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Thread: Homemade Packet Modem

  1. #1

    Default Homemade Packet Modem

    I'm doing research on building a modem for packet radio modes like APRS. My question is, there seems to be a ton of options in terms of modulations. I wanted to use Bell 103 for HF and Bell 202 for VHF, would this be sufficient? Should I add any other modes? Once I get the modem ready to test, would anyone be willing to organize a test session for diagnostics with a real setup?

    For the modem part, I'm thinking about using a simple PLL circuit, should be straightforward. A few more questions, now that I think about it:

    Bell 103 has an audio tone offset of 1000 - 2000hz (roughly), are frequencies referenced based on what frequency the mark or space tone is on or are they offset? (I.e. if someone says they're on 10Mhz are the tones at 10Mhz or 10.001Mhz?)

    Is there a preamble for the packet data so I can sync my clock to the packet or are the bits just sent?

    This is alot to ask all at once and I apologize for the long post. If someone can help answer I would appreciate it!

  2. #2
    Super Moderator 5B4AJB's Avatar
    Join Date
    Mar 2008
    Meneou, Cyprus


    Quote Originally Posted by KC3IBY View Post
    Is there a preamble for the packet data
    Yes, it has one of the best names created by mankind, called diddle.

    Try to find a Kantronics manual, the one(s) that came with my KAM explains everything. A lot of it is KAM-specific, but other packet modems use a similar subset of commands...

  3. #3


    Hi, for test data, there’s a well known APRS test CD:

    It’s half hour or so of driving around LA, but the data is old, so doesn’t contain any APRS updates, even up to APRS v1.1,
    but it’s a great starting point for decoding.
    There are at least a couple of errors in it. One missing waypoint symbol (making the information body payload too short),
    and an out of range latitude byte resulting in a latitude error that depends on your implementation.
    Both of these are Mic-E position packets.

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