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Sunday, August 28, 2016


TekTronix Model 1502 TDR
Roberta X hacks together a power supply for an old Tektronix TDR. TDR stands for Time Domain Reflectometer. Reflectometer can be broken into reflect-o-meter, which is how all meters used to be named (speedometer, tachometer). A TDR measures the condition of a cable by sending electrical signals down the wire and then looking at what gets reflected back from the far end, or any defects along the way.
    Electricity travels at roughly the speed of light, so for any cable you look at on the surface of the Earth, the time it takes for electricity to travel from your end to the far end and back is going to be so tiny as to be invisible, but this kind of information can be very useful for people who are concerned with making cables carry signals properly, so we have the TDR.
    Seconds can be broken into tenths, which you can measure with a stop watch, or hundreths of a second with the electronic timers used for sporting events. You can even measure sporting events to the nearest millisecond, but you would need to be travelling 60 MPH before the it would be worthwhile (at 60 MPH you will travel one inch in one millisecond).
    Below that (times less than one millisecond), we are talking imaginary numbers. There is nothing to compare with in normal, everyday events. But some people insist on poking around into even smaller divisions of time. Beyond (or below) milliseconds we have microseconds, and beyond that we have nanoseconds. Nanoseconds are kind of cool because light can travel one foot in one nanosecond.
    This is the realm of modern day computers. There are a billion nanoseconds in one second. Likewise there are a billion cycles in one gigahertz. So a computer that operates with a clock speed of one gigahertz, bits of data flowing down the data bus are going to be about twelve inches apart. In a computer that operates at 3 gigahertz (which is the last speed I remember hearing about), bits are going to be about four inches part, traveling at the speed of light. How is that these machines can operate at all?
    But that's nanoseconds. Beyond nanoseconds we have picoseconds, which are one trillionth of second, which is a TDR's stock in trade. Reading a description of the TDR Roberta was using is like reading the script to an episode of Star Trek.
. . . fire the tunnel diode pulser. The later models (1502B/C) used an LCD display and a microprocessor. The other major difference is the line charging method. The 1502 uses a fast (36 ps) tunnel diode pulser, the later models used a half sine wave to charge the line. The TD pulser, with its Dirac delta edge gives much better short range sensitivity although it is much easier to destroy. - TekWiki
P.S. Looking for pictures of picoseconds, the hot topic seems to be Picosure lasers for tattoo removal.

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