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Posts posted by trigger

  1. I think it is not that easy to build in just using bandpass filters.

    And the sound frequency generated by the vehicles are not always the same and you have to take care of the "sound" generated by other sources, these are the most critical points!

    May be you have to analyze the sound frequency spectrum in realtime and look for some peaks..... but will that be working..... I don't know.

  2. FM wave will reflect by ground...... why not? ???
    And the destructive interference is mainly caused by the delayed reflected wave (multipath) + original wave received by the receiver. (for example, in some area, TV set will get some "ghost image" due to the delayed signal mixed with the original signal)

    I think this book Principle of Mobile Communication by Gordon L. Stüber is a very good reference. It has mentioned some RF propagation modelling (like OKUMURA-HATA AND CCIR MODELS), where can provide you more deeper understanding in RF propagation.

  3. Just give you some hints:

    1. Try to make use of the ohm's law.
    2. Do not calculate the total current in only one equation.
    3. Try to divide the whole cirucit in parts and calculate their voltages and currents.
    4. Note that at R6, the voltage across it is the same as the battery voltage. Other parts will be more or less the same.
    5. Total current = current flow to R6 + current flow to R1 (and the branches)

    (try to name all node with voltages and currents so that will let you easily get the answer)

  4. It is very strange that you can build the hardware without a software consideration....... :o

    No one can able to help for no circuits, no general info........

    And no one can help if you don't give any try on your own coding!

  5. Alun,

    I don't have much frequency drift for my 453MHz LC transmitter with voltage variation......

    I found that if I use normal variable capacitor to the LC tank, it will drift about 2 to 4 MHz..... but when I use a trim capacitor (I think with better accuracy and temperature variation ) the transmitter frequency is just fine with just 500KHz drift.... even my hands get close to it... it still works within my expected frequency range.

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