I've got an ID-3LA chip wired up to an arduino and an antenna I wound myself with 30 AWG magnet wire. It works, but I've read that I should be able to tune the antenna by connecting it to an oscilloscope, and then gradually adding or removing turns to improve the signal. However it's been decades since I've used an oscilloscope and I can't find a good description of where to connect to. I've managed to connect the probe across the coil and see the 125khz sine wave, but adding and removing turns doesn't cause any changes, so I think I'm just reading what's coming out of the ID-3LA. Does anyone have experience doing this that can walk me through getting a usable signal to be able to tune this? Thanks!

  • \$\begingroup\$ Try moving the tag further away and repeat your test. \$\endgroup\$
    – Andy aka
    Oct 26, 2015 at 22:46
  • \$\begingroup\$ Could you sketch how you have connected the antenna, chip, scope and Arduino? Is the goal to transmit more power from the chip into the antenna? For that you need a resonant circuit - a capacitor in parallel with the turns of the antenna. \$\endgroup\$
    – Floris
    Oct 27, 2015 at 0:04
  • 1
    \$\begingroup\$ Have you read the "Fine Tuning" section on p. 10 of the ID-3LA datasheet ? \$\endgroup\$
    – davidcary
    Oct 27, 2015 at 2:17
  • \$\begingroup\$ @davidcary Ahh, geeze. I skipped over section 15 thinking it was still talking about external capacitors. Looking closer, that doesn't seem to be the case. If the last sentence in section 12 had included something like (see section 15), that might have saved me some time. I'll give that a try next time I can get on the scope. Thanks! \$\endgroup\$
    – Jon Garvin
    Oct 27, 2015 at 15:27

1 Answer 1


Your suspicion is true! That 125Khz signal one may observe is the frequency from the chip. For obtaining maximum range between the RFID reader/interrogator and transponder chip, there has to be impedance match and yes, frequency match too.

One can simply use the LC tank circuit formula and and try to match it with the required frequency(If there are no internal protective circuitry and capacitors in the chip, this frequency should remain same as 125KHz.) Also in some cases the chip manufacturer would recommend ideal inductor and ideal capacitor values.

f = 1 / 2pi*sqrt(LC)

With this formula obtain the right L value for 125KHz or whichever. One can measure inductance with a simple LCR meter (available option in many advanced multimeters)

More than a decade ago, I built LF&HF RFID readers with this Swiss company parts EM microelectronics. For the inductor situation (125Khz-137Khz) it was EM4095 chip. We implemented a flat spiral coil on PCB, using calculations and verified it with a simple inductance meter.

If one is also designing tags/transponders, match up the LC values to 125Khz or whichever is required.

Inductance meter is accurate and easy compared to setting up a new circuit for measuring frequency in oscilloscope and and re-calculating it for inductance.

Also if C value is not given, one may want to check the internal parallel capacitance of that chip's coil pins too. Measure this C with an LCR meter, before soldering/wiring.


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