2.4GHz PCB antenna. What now? - Page 1
Forum section: electronics / rf-microwave

author: mayor
author_rank: Regular Contributor
timestamp: April 01, 2019, 03:26:09 pm
content: Hi,I've designed a PCB antenna following stackup guidelines and dimensions for use with a 2.4GHz BLE chip. Target impedance is 50 ohm. I have used the xaVNA to tune the antenna by:0. Calibrating VNA (S/O/L) at the port1. Cutting off the end of a 50 ohm SMA cable2. Soldering the shield to gnd near the antenna feed point3. Adjusting extension for Open to 345 PS (looked good, grouped at the far end)4. Soldering connector of cable to antenna feedline5. Measure impedance6. Adding passives to match antenna7. Measuring results by soldering cable before the filterSee attached images for reference. VNA plot for antenna after match and board layout. Match looks good to me!Now, the noise floor is good at -120 dBm RSSI. My problem is that RSSI is pretty bad when communicating with a known good board (like -90 dBm at some 25 feet, through walls, but still too low).The question : what now? What tools or steps do I need in order to understand what is going on and fix the problem? As a home gamer, I don't have very much hardware (ie, the VNA and a scope's where I'm at!).Thanks for any and all advice!

author: tautech
author_rank: Super Contributor
timestamp: April 01, 2019, 10:02:04 pm
content: This:QuoteYou need do S/O/L at the end of the cable of to exclude the cable from the measurement.

author: mayor
author_rank: Regular Contributor
timestamp: April 02, 2019, 12:08:40 am
content: Hi, I'll give that a try, but TI doc says otherwise. Cal at port, then apply port extension.

author: E Kafeman
author_rank: Regular Contributor
timestamp: April 02, 2019, 08:27:25 am
content: What TI says is correct if cable is an ideal transmission line.It depend on how ideal your cable is, if you need to compensate for its losses.Short while cable is soldered at PCB with coaxial center wire soldered to actual pad. I use an exacto or a solder blob for shortcut. Use same pad for 50 Ohm resistor and open.If PCB ground length is short, use ferrite tubes at cable to avoid that it acts as an extended ground plane, affecting antenna impedance. Use several tubes as most types of ferrite not are good absorbers at 2.4 GHz.Target impedance is seldom 50 Ohm. It is an complex load that can be much but it have never ever happened to me to be 50+j0 Ohm for an active radio circuit,What shows your measurement at Rx and Tx? Measure at same point at PCB as you measured antenna impedance. No antenna connected during radio measurement.Use a serial cap if there is DC-bias at chip RF out and check that VNA can handle Tx power level. without additional external attenuators.

author: Psi
author_rank: Super Contributor
timestamp: April 02, 2019, 08:37:09 am
content: Quote from: mayor on April 01, 2019, 03:26:09 pmDo all testing 100% line of sight.As soon as you add a wall to the test you will get somewhat random results.Sometimes it will work better/worse for no apparent reason, and this will make measuring the effectiveness of other changes very difficult.

author: mayor
author_rank: Regular Contributor
timestamp: April 02, 2019, 08:53:28 am
content: Quote from: E Kafeman on April 02, 2019, 08:27:25 amCool, I'll do as Tautech suggests.Quote from: E Kafeman on April 02, 2019, 08:27:25 amGround is short. I looked on Digikey for ferrites. The highest frequency ones were 900 MHz. Thoughts?Quote from: E Kafeman on April 02, 2019, 08:27:25 amNot 100% sure how to do that. So I leave the SMA cable in place and, say, transmit from the chip. It's not clear to me the VNA should be set up? Usually, IT is the generator, so any clarification is appreciated!

author: mayor
author_rank: Regular Contributor
timestamp: April 02, 2019, 08:54:51 am
content: Quote from: Psi on April 02, 2019, 08:37:09 amMakes total sense.

author: mayor
author_rank: Regular Contributor
timestamp: April 02, 2019, 08:57:17 am
content: Following up with a question. Can a spectrum analyzer help in a situation such as this? Would a mismatch or otherwise ill-designed board produce a frequency offset or attenuation which could be measured and remedied? I know microwave transmissions aren't exactly home gamer stuff. Still my first revision of this board produced way better results, so I want to understand why.

author: tautech
author_rank: Super Contributor
timestamp: April 02, 2019, 09:19:37 am
content: Quote from: mayor on April 02, 2019, 08:57:17 amTo some degree in that with a return loss bridge they can show what frequency your antenna has the best emissivity but a VNA sweep can do that too.Not that it'll be much help other than maybe some better understanding, have a look an my antenna build project log:https://www.eevblog.com/forum/rf-microwave/antenna-project-log/

author: mayor
author_rank: Regular Contributor
timestamp: April 02, 2019, 12:32:34 pm
content: Quote from: tautech on April 02, 2019, 09:19:37 amGood stuff, but this excerpt from your thread:"Could I have done it differently, maybe but not without a SVA1015X."Hmmm, well, I gotta figure out what's going on without the SVA1015X :-)The chip I'm using has internal tuning for the Xtal through a cap array. Since the "known good board" is different from my custom board (I will have to populate another to test between two of the same), I am thinking there might be some frequency offset. I'll try adjusting values blindly and check effect on RSSI.

author: Gribo
author_rank: Frequent Contributor
timestamp: April 02, 2019, 02:27:14 pm
content: Get your known good device to output CW. Measure frequency and amplitude. Do the same with the unknown device. Make sure the frequencies are within the transceiver's spec. Do a site survey, you might have interference issues. If you have an RF test chamber, use it.

author: tautech
author_rank: Super Contributor
timestamp: April 02, 2019, 06:08:42 pm
content: Quote from: mayor on April 02, 2019, 12:32:34 pmNah, it was a bit of a sales plug but then you know that.The point of the thread was just to share my (very) little experience when building and characterizing an antenna. I tried a short leaded BNC adapter for the feed point and it wasn't til the obvious hit me that the installation should be tested with the coax feedline attached, then any S O L can be done directly at the VNA.Yeah antennas can be damn fiddly to get right even when you can trim them and adjust the feed point but in this case what I learnt most was to get the best Log Mag possible (3rd screenshot) and then tweak for a good SWR and Smith chart values.

author: Ice-Tea
author_rank: Super Contributor
timestamp: April 02, 2019, 06:42:21 pm
content: Your dip seems to be roughly at 2.4GHz. It should be a bit further up in frequency. Bu 40-50MHz or so.

author: mayor
author_rank: Regular Contributor
timestamp: April 02, 2019, 07:20:19 pm
content: Quote from: Gribo on April 02, 2019, 02:27:14 pmHome gamer here. I assume you're talking about at least having a spectrum analyzer, huh?

author: mayor
author_rank: Regular Contributor
timestamp: April 02, 2019, 07:23:00 pm
content: Quote from: Ice-Tea on April 02, 2019, 06:42:21 pmThat's correct. I will re-tune the antenna. It does seem to behave better when the IC transmits at 2402 MHz. I'll offset it and try again. Dunno if it can explain the RSSI delta with the known-good device though. But it's a concrete step I need to take. I'll report back.

author: E Kafeman
author_rank: Regular Contributor
timestamp: April 02, 2019, 09:34:24 pm
content: QuoteYou measure TX/RX impedance exactly same way as you measure antenna impedance.If Tx power is more then what VNA can handle must you add an attenuation at measurement cable and calibrate SOL including this attenuation.Rx impedance should not be any problem to measure as long as its input not is saturated by VNA signal. -10 dBm setting at VNA does in most cases works fine.Have attached an example of measurement, it is a TI BT-chip during RX. Circuit is matched according to an application note. Declared as a 50 Ohm impedance.Measured Z ranges from 35 to 45 Ohm but reactance part will be conjugate matched for by me, so it will remain a resistive load ranging from 12-17 Ohm. Not exactly 50 Ohm.TX impedance is a bit different but no problem to measure.The tricky part, depending on type of VNA, is that TX signal will be mixed with S11 reflection causing a bit unstable reading. Using S11 software averaging function or window function can hide that problem.TX and RX complex impedance must always be measured when doing serious impedance matching. It is this that result in what is called conjugate matching.It have so far never happened for me that a system have a measured impedance 50+j0 Ohm, and never are TX and RX impedances same. Impedance is always depending on frequency for Rx,Tx and antennaso a wide band conjugate matching is needed for anything that use more then a fixed frequency.

author: mayor
author_rank: Regular Contributor
timestamp: April 02, 2019, 11:03:27 pm
content: Quote from: E Kafeman on April 02, 2019, 09:34:24 pmOK, so I leave the cable attached to the board, just after the balun, before matching network. I assume I disconnect the matching network and antenna in this case, huh?And then transmit and measure? Not sure how I can tell what the VNA can handle, to be honest.

author: E Kafeman
author_rank: Regular Contributor
timestamp: April 03, 2019, 06:43:37 am
content: Yes, you can not measure both directions, antenna and radio at the same time.Simplest is center-wire moved 1-2 mm between two pads of a serial component, or, is cu trace temporary split with aid of an exacto for simple measurement in both directions.Your VNA can handle max -10 dBm when measuring S11 according tohttps://xaxaxa-dev.com/xavna.cppspDynamic range is 50 dB.A typical 2.4 GHz something, can emitt up to +10dBm. so then is a20 dB attenuatorneeded, or better if you not have any collection of these attenuators in your toolbox, buy three 10 dB attenuators for more flexibility. You will still have enough measurement range to measure TX impedance.It is even possible to build your own attenuators as introduced stray losses any way is compensated for when doing SOL calibration, but these noname attenuators are cheap so it is not much worth doing it your self.A quality SMA attenuator with certified db tolerance over whole freq-range will cost x10 linked prices or more but that is of limited value when calibration is done after attenuator is inserted.An alternative to attenuators, these 2.4GHz transmitters do often have a programmable output Tx level. Set output level temporary ta a safe low level when doing measurement. Reducing Tx level do often affect actual Tx impedance but it is mostly a minor effect.

author: OwO
author_rank: Super Contributor
timestamp: April 03, 2019, 06:49:31 am
content: Measuring the transmitter impedance won't usually give you the right numbers because power amplifiers are rarely conjugate matched but a load impedance is chosen for maximum efficiency or output power. The impedance looking into the amplifier's output port won't be 50 ohms.Also -10dBm is the maximum power the VNA can generate, not the maximum tolerated incident power on the ports. I recall seeing a 20dBm spec for this one somewhere.

author: mayor
author_rank: Regular Contributor
timestamp: April 03, 2019, 10:03:41 am
content: Wondering whether any of the measurements from the generator are truly necessary? I am using this balun:https://www.johansontechnology.com/datasheets/0900PC15A0036/0900PC15A0036.pdfIs the 50 ohm claim also misleading?

author: E Kafeman
author_rank: Regular Contributor
timestamp: April 03, 2019, 11:35:05 am
content: >Is the 50 ohm claim also misleading?A balun can in some cases hide impedance behind by adding transfer losses.If they claim 50 Ohm, is it probaly in that range with tolerances and in some amount do it probably mirrors impedance at other side of balun.Simplest to judge how it behave in your circuit is if full S-parameters exist for that component. It does probable exist parameters, if it is available for download is another thing.S11 in both direction can be measured before and after balun. A cut in PCB near balun is all that is needed,>Measuring the transmitter impedance won't usually give you the right numbersWay more worse then that with some bad luck. And maybe a bit above level for this tread. Can however write several pages about this and all variables and possible mistakes I have done.If you know what you is measuring and why is it however a minor problem. Final tuned result will most likely be a functional improvement for both efficiency and max output level with least amount of harmonics compared to unmeasured not conjugate matched circuit.It is true that the load impedance that results in max output power not is same impedance as impedance resulting in best DC power efficiency. A Tx circuit have a number of optimal impedance curves from different aspects such as best gain, best stability DC efficiency relative output power, dynamic range or less harmonic but they are all close to each other at Smith chart.Conjugate matching of TX is of course still correct to do but depending on type of goal is conjugate matching done with an offset in Smith chart.Neither of these optimal impedances are given by most chip manufacturers even if they know them. You have to measure it your self as each PCB design is unique.It is rather easy to measure TX DC current and plot it against actual TX load and TX power to find optimal load for different situations.Main reason that a TX chip can be heavily mismatched at 2.4 -5 GHz is that even if an application note is followed very close are many minor changes done in PCB pattern, different type of PCB material, different type of ground layers, layer flooding, other brands of inductors, heat shield at component pads, are some factors that can cause very big impedance difference between two else relative similar PCB and for same type of radio chip and component values.Possible reason that a problematic radio-chip remains poor performing after antenna matching is done (50 Ohm?), is that nobody measured radio impedance. Measure and matching antenna for assumed 50 Ohm have then no value at all. Useless spending of time.Another TX impedance to consider is impedance at harmonic frequencies. Here is goal the opposite, to avoid power transferring to antenna and should be included in total goal impedance as a S-parameter, especially if FCC certification is actual can a few dB improvement be of high value. That optimal suppressing curve must be combined with the other curves from TX/RX and antenna impedance curve.Weighting all factors for an optimal conjugate match, can be a very interesting job.A much more time consuming project then Tx optimal conjugate impedance is how to find optimal Rx impedance.Impedance match for optimal delivered power from antenna is not same as impedance resulting in optimal S/N, optimal stability or optimal input dynamic range.In best case can signal quality, RSSI and BERR be read from chip as values to judge if an parameter is improved while adjusting antenna shape or matching network.As a starting point, measuring Tx and Rx impedance and perform conjugate match based on this is more worth then not measure at all or assume 50 Ohm.There are some exceptions where a measurement can be very misleading, such as directly at output from an class E transmitter or some complementary outputs without a real balun converting signal to single ended.Too much VNA power can saturate both Tx and Rx circuits but it is often very visible.Matching Rx for optimal power transfer from antenna will not result in optimal S/N but it is way better for best S/N to conjugate match Rx circuit then not measure and doing no matching at all.These different Rx and Tx curves can not all be fulfilled, a weighting must be done, which parameter or if optimal Rx or Tx is the most important function.More about this kind of measurements can be read by searching for sites writing about TIS (Total Isotropic Sensitivity) and TRP (Total Radiated Power).Manufacture impedance s-parameters are in rare cases available but in most cases are NDA then needed and it must anyway be measured for each specific PCB.A much bigger problem then select impedance for best TX or RX is the antenna impedance.For many of these handheld GHz-units can that impedance vary very much depending on if handheld or placed in free space for embedded antennas.Situation is often just as problematic as for a radio chip if buying a produced antenna.Data sheet may state 50 Ohm and a single efficiency number for a wide band antenna.In best case is VSWR given with a well described measurement setup, size of PCB, how antenna was connected , if ideal ground and so on.With some experience can such curves say something of value or at least tell if antenna seems very deaf for all frequencies or performs well.These values can be very different when same antenna is implemented at own PCB with different PCB size, populated and with far from ideal ground plane, battery and ferrite inductors placed in a from antenna view bad distances from antenna main connection position and less free nearfield horizon.Not measuring TX and RX impedance and more then half the job is not done. If not knowing else is simple conjugate matching best choice.A exception is if a handheld unit has an optional external antenna connector for an unknown antenna. Then must 50+j0 Ohm be assumed even if measurement not is possible.In lack of other input do I always start measure TX and RX impedances, weighting them, and conjugate to find what impedance the antenna+matching network I intend to design need to meet.Typical units where customer specific want to improve battery life time is for Bluetooth headset.Small batterys and TXx efficency is important. Max output is less intresant as needed coverage range is short.Efficiency can be increased with as much as 30% compared to a 50 Ohm match.Peak power is however reduced with 1-3 dB.Max input level of VNA, if it not is known is it better safe then sorry. -10dBm may seems a bit low but starting with some attenuation will not harm.Attenuators do I often use as I measure cellphones as a part of antenna design jobs as well as tuning for best TIS TRP HAC SAR and antenna isolation.Tx power can be above 30 dBm and my VNAs can as best handle +20 dBm according to data sheets.It is probably not an absolute limit but a VNA is complicated and expensive instrument to repair so I prefer to have good margins.Assume that we have measured RX impedance, which conjugate matched will result in max power transfer from antenna or best S/N? How long away can optimal curve be that result in best SNR compared to measured impedance, how wrong will the matching become as seen in a Smith chart?See attached Smith chart which shows these both curves. As can be seen is it less impedance difference then impedance variations due to typical handhelds/free space antenna impedance variation.Add that you can never expect to match or tune components and antennas more then within some tolerances.If tuned for max power transfer and that goal is reached within +/-10% of total impedance is tuning job well performed also from best S/N view. Not optimal but acceptable good.Tuning for max peak TX power or for best TX efficiency is in average a bigger difference between such curves based on what I have seen so far, but still rather ok if conjugate matched according to measured impedance.Biggest difference I usually see is difference between TX and RX impedance and antenna impedance variations depending on near-field variations. It is between these curves biggest weighting must be done.Is TX more important then RX, unit will maybe seldom be handheld, a certain situation is more worth to optimize for.Matching network have its own limitations and need for weighting. Too many components cost more in losses then what the component adds in improvement. A high Q network is more critical and need components with tighter tolerances.Prefer to work with antenna design and matching network in parallel.If antenna shape is adjustable is that often best way to allow matching network to add biggest performance improvement.