RFID signal measurement with oscilloscope
I'm working on a RFID project and need some help about measuring with an oscilloscope RFID signals emitted from tags and reader. I use a reader compliant with ISO 18000-3 mode2 standard, thus operating at 13.56 MHz. I tried to put the oscilloscope probe near to the reader antenna and I displayed the carrier waveform at 13.56 Mhz , but I didn't manage to pick up tag answers waveform which, I think, overlaps reader signal.
Can anyone help me with this problem? Thanks in advance.
Could be up to 60dB difference between power of the reader and power emitted by the tag, so would be hard to use a scope and a probe for this application.
The RF in RFID: Link budgets
Mmm.. I think you are right! How can I pick up such signals then?
Use spectrum analyser for this purpose , or you should have a digital oscilloscope of greater resolution.
Near the tag, the load modulation is sufficiently high to detect it with an oscilloscope, may be up to 10% of carrier amplitude or even more. The subcarrier allows to identify the tag-to-reader communication in frequency domain, but I think, it's easier to decode from time domain waveforms.
I agree with FvM: the load modulated waveform is big enough to detector. If you want to know the bits, a Diode detecoter before the prober may work well. If you have a digital oscilloscope with 1Ms memory, you can also capture the waveform and demodulate it on a PC.
So you are suggesting me to use a diode detector before the oscilloscope probe. It could be a good idea, but I read from some papers related to the tag that I'm using that load modulation perfomed by the tags is a BPSK modulation of a subcarrier: is a diode detector suitable also for such a modulation?
I think, that the somewhat (excuse me) confuse 18000-3 mode 2 tag to interrogator modulation scheme isn't well suited for simple enevlope waveform demodulation with a diode detector. But it's not impossible with a steep low-pass, that can be passed by the subcarrier. Synchronous IQ demodulation, as performed by reader chips, in other words, digital signal processing seems more appropriate. It can be done from a recorded DSO waveform of sufficient length, as mentioned by kayaoo.
Unfortunately I have only a 1Ks memory DSO, but I'll try to record the waveform with some other tools, if I can.
You didn't tell about the exact purpose of your measurement, e.g. just checking some signal parameters, or decoding the data. In the latter case, you possibly want to design a receiver, e.g. based on a FPGA and digital signal processing.
I should display and measure both reader-to-tag and tag-to-reader signals so that I might afterwards make a comparison between them and a modified version of them (obtained adding some known noisy waveform). My purpose is to evaluate characteristics of rfid communication in presence of interferences and ability of tags to receive reader commands, replying even when the communication is made difficult
Sounds like a specification for a full featured contactless monitor. If you don't want to buy a commercial product, you have to design your own digital receiver. Good luck!