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Nice project. People interested in learning more about the diode
compensation circuit can find additional information in John Grebenkemper's article "The Tandem Match -- An Accurate Directional Wattmeter", QST, Jan, 1987, and a good deal more in my article "A Simple and Accurate QRP Directional Wattmeter" in February 1990 QST.. It turns out that you can reduce the diode drop to an arbitrarily low value by terminating it with an arbitrarily high resistance. But that's not the problem in detecting small signals. The problem is that the diode reverse leakage current becomes a bigger and bigger fraction of the forward current as the current gets smaller. That is, the diode becomes more and more like a resistor as the signal gets smaller and smaller. The problem, then, isn't a lack of forward conduction, but that the charge conducted into the load during the positive half cycle is sucked back out during the negative half cycle. The compensation circuit used in these two articles and in Jason's circuit only approximately match the RF conductivity characteristics of the diode, and the operating points of the diodes have to be carefully chosen to insure tracking with temperature changes as well as signal level. Roy Lewallen, W7EL Jason Hsu wrote: Last semester, I designed and built the QROP Meter for my graduate school independent study project. Thanks to those of you who helped me with certain thorny issues, such as transformers and DC amplification. Go to my web page at http://www.jasonhsu.com/ee.html to learn how to build the instrument. What makes my SWR/wattmeter different from the numerous versions available from MFJ and the numerous homebrew designs out there? 1. Works from 200mW to 100W! You get the best of both worlds! MFJ SWR/wattmeters don't work at QRP, and QRP SWR/wattmeters can't handle 100W. Tuning up at 1W instead of 100W reduces the QRM you cause by 20dB, or over 3 S units! You can also safely change the tuner's inductance taps WHILE transmitting at 1W. 2. GOOD resolution at SWR values above 3 and even above 5! This can be very handy at 160m and 80/75m. 3. 10-LED display for the SWR meter and another 10-LED display for the wattmeter: No more squinting at cross-needle meters! 4. More accuracy at low power levels: Most SWR meters understate SWR at low power levels due to the infamous diode drop loss. My design compensates for this and allows accurate SWR measurements at QRP power levels. Let me know what you think, and feel free to contact me if you have any questions or suggestions. If you decide to build the QROP Meter, let me know how it goes. Jason Hsu, AG4DG |
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