Old Tyme measurements

Discussion in 'General Electronics Chat' started by WBahn, Jun 10, 2012.

  1. WBahn

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    Mar 31, 2012
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    I am starting to get serious about getting into RF and want to do it step by step because one of my dreams is to develop kits, tutorials, and projects for people (high schoolers that want to see if electronics is fun, college engineering students that want to gain hands on knowledge and skills, etc., etc.).

    Because of this goal, I also want to do things in a way such that people without oscilloscopes or decent meters can work their way through the projects successfully.

    This got me to wondering about how the early tinkerers (I'm not talking about the big players, but they interest me to) built things. For instance, you want to tune a tank circuit to 850kHz using a self-wound coil (and perhaps a handmade capacitor). How did you know the inductance of the coil? how did you know when the circuit was tuned to 850kHz? I realize that, in general, you designed things to be adjustable. But how did you go about adjusting it? How did you know that your oscillator was wiggling at 850kHz?

    Today, of course, there are numerous ways to make these measurements, many of which involve test equipment that has become quite inexpensive. But, assuming you didn't have that, are there techniques that people used back in 1900 that a kid could use today?
     
  2. absf

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    May be you can use an AM transistor radio as a simple signal generator. There is a local oscillator in the Mixer stage. If you need 850 KHz, just tune the radio to 1.305 MHz.

    Pass both frequencies to a comparator (analog or digital)....... and connect the out put to a LED. Not sure if it is too much work for a beginner.

    Allen
     
  3. Ron H

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    I did a little bit of research (Googling). The attachment is from TIME AND FREQUENCY MEASUREMENT AT NIST: THE FIRST 100 YEARS.
    Farther down in the document, there is a discussion of using 100khz quartz crystal oscillators (in 1929) as the frequency standard. They don't describe how they determined the frequency of these oscillators.:confused:
     
    Last edited: Jun 10, 2012
  4. K7GUH

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    I don't know how they calibrated the very first oscillator ever, but the 2nd through nth ones probably used the zero beat heterodyne method as detected by even an insensitive receiver (probably a crystal set would do, neh?)
     
  5. WBahn

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    But I don't see how that solves the problem since how do I know when I have tuned the local oscillator to 1.305MHz?

    Perhaps I am missing somethingm but I don't see what the comparator and LED accomplish since I would expect the comparator to be HI half the time and LO half the time regardless of the relative frequencies, phases. or amplitudes. The only thing that would vary would be the frequency and structure of the pulses, but those wouldn't be discernable in the pervceived brightness of the LED.
     
  6. Ron H

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    Perhaps this is why most electronics hobbyists start out with LEDs and 555 timers.;)
    My first project (circa 1954) was an oatmeal box crystal set, with a galena crystal and a catwhisker. I didn't need any test equipment.
    You can generally build basic receivers without requiring test equipment, although troubleshooting is much easier if you have a cheap multimeter at minimum. Transmitters are a lot more difficult, although AM band transmitters (and even FM) can be tested by trial and error if you have a radio on hand.
    I'm pretty sure I'm not telling you anything you don't already know.:)
     
  7. crutschow

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    Perhaps one of your kits should be a simple frequency counter. They can be rather cheaply made (at least for frequencies up to a few tens of MHz) with a crystal oscillator, some digital counters, and a digital display. A high frequency preamp at the input would allow the measurement of low level signals.
     
  8. WBahn

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    Here's the History of WWV. It is rather interesting.

    I supposed one could bootstrap by taking their best shot at the tuned circuit and then adjusting it until you can receive WWV and then use that signal as you reference.

    It doesn't sound to hard to get to that point and it sounds like something that a DIY kit should permit a noobie to do and, I think, hearing the NIST broadcast on a radio that you built yourself, including winding the coils and building the antenna, would provide a spark of excitement.

    But then what? Here is the WWV Broadcast Format

    It's pretty complex, so getting the frequency you are interested in might be easier said than done, at least at first. But let's assume that you can deal with knowing that you can hear the, say, 600Hz tone only for 45 seconds every other minute. How does that (or any other part of the signal) help me tune an oscillator to 850kHz?

    An alternative for the kit would be to include a quartz crystal and have them build a circuit that gives them a "reference" that is nominally at 1MHz. How could you use that to tune your other oscillator to 850kHz?
     
  9. WBahn

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    That may be the direction to go, and the concepts are simple enough that they can be explained to a high schooler. That's one of the big things I am trying to achieve -- that they proceed at a pace such that they understand all of the circuits they are building, at least from a reasonable intuitive standpoint if not from a detailed mathematical standpoint.

    But, if nothing else, I would also like to include a description of how the early tinkerers did it.
     
  10. shortbus

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    Have you ever seen the Lindsay Publications catalog? Lots of books on the old ways of radio(1920's era). Heres one on the GDO and hoe to build one- http://www.lindsaybks.com/bks5/rexp2/index.html Would that help for what you want to do?
     
  11. WBahn

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    I don't know if I've run across Lindsay Publications or not, but it looks like it's worth checking out. I've found that so many books aimed at the hobbyist are full of crap that's just plain wrong, that it's hard to find the good ones. In part, that's because you generally don't know it's crap until you have invested more time than it was worth. I have no idea which side of that divide these fall into, but it does look worth a gander. If nothing else, they will definitely expose me to ideas and techniques that I can explore elsewhere even if it turns out their take on them is way off.
     
  12. WBahn

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    I've looked at some stuff on the GDO and, for my purposes, it has the same basic problem, namely that you have to be able to tune it to a particular frequency of interest or determine what frequency it is tuned to when it matches up with the unknown circuit. As such, it might well make an interesting project, probably a side project, for what I have in mind.
     
  13. shortbus

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