The Factory | Designing an RFID Reader

g'day and welcome to this week's episode of the factory this week we're interviewing peter who's been working on an rfid reader how you going peter i'm doing great awesome let's get stuck into it [Music] so yeah we're talking about rfid firstly peter what is rfid i know some people might have heard of rfid in the context of you know maybe they know their cat has rfid in it what sort of problem is this solving well this solves the problem of uh not having to put wires into your cap yes that's a good thing yeah so you can read um tags that you can embed in your cat or um or any device that you might have tags like this an rfid allows you to read these tags get the data off them without actually touching them and that can be really useful if you've got some sort of something in the way between the two devices or you want to keep something waterproof um wherever you can't have an electrical physical electrical connection yeah that's right electric electrical connections can be tricky in the first place so um this solves that problem so that's really cool we've got some applications for rfid how does this work how do we get communication without a physical wire well it all starts with the rfid chip and it generates a high frequency signal which goes into a coil which is embedded in the printed circuit board of this board that coil radiates a magnetic field that can be picked up by a reading card picks up the um the signal and energy harvests that signal so there's enough power in that signal to start the chip and for the chip to read whatever information is coming from the rfid reader and then respond accordingly and send data back to the rfid route so that's really cool we've got this working board how did we get there there's lots of prototypes on the board what were the design goals what were the challenges what was the process for getting there all right so the challenge here is to get as much power as possible from our driver chip and into our tag that we're reading um so uh and bridging that air gap getting that power through enough to actually start the chip and and allow it to this thing actually has to turn on doesn't it that's right it's gotta be able to actually run digital logic so the the driver ic chip uh is uh outputs its power via like an inbuilt h bridge and that's a very digital type of circuit there's gonna be lots of harmonics coming out of that that's right so it's rough it's just on or off and it's just pushing and pulling at this at this coil that we've got here and the first thing that we need to do to in order to suppress the electromagnetic interference is to put a low-pass filter in to stop stray components above that 13.56 megahertz that we're actually interested in yeah cool so we have to design a low-pass filter after we've done our low-pass filter then we need to do impedance matching from the the low impedance uh driver i see to the high impedance or reasonably high impingement relatively high impedance of that antenna and so we have to build a matching circuit using passive capacitors and inductors and also make sure we don't burn out the chair yeah that's right yeah we don't want to make smokes not smoke generators so we want to get as much power transfer as we can but we don't want to just burn the whole thing up yeah cool so peter what's the design process how do we get this low-pass filter there how do we work out the matching circuit okay so the design process is fun what we have to do is uh the first thing that we need to do is to have a look at our at our requirements for pika dev the pika dev has a very specific um dimensions so the first thing to do is just design a coil that matches so we're not even designing a matching circuit first you're choosing your coil no you start with the coil and so you design a coil that you go this coil looks like it might just work you come up with any confidence but it might just work so designed to coil so could you say it was an educated guess we have the dimensions we guessed what went into that educated guess what made us what made us choose a particular dimensions for our coil or a particular number okay so more turns means that you get um potentially well bigger inductance can allow you to to get better coupling to to the device so you're thinking like stronger magnetic fields with more turns yeah more turns equals more more of an inductor more is better right yeah we'll just throw everything out and see if it works so so we started off with um with this one here uh this one has three turns on the top and three turns on the bottom so six turns total and great that's a great starting point do you remember the the trace thickness on that one or that point two millimeters two yeah pretty thin traces yeah exactly then we started doing our measurements and found that the inductance was fine the resistance was just too high yes well basically we made a smashing circuit for it and the bandwidth was massive yeah you know we had this thing operating at 13.56 megahertz and the bandwidth on our matching circuit was like five megahertz or something we could we could barely see any energy being absorbed at all yes um the queue was just way too low and so the the problem with the large bandwidth is that that there's less energy to actually be captured by the the receiving coil yeah lots of energy going into heating up the coil not very much energy actually turning the tag on so didn't get very far with that one um so we did so we did our first iteration and it was great because we learned a lot about about going through the process of making we still made a matching circuit it just was not very just didn't work but we still learned how to measure inductance with an oscilloscope that is basically just putting a resistor in series within the inductance and picking a frequency picking several frequencies and doing a phaser division of the voltage and the current to work out what the impedance was of the whole circuit and pulling the inductance out of that impedance calculation so what's the actual design process for this matching circuit we need some component values how do we get component values well what you do is once you've actually measured your your inductance of your coil then you go to the data sheet and just look at all the formulas just to start plugging values in that we know about put them into the into the formulas calculate capacitor values and inductor values keeping in mind that you want to be within certain parameters you know you want you want your inductor to be more of an inductor than anything else more inductive than resistive is a good start that's right more inductive than capacitive got to stay well below is resonant frequency yeah all of the yeah all of those things yeah really just turn the handles take a measurement turn the handles on the maths get some component values see if they work lock them on the board so first revision had a crack at it didn't really work what did you change why do you think it helped okay the first thing that happened is okay we're going to get this resistance down let's make the coil shorter so we took out 50 of the coil by just putting the coil on one side of the board the other thing that we did is double the the trace width or maybe more than doubled okay just make it bigger just get rid of those copper losses yeah so we did those two things and then we then had to then redo all of the calculations again thanks spreadsheets and that got the first thing we had to do was to measure the inductor the new inductor that we just created using the process that was just described oscilloscope resistor phase changes all of that sort of stuff lots of maths and then we had to do more mass to get these capacitor values plonk them on the board and um well then it what happened it just worked yeah we got lucky it worked get those values right and then suddenly it just brings to life gotta love reading the manual so we've got the current working design revision here peter one final question though why is the coil on the back i'm not seeing any traces on the top side of this pcb well we've designed this so that people might mount this on their projects so that it's just flush with the um outside of the enclosure like this so this is a simulation of the enclosure we've got um enclosure figuradev device our coil is as close to that as possible so you get minimum interference yeah nice just let you really just literally glue it to the inside of the box yeah and hopefully the other side is close enough to the call that it will still read yes that's really cool well thanks for that discussion about the rfid project peter um you'll be seeing this eventually well we've still got some design work to go so in the next couple of months maybe you'll uh be able to grab one of these and play with it yourself yeah that's right thanks a lot cheers [Music] you

Channel: Core Electronics

Views: 7,164

Keywords: How To Design An RFID Reader, RFID reader, RFID Module, Radio Frequency, Matching Circuit, Impedance, Engineering, RF Coil, Low Pass Filter, PiicoDev, I2C, High Frequency, Oscilloscope, How To Design An RFID Device, How To Design An RFID Module, How To Read An RFID Tag

Id: JXwvcSJjhPI

Length: 8min 57sec (537 seconds)

Published: Wed Dec 08 2021