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I'm using a low-frequency RFID IC that handles a bunch of the low-level signal decoding, but I'm hoping to get more range out of it. It has two TX pins (diagram below) that it describes as a "full bridge" to oscillate the antenna at 125 kHz, 10 Vpp (when supplied with 5 V).

Unfortunately, I'm not getting the range I want out of the antenna, and I was thinking that upping the voltage might help get some more power into the whole thing, thus increasing transmit range. My questions are, then:

  1. Can I use these two TX output pins to drive an H-bridge - or another full bridge? (A provided schematic would be really appreciated)

  2. Will increasing the voltage help with the transmission range?

  3. If this approach won't work, how else might I increase the range? A different IC? Etc.

The specs I'm using for the antenna are:

22" diameter, 20 gauge wire. 50-150 wrappings are what I've tested, but I'm fine with going outside that range. DCR is around 6-7 ohms, and the coil is spread about 3-5 inches producing inductance in the 10-50mH range (again, depending on the exact configuration).

IC block diagram

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Helpful
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    How much range are you expecting? The thing about *near* field communications is that they only work over very short distances - your best bet is usually to make the antenna bigger. – pjc50 Mar 08 '21 at 15:53
  • I'm currently iterating through several different antenna parameters - number of wrappings, spacing, etc - but I've got a 22" diameter antenna (just a coil, really) and I'm hoping to read a tag anywhere inside it. Tried with anywhere from like 50-150 wrappings so far, I think the reactance is getting too high. – Helpful Mar 08 '21 at 15:58
  • What are your coil L & turns DCR estimate, details in question please. I’m thinking of suggesting a tapped step up resonant design to get you 10x – Tony Stewart EE75 Mar 08 '21 at 16:32
  • Measured a range, usually varying 10-50mH. EDIT: putting details into the question, sec. @TonyStewartSunnyskyguyEE75 Put that into the question. – Helpful Mar 08 '21 at 16:38
  • What range now? And want? Again into Q. – Tony Stewart EE75 Mar 08 '21 at 18:07
  • Next compute tap into coil to make 640uH+10nF+10 Ohms (incl DCR) series as your primary and leave secondary (autotransformer) open, tuning spread for interwinding capacitance to resonate in air to 10 to 100 pF with 5Vdc differential bridge drive. – Tony Stewart EE75 Mar 08 '21 at 18:31
  • I... don't understand what you are asking. :/ – Helpful Mar 08 '21 at 18:44
  • I asked what range do you get now, and what range do you expect for signal. You have to increase power by stepping up voltage with autotransformer both resonating with cap and secondary., can be an air cap so coils may be compressed. Like 1: 30 turn ratio. – Tony Stewart EE75 Mar 08 '21 at 22:28
  • Gotcha. Right now we've got as far as 3-5 inches from the coil, but I want to get all the way into the center (11 inches) with no reliability issues. I was thinking I might step up the voltage by using an h-bridge on top of the output my chip produces, but I don't know if that would work. – Helpful Mar 08 '21 at 23:18
  • No it might get a bit more, try my method . It should work all the and off axis D/2 or more – Tony Stewart EE75 Mar 08 '21 at 23:34

1 Answers1

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  1. Can I use these two TX output pins to drive an H-bridge - or another full bridge?

Yes. According to AN98080 page 16,

"For long range systems, external power MOSFET-transistor pairs can be connected to TX1 and TX2 to allow for even higher currents."

  1. Will increasing the voltage help with the transmission range?

Yes, but increasing the current can also help. With a higher current driver you could reduce the number of turns to reduce inductance and increase inductor current, producing a stronger field. Page 17 of the app note says:-

"La ≤ 800 μH when operating the HTRC110 in normal mode at I antmax = 200 mA and La ≤ 400 μH for the burst mode at I antmax = 400 mA respectively. If using an external antenna current boost stage, even lower inductances are required."

Provided the 5 V power supply can handle it, increasing current will be easier because you can drive the external MOSFETs directly without needing level translators. Ultimately you are limited by the coil needing at least 1 turn. If you can't get enough field strength with that then you need to raise the voltage.

  1. If this approach won't work, how else might I increase the range? A different IC? Etc.

If this doesn't work it's either because you are doing it wrong (not tuning for resonance, wrong Q, wrong coil dimensions, too many turns) or you are just expecting too much. You can always increase the field strength to get enough power to activate the tag, but if it's not close enough to significantly influence the transmitting amplitude it won't be detected. For longer range you may have to look at a different technique.

Bruce Abbott
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