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Radio controlled toys

Discussion in 'Electronic Design' started by Roberto, Oct 12, 2003.

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  1. Roberto

    Roberto Guest

    In the manufacture of radio controlled toys, how do they make transmitters
    and recievers which operate on the same frequencies? Also, how do the
    transmitters (more specifically, the oscillators) actually work?

    Thanks in advance,
  2. Ian Stirling

    Ian Stirling Guest

    They use little bits of crystal.
    If you take two tuning forks, and ring one, the other will begin to ring too.
    The crystals in the transmitter and reciever ring at a specific frequency,
    and helps select the desired signal.
    As to how it actually works, it's sort of like a swing works.
    If you push the swing at the top of its swing, then it tends to swing at
    one frequency.

    For more details, you might want to look at any electronics textbook.
    I'd recommend "Art of Electronics" by Horowitz and Hill,
  3. Roberto

    Roberto Guest

    They use little bits of crystal.

    Are there crystals which 'ring' in the GHz range? Also, can you buy pairs
    crystals which have same resonant frequencies?

  4. Ian Stirling

    Ian Stirling Guest

    Sort-of, but not very well, and yes.
  5. Dave VanHorn

    Dave VanHorn Guest

    Not as such.
    Microwave oscillators are usually phase-locked to a reference oscillator
    operating at a much lower frequency.
    Of course, bags full of them.
  6. Roberto

    Roberto Guest

    Also, can you buy pairs crystals which have same resonant frequencies?
    So are you implying here that you don't actually buy pairs, but crystals are
    mass-produced in specific frequencies so that they all "in-tune" with each
  7. Hal Murray

    Hal Murray Guest

    So are you implying here that you don't actually buy pairs, but crystals are
    Yes, For example most watches run at 32.768 KHz using mass
    produced crystals.

    There is an amazing amount of info available on the web. Try google.

    Roughly, computer grade crystal oscillator packages normally are
    within 100 ppm of the number stamped on the case. That covers the
    initial manufacturing tolerances, temperature changes, supply voltage,
    and probably other things I haven't thought of. (long term aging?)

    For a given crystal, the temperature is normally the most important
    error factor. If you have a good reference, you can usually measure
    the temperature quite accurately by measuring the frequency of a
    crystal. (after calibration)
  8. Active8

    Active8 Guest

    long term aging is given as a separate spec.

  9. Dave VanHorn

    Dave VanHorn Guest

    Within their specified accuracy, and when put in a properly designed
    circuit, yes.
    A small bit of trim might be required, depending on the application.
  10. Crystals are mass-produced for certain frequencies - particularly
    those commonly used with microprocessors, nice round numbers like
    10.000 MHz, frequencies that make the generation of common baud rates
    easy (like 11.0592 MHz).

    For crystal controlled radio transmitters and receivers, the crystals
    can be custom made for the application and frequency to be used. If
    you are building a transmitter and superhetrodyne receiver, the
    receiver crystal frequency will differ the transmitter crystal
    frequency by the IF frequency.
  11. Active8

    Active8 Guest

    Sort-of, but not very well, and yes.
    YIG and YAG are crystals.

  12. Active8

    Active8 Guest

    i "only" paid $7 each(?) for two 10.245 MHz XTALs from Jan. you'd think
    that would be off the shelf for 10.7MHz to 455kHz conversion, but no. I
    only see 10.240. the 10.245 unit is for those (here i go again)
    freakin' Motorola Mosaic chips that you can't find any more. a NBFM dual
    conversion chip once found in every cheap 2-way HT from Maxwhatever to

    but they made the things for me and sent the calibration data, too. i
    suppose the have a bunch of blank slabs that they trim to freq. probably
    takes a few minutes and then some to calibrate. they oscillated. i glued
    a cheap npn (2n2222 IIRC) upside down on an empty cigarette pack with
    hot melt and ugly-bugged 'em to test.

  13. Alan

    Alan Guest

    Ug, I only have a few hundred 10.245 MHz crystals on hand, too bad I
    didn't see you looking. Just happens to be one of the values I got,
    probably didn't pay that same $14 for the lot. Only got them because
    they were near a round number, and close enough to 10 for my PICs to
    run for 40 MHz. Just say so if you want some backups..

  14. Fred Abse

    Fred Abse Guest

    10.240 used to be (probably still is) used in CB radios. It makes
    designing a synthesizer for 10 kHz channel stepping easy, and they just used
    a first IF of 10.695 instead of 10.7, so you have a multi-channel radio
    using just one crystal for both receive and transmit. Most of the
    selectivity was at 455 kHz, so the 10.695 filter could be cheap and
    crappy, if there was one at all.
  15. Gary Tait

    Gary Tait Guest

    Whereas On Sun, 19 Oct 2003 15:23:18 +0100, "Fred Abse"

    Nearly every fixed channel 49 Mhz cordelss phone I have uses one of
    those chips. Some use Rohm and others clones of it. Most of my analog
    cell phones use a variant of the same chip also.
    I'd have to look, to be sure, but most of the CBs I have have 3
    crystals, newer ones with CB specific PLL chips one or two.
  16. Active8

    Active8 Guest

    On 18 Oct 2003 17:43:11 -0700, Alan said,
    i was sneaky :)
    thanks, if i can find out about those vanishing NBFM chips i'll think
    about it. i'll have to rip open an old cordless and check Rohn, etc.

  17. Active8

    Active8 Guest

    thanks for the tip.

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