There was an urban legend back in the days of mechanical electricity meters, that there were “lucky” appliances that once plugged in would make the meter go backwards. It probably has its origin in the interaction between a strongly capacitive load and the inductance of the coils in the meter but remains largely apocryphal for the average home user. That’s not to say that a meter can’t be fooled into doing strange things though, as a team at the University of Twente have demonstrated by sending some more modern meters running backwards. How have they performed this miracle? Electromagnetic interference from a dimmer switch.
Reading the paper (PDF link) it becomes apparent that this behavior is the result of the dimmer switch having the ability to move the phase of the current pulse with respect to the voltage cycle. AC dimmers are old hat in 2021, but for those unfamiliar with their operation they work by switching themselves on only for a portion of the mains cycle. The cycle time is varied by the dimming control. Thus the time between the mains zero-crossing point and their turn-on point is equivalent to a phase shift of the current waveform. Since electricity meters depend heavily upon this phase relationship, their performance can be tuned. Perhaps European stores will now brace themselves for a run on dimmer switches.
If you’re curious about these old-style dimmers, take a look at some of their basic functionality.
Thanks [Dorus] for the tip.
These days we are spoiled with a lot of cheap test equipment. However, you can do a lot of measurements with nothing more than an oscilloscope. Add something like a signal generator and you can do even more. One classic technique for frequency measurement, for example, is using a scope to display a Lissajous pattern. [Franz Schaefer] has a video showing how to generate these useful curves with GNU Radio.
As we pointed out earlier, GNU Radio doesn’t have to be about radio–it is really just a Python-based signal processing laboratory. [Franz] uses GNU Radio Companion to create blocks which in turn create the patterns on an old analog scope.
Continue reading “GNU Radio Drives Oscilloscope”
In 1971, the United States Navy launched the Omega navigational system for submarines and surface ships. The system used radio frequencies and phase difference calculations to determine global position. A network of eight (VLF) transmitter sites spread around the globe made up the system, which required the cooperation of six other nations.
Omega’s fix accuracy was somewhere between one and two nautical miles. Her eight transmitter stations were positioned around the Earth such that any single point on the planet could receive a usable signal from at least five stations. All of the transmitters were synchronized to a Cesium clock and emitted signals on a time-shared schedule.
A ship’s receiving equipment performed navigation by comparing the phase difference between detected signals. This calculation was based around “lanes” that served to divvy up the distance between stations into equal divisions. A grid of these lanes formed by eight stations’ worth of overlapping signals provides intersecting lines of position (LOP) that give the sailor his fix.
In order for the lane numbers to have meaning, the sailor has to dial in his starting lane number in port based on the maps. He would then select the pair of stations nearest him, which were designated with the letters A to H. He would consult the skywave correction tables and make small adjustments for atmospheric conditions and other variances. Finally, he would set his lane number manually and set sail.
Continue reading “Retrotechtacular: The Omega Navigational System”