evandude
New Member
as part of an RFID system, I have a couple of coils, one on a reader and one on a tag. each one is tuned with a parallel capacitor to resonate at the carrier frequency of 125KHz. the reader coil is alternately pulled to 12v, and then open circuited, through a 100 ohm or so resistor, and since the reader coil is resonant it ends up with a decent sine wave on it (slightly distorted but good enough)
so the general theory is that when the terminals of the tag coil are shorted together while it is near the reader coil, it will attenuate the signal on the reader coil... effectively, the two coils together act as a weak air-core transformer, so shorting the output terminals loads the input down.
However, the reality is this: when the tag is placed in the reader field, but not transmitting anything, the reader coil signal is attenuated somewhat, which makes sense since the tuned tag coil would load it to a degree. however, when the tag is transmitting (alternately "shorting" (well, attenuating, not a real short) and then open-circuiting its coil) instead of seeing corresponding attenuations on the reader coil signal each time the tag coil is shorted, I see the opposite: the signal on the reader coil is INCREASING with each attenuation on the tag coil instead.
My theory is that the tuned tag coil loads the reader coil a lot, and that when it is alternately attenuated it ends up being temporarily de-tuned; and that the de-tuning reduces the load on the reader coil significantly enough to outweigh the attenuation caused by "shorting" the tag coil terminals, thus resulting in a temporary net increase in the reader coil signal. But I could be way off, and this semi-educated guess of mine is far from being a solid enough answer for me.
Anyone have any ideas for me? (or did my description of the system just confuse everyone too much? it's late...)
so the general theory is that when the terminals of the tag coil are shorted together while it is near the reader coil, it will attenuate the signal on the reader coil... effectively, the two coils together act as a weak air-core transformer, so shorting the output terminals loads the input down.
However, the reality is this: when the tag is placed in the reader field, but not transmitting anything, the reader coil signal is attenuated somewhat, which makes sense since the tuned tag coil would load it to a degree. however, when the tag is transmitting (alternately "shorting" (well, attenuating, not a real short) and then open-circuiting its coil) instead of seeing corresponding attenuations on the reader coil signal each time the tag coil is shorted, I see the opposite: the signal on the reader coil is INCREASING with each attenuation on the tag coil instead.
My theory is that the tuned tag coil loads the reader coil a lot, and that when it is alternately attenuated it ends up being temporarily de-tuned; and that the de-tuning reduces the load on the reader coil significantly enough to outweigh the attenuation caused by "shorting" the tag coil terminals, thus resulting in a temporary net increase in the reader coil signal. But I could be way off, and this semi-educated guess of mine is far from being a solid enough answer for me.
Anyone have any ideas for me? (or did my description of the system just confuse everyone too much? it's late...)
