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Project #1: 5 kbaud BBC modem in the 125 kHz RFID Band

(Last Mod: 04 November 2010 18:12:50 )

Project Overview

The goal of this project is to create a transceiver capable of sending and receiving BBC-encoded messages at 5 kbaud at a carrier frequency of 125 kHz.

This frequency was chosen because it is one of the commonly used carrier frequencies for RFID applications and one of my interests, down the road, is RFID. In addition to building up practical, hands-on experience in this band, the frequencies in use are low enough that lumped component models should be adequate, rigorous layout techniques should not be critical for most of the circuits, and I have scopes that can handle the frequency range very well.

It should be noted that the claim that this is the RFID "band" is somewhat misleading. As near as I can tell, it is not a recognized ISM (Industrial, Scientific, and Medical) band but it is one of the FCC Part 15 bands authorized for unlicensed use provided, among other things, that it doesn't cause harmful interference to licensed services.

Given the very long wavelength of a signal at 125 kHz (more than two kilometers), the antennas involved will not be producing a propagating wave but, instead, an attenuating wave because they will be working with near-field effects. Such signals die out as the cube, not the square, of the distance, and so these radios will have very short ranges. But, for my purposes, that's not at all important. In fact, it is a benefit because it means that there is little chance that I will interfere with anyone else as I rather clumsily make my first foray in the world of radio electonics.