13.56MHz PCB Coil Antenna Impedance question - Page 1
Forum section: electronics / projects

author: steaky1212
author_rank: Regular Contributor
timestamp: January 15, 2014, 02:21:55 pm
content: Hi,I've got a project where I'm using a TI TRF7970A to read an RFID tag. I am using an RF multiplex chip to switch between multiple small (20mm diameter) circular antennae (about 5 turns).Theoretically the coil inductance should be around 1.3uH based on number of turns, diameter etc.I've checked the DC resistance of the switch using a Fluke 175 DMM and got results around the 5R mark.I've measured the complex impedance using a VNA and I am getting results of around 26+157j, which is an inductance of around 1.8uH.I need to know the complex impedance so that I can tune the circuit, but I dont feel that I can trust the measurements.I thought that the real component of impedance was frequency independent but when I look at the real component (between 10MHz and 30MHz), at my frequency it is 26R, but it peaks to 1450R at 22MHz.Does anyone have an ideas on why I've got a frequency dependent real component, and if I cant trust this measurement where do I go from here?

author: KJDS
author_rank: Super Contributor
timestamp: January 15, 2014, 02:24:03 pm
content: Can you post up a picture of the smith chart with the result and also of the circuit?

author: steaky1212
author_rank: Regular Contributor
timestamp: January 15, 2014, 03:42:24 pm
content: Hi,I've attached the smith chart as requested.Theres nothing to the circuit. I go from 50R SMA connector, through an RF multiplexer IC and out to one side of a coil. The other side of the coil is GND. I've got no other components populated yet as I first need to get the characteristics of the coil first.

author: KJDS
author_rank: Super Contributor
timestamp: January 15, 2014, 04:03:55 pm
content: the real part of the impedance is not constant.Where is your VNA reference plane and how far from the coil is it?

author: steaky1212
author_rank: Regular Contributor
timestamp: January 15, 2014, 04:10:59 pm
content: Is the real part of the impedance supposed to be constant? As I was under the impression that it would be... as the real part is Resistance.My VNA reference plane is just the RF ground pour of the circuit - in this case a copper pour with approx 9mm clearance around the coil. I don't have it sitting on a grounded copper sheet or anything like that.I've not got any tuning components yet, I am just trying to measure the impedance of the coil so that I can select the tuning components.

author: KJDS
author_rank: Super Contributor
timestamp: January 15, 2014, 05:06:14 pm
content: I've got to ask is this a work project? If so, you've either got a lot of understanding to do about RF circuits and VNAs or it would be much cheaper to hire a consultant for a day.Have you calibrated the VNA? if so how, OSL at the end of the test lead where it meets your SMA connector? If so that sets the VNA reference plane.Now add in a length of cable, if it's a quarter wave long at 22MHz then it will transform the near open circuit to the short circuit point.http://en.wikipedia.org/wiki/Quarter-wave_impedance_transformerHere's part one of what is probably the most straightforward article on matching using the smith chart.http://cp.literature.agilent.com/litweb/pdf/5989-9015EN.pdf

author: calexanian
author_rank: Super Contributor
timestamp: January 15, 2014, 06:20:20 pm
content: Find a ham with an antenna analyzer you can borrow. They are more common that you probably think.

author: JackOfVA
author_rank: Unknown
timestamp: January 15, 2014, 11:03:47 pm
content: Since you have a narrow band operating range, is there a reason you can't just match the inductor/antenna at 13.56 MHz? Tune out the reactance and match the resistive component to whatever your circuit requires.The resistance component consists of some radiation resistance - probably close to negligible given the physical size of the coil compared with a wavelength - and loss resistance from the copper, dielectric loss in the PCB substrate and who knows what else. And these are not constant with frequency.In addition to the inductance of the coil, it has distributed capacitance so that you have an RLC parallel circuit. At some frequency this will become self-resonant and I suspect that's what you see at 22 MHz. You might look at your elementary network analysis texts to see what happens to a parallel RLC network impedance as it is swept from below to above the parallel resonant frequency. When you look at the VNA results, you should see the sign of the X component flip from inductive (+) to capacitive (-) at the SRF. Exactly at the SRF, of course, there will be no reactive component to the impedance.And as has been said, it's important to perform the VNA calibration at the measurement plane which presumably will be somewhere on the PCB near the coil.

author: G0HZU
author_rank: Super Contributor
timestamp: January 15, 2014, 11:44:48 pm
content: Can I ask: why are you connecting the hot end of the coil direct to a multiplexer IC? The multiplexer will presumably have a high enough ESR to dominate/ruin the real part of the coil impedance? It will also introduce some stray reactance.If you run it like this there will be high RF voltage across the coil when transmitting and the multiplexer may not be rated for high RF voltage. What kind of multiplexer are you using?I think you may need to come up with a better strategy for the aerial multiplexing (assuming I've understood what you are trying to do at present). I would expect you to resonate and match each coil down to a low impedance and THEN worry about the multiplexing of each aerial.At a guess, a 1.5uH coil like yours (measured on its own) will have a reflection coefficient in the ballpark of about 0.997 / 37deg at 13.6MHz because the real part of the coil impedance will be a fraction of 1 ohm. This is right by the edge of the smith chart and will require a fair bit of operator skill to measure on a VNA without introducing lots of uncertainty and noise on the measured real part of the impedance. You will see this when you then display the real part of the impedance from the VNA raw data as it will look very noisy unless you use the advanced features in the VNA to reduce the noise.Also, your coil dimensions and inductance don't seem right. A 20mm diameter aerial seems really small for a reader. Did you mean to type 20mm radius? This would also get you closer to about 1.5uH with about 5 turns.If you design/multiplex it the way I suggested above you wouldn't really need a VNA to measure anything as the whole process becomes a fairly basic impedance matching exercise.

author: skipjackrc4
author_rank: Regular Contributor
timestamp: January 16, 2014, 02:29:46 am
content: Quote from: G0HZU on January 15, 2014, 11:44:48 pmI've successfully tested a 20mm diameter antenna with the TRF7970 when trying to make the antenna as small as possible. Performance was surprisingly adequate.

author: steaky1212
author_rank: Regular Contributor
timestamp: January 16, 2014, 03:51:08 pm
content: Quote from: KJDS on January 15, 2014, 05:06:14 pmI used to use an E5071C pretty much daily as my last job, but that was for characterizing Ethernet cable so run through a balun and calibrated to 100R. This is my first attempt at doing actual 50R measurements. Also, I'm using a miniVNAPro USB unit for testing.Sorry when you said reference plane I thought you means ground reference, but the calibration plane. I've performed an OSL at the end of my feeder cable (32mm RG174), and then taken measurements from the TRF7970A side. I know its best to measure at the coil and tune there, but I wanted to reduce the number of components as I am switching across a number of coils.I've repeated the test, but I've cut the traces to the switch and bypassed it with wire. I still get a peak in the real component of impedance. My trace length from where I measure to the coil is around 235mm.The coil has an inner diameter of 16mm, outer diameter of 21mm, and 6 turns. Giving an approximate inductance of 1.3uH.

author: JackOfVA
author_rank: Unknown
timestamp: January 16, 2014, 05:20:43 pm
content: Quote from: steaky1212 on January 16, 2014, 03:51:08 pmLet's try this again. All inductors have associated parasitic or distributed capacitance and also some degree of resistance. In the real world, there is no such thing as a "pure" inductance with 0 distributed capacitance and 0 ohms resistance.Hence, your coil/antenna is really a parallel RLC circuit.A parallel RLC circuit impedance as measured will have an R and jX component that varies with frequency - a 1 mH choke that I measured is in the figure below.If you derive or locate in an elementary network analysis book the equation for impedance of an RLC circuit you will find exactly what you measured and what I measured. So there should be no mystery associated with your measurements. You have the data to derive a model of your RLC network and de-embed the pure L, R and C. Many modern VNAs will do that for you by pressing the correct button.Assume L=1.3uH and SRF (self-resonant frequency) is 22 MHz. and you can calculate the distributed capacitance. It is 29pF, which is not an unreasonable level of distributed C for a spiral inductor on PCB, particularly if the ground plane below the coil has not been removed, plus other stray C.