Problem with radio frequency custom board. Balun mismatch?

RE: Problem with radio frequency custom board. Balun mismatch?

Andreas — Tue, 31 Jan 2017 07:33:07 GMT

Glad to hear you figured it out. Remember though that tuning the current will only improve the range so much. To achieve the maximum possible range a PCB redesign has to be done.

RE: Problem with radio frequency custom board. Balun mismatch?

angel — Mon, 30 Jan 2017 15:54:45 GMT

Thanks a lot Andreas, in the end we finally found the problem thanks to your last message :). We had changed the 16MHz capacitors to a slightly smaller value (8pF instead of 12pF) to try to solve the problem, and then we forgot to change them back once we had the correct balun. Then we tried to put them again to 12pF and it worked!

Now we have the problem of the range of the antenna, which is significantly smaller than the ones in the DK, but this we will solve tuning the antenna.

Thanks a lot!

RE: Problem with radio frequency custom board. Balun mismatch?

Andreas — Fri, 27 Jan 2017 14:34:42 GMT

Alright, it should not be the LFCLK then. Could you describe how you tested the TX and RX? i.e what kind of device did you measure with or test against? Did you enable the DCDC converter when testing(if so, try disabling it)? Also what is C_L for the 16 MHz crystal in your custom board?

RE: Problem with radio frequency custom board. Balun mismatch?

angel — Thu, 26 Jan 2017 16:51:09 GMT

Hi again Andreas,

I have tried these two approaches:

err_code = softdevice_handler_init(NRF_CLOCK_LFCLKSRC_RC_250_PPM_250MS_CALIBRATION, NULL, 0, NULL);

and

err_code = softdevice_handler_init(NRF_CLOCK_LFCLKSRC_SYNTH_250_PPM, NULL, 0, NULL);

For neither of them the custom board is able to receive anything. Is this what you meant? Any other suggestion?

Thanks a lot for the help!

RE: Problem with radio frequency custom board. Balun mismatch?

angel — Thu, 26 Jan 2017 09:14:15 GMT

Hi Andreas,

I didn't try changing the LF CLK source, will try today. The code runs perfectly (TX and RX) in all kinds of DK, PCA10028 or nrf51 dongle. Does this mean that the problem is not in the software then? I assume it is not, but I want to have 100% certainty, no assumptions.

Thanks!

RE: Problem with radio frequency custom board. Balun mismatch?

Andreas — Thu, 26 Jan 2017 09:09:10 GMT

Hi Angel,

Alright, the increased TX range is a good sign. I can not remember having heard of a similar issue with RX being caused by HW. Regardless, I would imagine at least some of the power should seep through if the devices are placed right next to each other.

Did you attempt changing the LF CLK source or running the code on a DK?

RE: Problem with radio frequency custom board. Balun mismatch?

angel — Wed, 25 Jan 2017 18:18:30 GMT

Hi again Andreas,

We have already changed the balun for the one that is fully compatible and the results are:

We get more range in the TX way, i.e. we can send data further, but still we cannot receive anything through the RX line. This is really weird... I am trying to get the RSSI data as Michael said and I am getting from 0xD0, when both devices are really close to 0xAE, when the distance is like 1 meter. I know that the performance with two nRF dongles is waaay higher, but why are we not able to receive then?

I need to know something. Is there any possibility that the problem is software related, even if the same software works when we use two dongles? I need to separate the source of problems somehow, otherwise this is crazy...

RE: Problem with radio frequency custom board. Balun mismatch?

Andreas — Tue, 24 Jan 2017 07:52:18 GMT

Hi Angel, Good to hear.

While the correct balun might be easier to solder, having the solder pads on the side instead of on the bottom, it should be a little too wide for your current footprint. You might be able to fit it by making solder bridges or something, with some degradation in performance.

The noise due to the missing 15 nH should be easy to spot if you get a signal out, but you need a spectrum analyzer. Set up a TX carrier and look for spurious that pop up when you enable the DCDC converter.

The 16 MHz XOSC is only needed for a few modules, if not the internal 16 MHz RC oscillator will be requested. Still it might be incorrectly loaded. Timing is generally depending on the LF CLK though, you could try to simply change LF CLK source to either internal RC or synthesized from HFCLK.

RE: Problem with radio frequency custom board. Balun mismatch?

angel — Mon, 23 Jan 2017 16:39:59 GMT

Thank you very much guys. These are the steps we are going to follow now:

Short term (couple of days):

I will order the balun that you say
I will make sure that we solder it properly (by hand, difficult to make sure..)
Since I can get messages from my custom board (but not the other way around) I will check the signal strength as Michael suggested.
I will check if the voltage arriving to the balun has any noise at all
Since my application has timers, I2C communication and everything works quite good, I am assuming that the 16MHz crystal is working properly.

In the long term (1-1.5 weeks):

Redo the layout with all the details you suggest, using inductor and correct balun, removing vias and nets behind the antenna, etc..

RE: Problem with radio frequency custom board. Balun mismatch?

Andreas — Mon, 23 Jan 2017 14:04:54 GMT

Hi Angel, Michael has a lot of good points regarding the antenna. To sum it up:

There should not be any signal traces beneath the antenna.
There are no tuning components for the antenna. (this is increasingly important with a via in the RF path.
The antenna must be tuned using a network analyzer for optimum performance. This is hard without any tuning components though.

In addition I have some comments:

The balun you are using is designed for the WLCSP package version, and thus transforms the load impedance into a different one than the QFN package version requires for optimum performance. The correct Johanson part would be 2450BM14E0003 in your case.
This particular antenna is a meander antenna design, which relies on the geometry in order to radiate. The total length is not relevant as in a monopole antenna. It looks very much like an antenna which we have been using in a lot of our designs, but one of the sides is too short, which will decrease the radiation efficiency.
The antenna should be placed on the end of the board, not in the middle. Ideally (for this one at least) there should be free space on three sides, and a ground plane on the feed side.
The balun is not properly grounded, there should be a larger land with multiple vias to bottom/internal ground planes around the ground solder pad. The balun seems to be placed approximately the correct distance from the nRF though.
There is a 15 nH inductor missing between L1 and C11 for filtering noise from the DCDC converter.

As Michael said you might be better off redoing the layout. Prioritize the antenna and radio placement, place it where it will radiate freely, and where it is free from metal objects and e.g. not covered by a hand when used. Also, if you did not already, check out the nRF51 PCB design tutorial and the reference design with Johanson balun, which also contains Altium designer files that you can use. Using the nRF51-Dongle HW files you can also see how to use the nRF51 with this kind of antenna, however with a discrete component matching network.

And as always, please feel free to submit your design for review before shipping it off to production.

RE: Problem with radio frequency custom board. Balun mismatch?

angel — Sun, 22 Jan 2017 16:43:56 GMT

Thanks again Michael, will take a look to this paper. I have also tried to change a bit the frequency by software (2400-2524 MHz), but none of the extreme values work well

Thanks!

RE: Problem with radio frequency custom board. Balun mismatch?

angel — Sun, 22 Jan 2017 16:00:26 GMT

Here is a picture with the distance from the balun to the microprocessor. What are your thoughts about this?

http://imgur.com/a/OzYwy The sum of everything is around 32-33mm, maybe a bit more.

Thanks!

EDIT:

Two more pictures that show different planes, ground plane and signal plane

http://imgur.com/a/gaaKT

RE: Problem with radio frequency custom board. Balun mismatch?

Michael Antwih — Sun, 22 Jan 2017 15:52:31 GMT

helpful whitepaper above

RE: Problem with radio frequency custom board. Balun mismatch?

Michael Antwih — Sun, 22 Jan 2017 15:52:14 GMT

PCB-quarterwave-2_4GHz-monopole-jan05 (1).pdf

RE: Problem with radio frequency custom board. Balun mismatch?

Michael Antwih — Sun, 22 Jan 2017 15:50:55 GMT

Is the ground plane under the feedline + balun solid? This is another important thing to note because this would make up the impedance of the transmission line.

RE: Problem with radio frequency custom board. Balun mismatch?

Michael Antwih — Sun, 22 Jan 2017 15:47:03 GMT

You use measure from the antenna to the feed line, if the feedline is short. If the feedline to long you will need to add that into the measurement. feedline is balun to antenna. Easy way to look at it would be feedline = section above the ground plane.

RE: Problem with radio frequency custom board. Balun mismatch?

Michael Antwih — Sun, 22 Jan 2017 15:43:59 GMT

Traces underneath the rf antenna is a bad practice. The antenna will not be able to properly radiate. If you look at the dk you will notice it has a keep out zone around the antenna. If you were able to get a hold of a network analyzer you would basically perform a S11 measurement on the antenna. You would want to make sure you have good return loss -9 or higher in the freq range of bluetooth. 2.4-2.45ghz. I normally use the following markers; 2.4, 2.44 & 2.45 ghz.

Since you have traces under the antenna my recommendation would be to following. Redo the pcb with no traces around or under the antenna. You need a keep out zone. I would also place a pi network or some lumped configuration between the antenna or the balun to aid in matching.

For these boards you could cut the antenna and place a sma on top layer and use an external antenna for now.

Oh also try to avoid vias in the antenna path. With the proper keep out you should have no need to change layers.

RE: Problem with radio frequency custom board. Balun mismatch?

angel — Sun, 22 Jan 2017 15:40:29 GMT

We have also some length from the balun to the microprocessor. I guess this extra length cannot be directly added to our 24.025mm no? I guess we would have to tune our antenna with this logic analyzer?

Thanks!!!!

RE: Problem with radio frequency custom board. Balun mismatch?

Michael Antwih — Sun, 22 Jan 2017 15:35:27 GMT

Even though the dk board measures 27.8 it has a matching network before the balun which compensates in order to get the correct frequency response.

RE: Problem with radio frequency custom board. Balun mismatch?

angel — Sun, 22 Jan 2017 15:23:37 GMT

Yes, those traces are underneath. Do you think that they will interfere importantly in our communication?

We may have access to a signal analyzer, but I am not sure yet. If we had access, what is the procedure we should follow to test the antenna? Thanks a lot again!

RE: Problem with radio frequency custom board. Balun mismatch?

angel — Sun, 22 Jan 2017 15:21:36 GMT

Hi Michael, thanks for your fast and complete reply! Really helpful!

We have measured the length and it is around 24.025mm if I measure it until it is connected to the balun. The thing is that if I do the same measurement with the nrf51 dongle, I get around 27.8mm. But nothing is close to 31 or 32mm, not even in the devices that work perfectly (nrf51 dongle)

Here is a picture of other antenna in our design (the previous one was slave, this is the master)

http://imgur.com/a/tRUtf

In this case we have a ground wall, as you can see.

To get this ~31-32mm, from where to where should we measure?

Thanks!

RE: Problem with radio frequency custom board. Balun mismatch?

Michael Antwih — Sun, 22 Jan 2017 11:37:05 GMT

Also are those traces (gray lines) underneath your antenna? Another thing you would need to check to the impedance from the feedline to the balun and then the balun to chip. determine the transmission line impedance of ANT, ANT1 and ANT2. Those could be causing your mismatch as well.

If you have access to an network analyzer let me know. Problem is easy to resolve with one of those.

RE: Problem with radio frequency custom board. Balun mismatch?

Michael Antwih — Sun, 22 Jan 2017 11:32:32 GMT

It doesn't appear you have implements any matching between the antenna and the balun. I would assume some type of matching would be needed.

What is the trace length of the antenna? The total length should be ~31-32mm for a quarter wave antenna at 2.4ghz.

Your antenna may be around that length but then you introduce some parasitics because of the via which connects the antenna to the feedline / balun.

This via might be causing added reflections but hard to say without measuring it.

Here are some things you can try.

You can verify your RSSI by using the nrfConnect tool of your smart phone. Basically have your custom board advertise while you listen with you smart phone place directly next to the board. Normal you would expect rssi any where between 30-50 if the units are directly next to eat your; if the lower number being better. In your case you might see values much higher like 70-80's which would confirm your matching findings.

To correct the matching if you don't have a network analyzer you can measure the antenna trace length to see how close it is to 31-32mm. If you are less than, remove some of the solder mask on the antenna and add length by solder copper tape to make up the delta. If you are greater than try trimming the antenna by cutting the trace to account for the delta.

The above might see not work because of the via. So what you can try next is to remove the via from the circuit. Basically make two cuts, one disconnecting the feedline from the via and the other disconnecting the antenna from the via. On remove some solder mask from the feedline and then add on copper tape in order to get the correct trace length.

if it is matching you should see some type of change.

Assuming you have already looked into the balun soldering and chip soldering.