check the web page of the company i work for (www.baumerident.com), i develop this stuff @125kHz and 13.56kHz :-) also 433,92MHz 2,45GHz and 5,8GHz ID-Systeme as well as barcodereaders. okay enough advertising :-)

most companies want you to sign a NDA before you get more informations. if you only want to implement RFID read only feature running at 125kHz carrier frequency, then this is a very easy HW implementation, but you need to take much care of the software that you do not read "ghost-tags" i mean data that is not sent. most of the easy and cheap solutions give false id-codes and it makes much fun when i am visiting such ID-Fares showing them, that the system reports wrong data. this happens mostly if you are at the end of your comunication range and/or your tags is moved during data transmission.
you should also be aware of the maximum transmitting power that is allowed in your country (CE-marking).
so what you need is a antenna coil, a cap giving the resonance circuit and a receiver, but you will see with a scope that the data from the tag is somewhere inside the noise, so will need a good analog filter (or DSP if you can) and an amplifier (we have a gain of about 100dB) and a simple comperator at the end. that's it. well the filter is tag depended check the data rates and determine the cut-off frequencies.

if you what read/write feature then you need a better HW, as the modulation from sending data to the tag will also influence your receiver and has a much much higher signal level than the data from the tag.

the reading distance depends on the geometrie of the tag antenna combined with the antenna of the reader as this is a transformer .... (hint .. hint ...)


have fun, i had mine during 8 Years of developing ID systems.

Michael