Oscilloscope!

Matt lent me his oscilloscope, after a bit of fiddling around I’ve recorded both what I’m transmitting and what I’m receiving on the other end. This is using Z6900 transmitter and a Z6905 receiver. That’s hooked up to an AVR transmitting the bit pattern 0101010 1110011 1000011 at 976.5Hz. The receiver data out pin is connected to ground via an 8K resistor.

Transmitted data: (CSV)

Received data: (CSV)

You can see something odd is going on. I don’t know where that noise just before the message is coming from; it’s not present on the transmission end, and it’s not present after the message ends. Anyone got any clues? I’m a bit stumped. You can clearly see the transmitted signal in the received data, it’s just with that horrible crap sprayed over the start of it. Would putting an appropriate cap in as a filter be a decent fix?

(Sorry the screenshots from the scope have different scales, the software is a bit fiddly to use and I’m not sure how to turn fix the scale.)

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One Response to Oscilloscope!

  1. Cary says:

    It looks like it’s being generated by the receiver as its envelope detector gets tuned to detect the newly arriving data (Assuming your radio modules are of the garden variety using ASK modulation, I haven’t looked at the links to see).

    You might try adding a preamble of 10101010 repeated as long as you need to get the receiver locked-on and spitting nice edges, maybe 2 to 5 bytes worth, remembering that the lock time will increase with range.

    You might also like to investigate using a Manchester code, or some other asynchronous encoding scheme, which would greatly simplify your receiving algorithm (this bit is in reference to what we talked about last week when you were describing triple ring buffers and trying to find phase lock).

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