Design Review of NRF51822-QFAA board

RE: Design Review of NRF51822-QFAA board

Andreas — Thu, 01 Nov 2018 15:35:21 GMT

Hi,

Both antennas are fine, just make sure you add a couple of mm to the length as it is easier to make is shorter (cut) than make it longer (copper tape). It is hard to say which one is better, both should be very good and there should be little difference between them. If you need to know which gives 2% more range than the other, you will have to make one board with each, and compare them. I suspect the reason why you can not find any post regarding successful use of the IFA is that most people make a post when there is an issue, not when everything is working as it should.

Best regards,

Andreas

RE: Design Review of NRF51822-QFAA board

tamojit — Thu, 01 Nov 2018 07:47:02 GMT

Thanks Andreas for your reply. But I am worried about the F antenna I am using. I haven't seen any proper results or post regarding the successful use of the inverted antenna with nrf51822.

So I again modified my design and copied the meandered antenna from the official NRF51822 EK Board. I have attached the zip provided by Nordic.
devzone.nordicsemi.com/.../nRF51822-Evaluation-Kit-_2D00_-Hardware-files-2_5F00_0.zip
I copied the design from the PCA10001 folder.

Here goes my updated design.

Can someone help if this is good ?
Or, the F antenna is better to go with in terms of efficiency?
Please provide me corrections(if any).

Thank you in advance for taking out some time to review the design.

RE: Design Review of NRF51822-QFAA board

Andreas — Wed, 31 Oct 2018 15:07:12 GMT

Hi,

Your design looks good now, you should be good to go.

Best regards,

Andreas

RE: Design Review of NRF51822-QFAA board

tamojit — Wed, 31 Oct 2018 09:11:20 GMT

I expected some suggestions...

RE: Design Review of NRF51822-QFAA board

tamojit — Tue, 30 Oct 2018 11:54:41 GMT

Thanks for your valuable suggestion. I just updated the board layout a bit.

I plan to use the balun, for now I am sticking with the discrete balun network provided here
In the design I just added a shunt capacitor [C25] for tuning.

The thick line in the design from the feed of the antenna to the common point of C6,L3 has a width of 1.42mm and clearance from the TOP GND plane is 0.254mm. However the bottom GND plane is present under that line.

As the board thickness will be 1.6mm, the impedance of the line becomes 50ohm.
I got the value from Saturn PCB Toolkit.

Screenshot of Saturn Toolkit:

Am I going right ?

What else is needed to be considered?
Please help!

RE: Design Review of NRF51822-QFAA board

AmbystomaLabs — Mon, 29 Oct 2018 21:16:59 GMT

The balun question is moot. You should use the balun since it will give a predictable result unlike a lumped element balun. So you should use the chip balun for either antenna.

On the F antenna, if you plan on using it you should model it or really trust the source that they modeled it. Sonnet is an easy app to use and the free version should take care of it.

If you still feel like you want a printed antenna without trusting your source for the F and modeling it yourself, then copy either the F-meander hybrid that both Nordic and TI have used. TI even has a whitepaper out on it. Then you are getting an antenna from a reliable source that if duplicated faithfully will likely work.

If none of the above options appeal to you then hire an RF engineer since you will have some matching to do with an unknown antenna and a lumped element balun.

RE: Design Review of NRF51822-QFAA board

tamojit — Mon, 29 Oct 2018 18:36:32 GMT

Please help!

RE: Design Review of NRF51822-QFAA board

tamojit — Mon, 29 Oct 2018 17:30:08 GMT

I am just worried about the inverted F antenna tuning. So can I avoid the discrete matching network and use this 2450BM14E0003T balun instead?

RE: Design Review of NRF51822-QFAA board

AmbystomaLabs — Mon, 29 Oct 2018 17:22:48 GMT

1. Yes I meant vias on the interior. Just because they are both ground doesn't mean they are at the same potential all the time.

2. I'm sorry I simply don't have the time right now to review individual parts. Printed antennae like your F antenna have much better efficiency than chip antennae therefore greater range. Chip antennae are easier to get to 50 ohm by rigidly following the design guidelines. Take your pick.

3. If you use a chip and rigidly follow the design specs yes you will probably get something that works well without a lot of effort.

4. Just google harmonic filter.

RE: Design Review of NRF51822-QFAA board

tamojit — Mon, 29 Oct 2018 17:00:02 GMT

AmbystomaLabs Thanks for your review!

[quote userid="7201" url="~/f/nordic-q-a/39947/design-review-of-nrf51822-qfaa-board/154918"]You failed to stitch the interior of the board. The whole things needs to be stitched.[/quote]

By stitching you mean to connect the 2 layers with via, right? Both the planes are GND.
I will do it eventually as I go through the design process.

[quote userid="7201" url="~/f/nordic-q-a/39947/design-review-of-nrf51822-qfaa-board/154918"]You should use the Johanson matched balun for it.[/quote]

Ok. I am getting these parts from Mouser, please verify the following:
1. 2450BM14E0003T
2. Chip Antenna
This is the antenna I prefer to use along with the balun. Please verify, if I need any additional filter/matching network along with the balun.
Actually I do not prefer to use the discrete matching network. They are pain in the a*s!

[quote userid="7201" url="~/f/nordic-q-a/39947/design-review-of-nrf51822-qfaa-board/154918"]You should model the antenna. Use the free version of Sonnet. It will easily model that antenna and the learning curve is easy. This way you can get the antenna close to 50ohm in the model and the board will come out close.[/quote]

Is this step necessary if I use the chip antenna?

Waiting for your reply.

[quote userid="7201" url="~/f/nordic-q-a/39947/design-review-of-nrf51822-qfaa-board/154918"]Also, you get a harmonic filter in the same package and a smaller footprint than all those discretes.[/quote]

I did not understand by what you meant by harmonic filter. Please elaborate as I am a noob in RF designing.

RE: Design Review of NRF51822-QFAA board

AmbystomaLabs — Mon, 29 Oct 2018 15:28:24 GMT

As mentioned earlier do not use 0603 for matching components. At 2.4GHz, it's about 10 degrees just going from one side of the device to the other. Matching is a pain. Use 0402 or preferably 0201.

You failed to stitch the interior of the board. The whole things needs to be stitched.

You should use the Johanson matched balun for it. Likely it will work without a lot of post matching. Also, you get a harmonic filter in the same package and a smaller footprint than all those discretes.

You should model the antenna. Use the free version of Sonnet. It will easily model that antenna and the learning curve is easy. This way you can get the antenna close to 50ohm in the model and the board will come out close.

RE: Design Review of NRF51822-QFAA board

tamojit — Mon, 29 Oct 2018 13:45:58 GMT

I have updated the board layout by adding a shunt capacitor. Please have look.

RE: Design Review of NRF51822-QFAA board

tamojit — Mon, 29 Oct 2018 13:04:08 GMT

The board is still in design phase, and I am pretty noob in RF antenna design. Can you help me in measuring the impedance of the antenna?
If after fabrication of the board it it turns out that the impedance of the antenna is less than 50ohm, what should I do?

RE: Design Review of NRF51822-QFAA board

haakonsh — Mon, 29 Oct 2018 12:42:10 GMT

No 0603 are fine as well. It's just more difficult to tune the balun/filter with components smaller than 0402 or bigger than 0603. You might not need to have the balun/filter tuned at all though.

RE: Design Review of NRF51822-QFAA board

tamojit — Mon, 29 Oct 2018 12:37:33 GMT

Is there any issue that I can face with 0603 components?
And about the impedance of the antenna. I haven't calculated yet.

RE: Design Review of NRF51822-QFAA board

haakonsh — Mon, 29 Oct 2018 12:29:15 GMT

I urge you to stick to 0402 components in the balun and filter network. Otherwise it looks good.

Have you calculated the impedance of the transmission line between the balun/filter and the antenna?