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...)

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EEgeek.net

 

Evandude, are you familiar with grid (or gate) dip oscillators? If not, try reading up on them. This article, in particular, may be helpful.

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Ron

 

Thank you Ron, that was a very informative document. Seems to reinforce my feeling that the resonant circuit formed by the tag coil and tuning capacitor is appearing as a pretty heavy load to the driven reader coil; makes sense if I think about the tag coil and inductor as a series resonant circuit, behaving more like a short.

Do you think that the other part of my speculation is correct? (that "shorting", or more accurately, loading the tag coil resonant circuit externally is at the same time producing modulation on the reader coil, but also reducing the static load on the coil by de-tuning the tank circuit of the tag coil, resulting in a net increase?)

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EEgeek.net

 

Well, I seem to have answered my own question. Went and tried the system today with the tag coil tuning capacitor removed, and it behaved like I expected; no static attenuation, only the attenuations corresponding to the data transmission.

Glad to have that sorted out :lol:

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EEgeek.net