I need someone to explain this to me.

Teensy board used as an AM radio transmitter

[Angus McInnes] has been working on AM radio transmission techniques. He tried out a method of using a VGA port for the task but found the vertical blanking was audible. His latest experiments use a Teensy microcontroller board as an AM transmitter.

This is not a standalone solution, but rather a hardware extension for his laptop. This is because the microprocessor doesn’t have enough cycles to do much more than read bytes over USB and push their bits out one of the I/O pins.

To get a steady stream of data he’s using¬†isochronous mode to push a steady data stream via the USB connection. Bulk transfer is another option but [Angus] found that it caused some jitter in the audio. Each byte is fed to the AVR SPI hardware once every eight clock cycles. His transmission can be picked up from across the room, but that’s the limit since the AVR doesn’t put out that strong of a signal. But it should be a rather trivial exercise to build a simple amplifier.

Comments

  1. Harvie.CZ says:

    I’ve been experimenting with such square-wave AM transmitters based on Arduino, CMOS ICs and similar, but i can’t figure out how to make output more powerfull.

    I can hear transmitted signal when transmitter antena is no more than 10cm away from receiver or if i connect transmitter output to metalic plumbing which is going trough whole house. But that’s not much satisfiing. I’ve tried using single transistor amplifier and totem pole push-pull half bridge to amplify output without any improvement in signal stregth. I’ve also tried various length and thickness of wire used as antenna.

    Any ideas how to build some ultra simple amplifier for such ultra simple square wave AM transmitters??

    BTW i’ve noticed that connecting two LEDs in series (originaly it was BJT transistor with only two legs connected) between possitive suply and RF output makes things bit better. Why?

    • adcurtin says:

      Maybe your supply is noisy, and the diodes help that (or change the voltage, if it’s just 2 LEDs with no resistor). Try putting a couple caps across the supply (a ceramic for high freq and an electrolytic for low freq)

      • Harvie.CZ says:

        I’ve figured this out by myself right in the beginning of my experiments. I’ve put ceramic and electrolytic capacitors across power supply. It reduced mostly all of noise that i’ve heard from receiver speakers. There was some noise left because circuit was not shielded, but that’s not big issue.

        Anyway… i still can’t improve the output power. I’ve been thinking about buying some monolythic RF amp like MAR-8a, but i think that this can be done using transistor and coil or something equivalently simple. Any ideas?

  2. einballimwasser says:

    Well .. It kinda works with the vertical blank (blank == sync?). We did this for fun in university on a boring evening – http://www.youtube.com/watch?v=ZS44zDSNzU4&feature=player_embedded

    Unfortunately I can’t remember what the reception frequency was :(

  3. dxa says:

    http://bellard.org/dvbt/ (using VGA card to transmit 4 DVBT video channel by air)

  4. randomerr says:

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