so desired. The problem is, if we close the loop with a gain of one, there would only be 13 dB of loop gain in the low audio frequencies, where most expected disturbances will occur. The 120 Hz spectral line is the big troublemaker and 13 dB is just not enough.
It was really beginning to look like a new design was essential. It was time for deep surgery—the power transformer had to come out! After ten minutes of wicking solder, the transformer was on the bench. Continuity check revealed that the factory Fig 1—Basic switch-mode power supply functions. ton Ts Typical values are ton = 8 µs and ts = 15 µs. As D goes from zero to unity, the dc output will go from zero to 28 V. Since the goal is constant output voltage, PWM control is used to overcome line and load disturbances to Fig 2—The buck regulator output stage.
This causes T2 to operate on a minor BH loop, forcing the magnetic flux to reverse on a cycle-by-cycle basis before the core saturates, taking only about 20 mA to flip the core. This is really slick. Whoever invented this circuit (see Fig 11) is a genius. This is a truly elegant design! I have seen its widespread use in the world of off-line switchers. Only the base drive transformer T2 was modified in this section, the rest of the factory circuit was unchanged. Incidentally, if you get the phase of the one-turn loop reversed, the oscillator will not start and it will not respond to external PWM drive.
