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Frequency Deviation and bandwidth in relation to data speed

Discussion in 'General Electronics Chat' started by mik3ca, Nov 13, 2017.

  1. mik3ca

    mik3ca Member

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    I have two radio modules HM-TRP's and I ran a test where each of these were connected to PC's through serial port adapters. and when I hold down a key on the keyboard, its like the PC wants to receive the data but chokes every 20th or so byte by not printing the character it's supposed to receive. The same happens in the opposite direction. I could blame the PC's themselves but because I'm dealing with wireless modules, and because I have so many wireless electronics around me, I want to rule the radio modules out first.

    My radio modules currently use the following settings:

    Frequency: 915Mhz
    Modulation data rate: 115200bps
    Receive Bandwidth: 105Khz
    Deviation: 35Khz
    Output Power: +20dBm (maximum)
    Uart data rate: 115200bps

    The manual states I can set the Frequency Deviation anywhere from 10 to 160 Khz and I can set the Receive Bandwidth anywhere from 30 to 620 Khz.

    I also use the converter found on the following page but using a 2.2uF capacitor instead of 100nF: http://picprojects.org.uk/projects/simpleSIO/ssio.htm

    I'm just curious. because I have so many wireless around, would lowering the bandwidth help? or would changing the deviation help? I am trying to exchange data at 115200bps with these devices but if I can't then I'll try 57600bps instead.
     
  2. dknguyen

    dknguyen Well-Known Member

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    It only happens when you're holding a key down? Not when you slowly tap? And not every nth character?

    Have you tried checking any flow control signals to see if you're just overrunning?

    Also, does it happen at 9600bps? (That would have been the first thing I tried before even posting). If 9600bps works just fine then I gather your airwaves are too noisy.
     
  3. mik3ca

    mik3ca Member

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    I use no flow control. when I tap slowly (with pause every second) the characters come out normal nearly every time. When I hold a key down, the error happens more often.

    I want to try a higher data rate because I want to ideally exchange 1600 bytes of data in one second (or at least 800 bytes) and I don't think this is possible with a 9600 baud.
     
  4. dave

    Dave New Member

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  5. mik3ca

    mik3ca Member

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    whoops. I meant I wanted to exchange 800 bytes in 1/4 second
     
  6. Pommie

    Pommie Well-Known Member Most Helpful Member

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    The key repeat rate is nowhere near your baud rate so I suspect noise. Try 9600 as suggested.

    Edit, 800 bytes every 1/4 second is 3200 bytes per second. At 8 bits with a start and stop bit will require 32,000 baud. You can easily manage with 57.6k baud.

    Mike.
     
    Last edited: Nov 13, 2017
  7. mik3ca

    mik3ca Member

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    Ok, so I decided to lower the baud of every device in the test from 115200bps to 38400bps and it turned out that the full data came out properly over a short distance. I'm still curious about the other settings I mentioned earlier.
     
  8. mik3ca

    mik3ca Member

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    I just ran the same test again but this time I moved the modules about 30 feet apart (up two flights of stairs) instead of 2 feet and I received nothing.
     

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