802.15.4 Antenna solution! - Page 1
Forum section: electronics / projects

author: Mark IV
author_rank: Contributor
timestamp: April 10, 2015, 04:26:57 pm
content: Hello!!I'm designing a board with CC2538 from TI:http://www.ti.com/product/cc2538I don't have lots of experience working with rf boards and I guess it's quite tricky. What is the best solution for antenna and antenna matching?? I'm reading some application notes and other online info but it's really hard to go through some concepts like calculating antenna matching or what is the difference between a ceramic solution for the antenna or an integrated one.Right now, I'm designing guided by the reference design of the evaluation kit, which has some inductors and capacitors in the RF routes and an integrated antenna. But also I see other references like:http://www.openmote.com/hardware/openmote-cc2538-en.htmlwith a balun and a ceramic antenna...Someone can help me with his good RF experience or can point me a good reference to make a good decision?Thanks!

author: Howardlong
author_rank: Super Contributor
timestamp: April 10, 2015, 05:45:08 pm
content: Without any doubt, base your design on TI's reference design if you can. Any deviation from that will have you scratching your head needlessly.

author: Howardlong
author_rank: Super Contributor
timestamp: April 10, 2015, 07:56:39 pm
content: Datasheet is vague, just says 66+j64 ohm is best match, and they supply a suggested lumped balun filter in the DS which is replicated in the reference schematic, and some gerbers. Looks like the reference design is four layer but I couldn't find the stack up, although I didn't look very hard.When the values are down at 1pF and 2nH, as in this case, layout and stack up becomes crucial. I did some calculations of my own and the values given seem reasonable assuming no parasitics.

author: Mark IV
author_rank: Contributor
timestamp: April 12, 2015, 10:29:20 am
content: Thanks for all the replies!!Yes, Johanson has a specific balun for the TI chip:http://www.johansontechnology.com/datasheets/baluns-matched/2450BM15A0002.pdfFor manufacturing, I guess a balun is a better option.I have a doubt about the passive components in the matching antenna circuit...In my PCB, I can design a route width for 50 ohms in the matching so...what is the need of other components? Something about the length of the route that I am missing?By the way, in the TI's reference design, the layer stackup is specified. And, yes, 4 layer: signal& rf layer at top + ground at mid + power at mid + ground at bottomThat RF...It is a headache!

author: freker
author_rank: Newbie
timestamp: April 12, 2015, 06:54:40 pm
content: Hi Mark,The matching components placed by the antenna are for matching the antenna impedance to your 50 ohm transmission line. The antenna will be detuned by factors such as the size and shape of your ground plane, objects in the near field such as batteries, housing, etc. and you will then have to use the matching components to tune the antenna impedance to 50 ohm. This work will require a network analyzer.For a 2-layer reference you can follow the CC2533EM design. The radio is quite similar in 33 and 38.You can also get support on the TI E2E forum.Cheers,Fredrik

author: wemme
author_rank: Regular Contributor
timestamp: April 13, 2015, 01:34:06 am
content: Normally the Johanson Tech Baluns have an 50ohm output impedance but as everyone has already stated Stack up and layout are critical! especially if you don't know what you are doing or if you don't have the required equipment.Normally it is standard to run a 50 ohm (or what ever you output impedance is) coplanar transmission line to a PI matching network and then to your antenna.the PI network allows you to match your line impedance to the antenna impedance.Depending on your antenna type and topology (pcb trace antennas in particular) it may also need to be tuned for the frequency you are operating this should be done before matching to your transmission line.All the matching and tuning should be done in the finial case/environment as this can impact on the capacitive loading on the antenna.An often overlooked issue is that the case can be a critical component affecting RF performance.you really need to have a good understanding of Rf layout and a good calculators, Saturn pcb toolkit is good for working out stack ups and impedances.You ideally need a Vector network analyser to measure the impedance and then calculate the matching circuit and test again with the VNA.Antenna tuning can usually be done on a decent Spectrum analyser with a Tracking Generator and a VSWR bridge. the Maximum return loss is usually the resonance point of the antenna by trimming the antenna you shorten the wave length and up the frequency of resonance.This is all a very simplified overview in reality a lot of time, effort and money can be involved to get a design perfect.The easiest (usually cheapest) option is just stick with reference designs and stack up and save yourself the grief.RegardsBart

author: Howardlong
author_rank: Super Contributor
timestamp: April 13, 2015, 09:14:05 am
content: To add a couple more comments...A two layer stackup is much easier to reproduce unless they're using exotic substrates, but make sure you use the correct substrate thickness, they're often thinner than 1.6mm! If it says 1mm, you must use 1mm! Double sided also usually means that the transmission lines are wider due to the increased dielectric distance to the ground plane.The problem of using thinner boards is that the physical strength of the board reduces, so care needs to be taken to avoid flexing of the board or your MLCC caps will crack over time, and during assembly the board may flex risking associated connection problems. I did a CC3200 prototype board a few weeks ago on 0.4mm board, it worked well except that for a production design it would be difficult to make rigid enough.I have started to use Murata MM8030-2610RJ3 RF switch/jacks as a matter of course on prototype RF boards, these are miniature RF sockets rated to 11GHz with passthrough when nothing's plugged in to them. This means you can easily probe in circuit with your VNA. They're reasonably cheap too, $0.20 in qty 1ku. For production, you can replace with a colocated 0402 0R or MLCC cap. I will admit that the probe adapters aren't very cheap though!

author: wemme
author_rank: Regular Contributor
timestamp: April 13, 2015, 09:42:26 pm
content: Quote from: Howardlong on April 13, 2015, 09:14:05 amI have to agree about the mechanical issues regarding the thickness of some Rf pcbs. I had a 0.8mm board that was forever causing issues in production due to connection failures due to flexing particularly when the board was broken out of the panel.We have also been looking at the ULF RF switch jacks but always ended up going without due to the high volume aspect of the product vs unit cost.I must say those jacks are a good idea if you want to add an external antenna option you can just plug it in. with the switch isolating the on board antenna.for prototyping you put it the other way around so you can switch out the RF port on the balun and terminate to the on board antenna for tuning and matching.RegardsBart

author: Howardlong
author_rank: Super Contributor
timestamp: April 13, 2015, 10:09:12 pm
content: I guess if the cost of placing a 0.1c MLCC cap is too much, what you could do is have the board passthrough the RF by default, and then simply break the track and place the RF switch/jack: in fact if there's enough CPW length left on the board you could do this.

author: wemme
author_rank: Regular Contributor
timestamp: April 13, 2015, 10:19:27 pm
content: Quote from: Howardlong on April 13, 2015, 10:09:12 pmYes we do butcher up a few boards in testing and complianceWe usually remove the Balun and terminate a IPEX connector to the feed pad and surrounding ground.RegardsBart

author: Howardlong
author_rank: Super Contributor
timestamp: April 13, 2015, 10:58:10 pm
content: The cost of u.fl has always made their use fairly prohibitive IMHO in a production unit. They are also enormous, in comparative terms of course, the w.fl is better in that respect. The w.fl is on a par size-wise with the MM8030 I mentioned, but the MM8030 is less than 1/3 the price of u.fl or w.fl.