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RC Transmitter problem

Discussion in 'General Electronics Discussion' started by camerart, Oct 10, 2013.

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  1. camerart

    camerart

    109
    2
    Aug 18, 2010
    Hi,

    I am trying to connect a radio transmitter to a TX module to get it to work as it should. There is something wrong with either the transmitter signal, or some other thing. I put the output of the transmitter into an oscilloscope and here is what I see:-

    This shows the channels signals. The first one is Throttle and the second one is Rudder. When I move these you can see the square wave moves in a way I would imagine. As I zoom in and do the same thing, there is a kind of ghost signal. Is this the signal or the oscilloscope?

    Cheers, Camerart.
     
  2. pebe

    pebe

    83
    11
    Sep 3, 2013
    Your scanning period is shorter than the period of the entire waveform sequence. Hence successive scans are overlaying earlier parts of the waveform.
     
  3. camerart

    camerart

    109
    2
    Aug 18, 2010
    Yes, I tried moving the other channels and indeed they moved independently from the first two.

    Thanks Pebe.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
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    Jan 21, 2010
  5. camerart

    camerart

    109
    2
    Aug 18, 2010
    The trigger hold off on my scope doesn't have the range to completely get rid of it, but now I am aware of it, I can 'see through' the readings.

    Thanks, Camerart.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,839
    Jan 21, 2010
    There are a number of other options.

    1) You could find some other signal that is generated at the beginning of this waveform and use that to sync your scope.
    2) As an extension to that you could build your own trigger delay circuit and use it to generate an external trigger based on the input signal (but with a longer holdoff).
    3) you can use a slower timebase

    Option 3 was shown in your video. The first signal seemed nicely synced. The problem occurred when you switched to a faster horizontal timebase.

    Some scopes have a horizontal magnification (aka sweep magnifier, or sweep expander) feature where you can (typically) pull out a knob and the middle portion of the display is stretched out. This is like increasing the speed of the horizontal timebase, but it allows you to use the horizontal position to scan through the entire waveform.

    Horizontal magnification also allows you to see "further to the right" than if you increased the timebase speed (which is what you were doing in the second part of your video.

    I believe that if you get a stable display of the entire waveform (as you did in the first part of your video) then use the horizontal magnifier (if you have one) you will be able to see a nice clear picture of the beginning of the waveform.

    Be aware that many of these controls (holdoff, horizontal magnification) can result in a dimmer display. You may need to turn the brightness up a little. If you turn it up a lot, remember to turn it down again before you change scope modes. Whilst a short duration of an over-bright trace is going to do little damage, I prefer to avoid it where I can.
     
  7. camerart

    camerart

    109
    2
    Aug 18, 2010
    My oscilloscope is quite old, and I use it as a guide, so doesn't have magnifiers etc. I'll have a look at what you suggest, to see if I can see later parts of the 'magnified' wave.

    Thanks for your suggestions.

    Cheers, Camerart.
     
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