Review of nRF 8001 schematic with Antennae

RE: Review of nRF 8001 schematic with Antennae

Garth Snyder — Fri, 21 Feb 2014 19:23:17 GMT

Excellent points, Ole. My board isn't bound for mass production, so the only concern was that it function as a BTLE device and not cause any obvious local interference. I'm not by any means recommending skipping the pi network, design review, or signal analysis for a production board.

I was mostly responding to thomas, who was having trouble getting the antenna to work at all. According to my very limited experience, yes, you should be able to follow the cookbook recipe blindly but carefully and end up with something that at least functions as a radio with reasonable range. And there doesn't seem to be any problem pairing the nRF8001 with that Johanson balun chip.

RE: Review of nRF 8001 schematic with Antennae

Ole Morten — Fri, 21 Feb 2014 14:50:52 GMT

Have you also had harmonics and spurious values measured? It doesn't really matter if the performance is great, if it won't pass FCC/CE testing... This is the primary reason for both having your design reviewed by a hardware application engineer (not me!), and to measure and verify the design.

I'd therefore strongly recommend you to post a support case, uploading complete schematics and Gerbers and asking for a design review.

RE: Review of nRF 8001 schematic with Antennae

Garth Snyder — Thu, 20 Feb 2014 20:24:44 GMT

I'm know very little about RF design, but I have used a Johanson 2450BM14A0002 balun chip along with a Johanson 2450AT18A100 chip antenna for an nRF8001-based board, and the combination worked great. I didn't use any kind of additional matching network beyond the balun chip.

I did, however, follow Johanson's layout suggestions pretty closely. There are a couple of different Johanson tech notes with not-entirely-consistent advice; I used page 2 of the antenna chip data sheet (which I'll attach below) as the model.

In particular, I did ground planes on both sides of the board with frequent vias on the periphery, a wide, 19-mm stripline "runway", a large keep-out area on the ground plane behind the stripline, a circular no-ground area behind the balun itself, and no ground plane on either side in the immediate vicinity of the antenna, using the recommended margins. My total ground plane area was only about 60% of the recommendation, but there wasn't much I could do about that.

As I said, performance was great. On the other hand, all that keep-out area is expensive; I'm not sure it's any more space-efficient than a PIFA or meander PCB antenna. Also, I'm hand-reflowing and that balun chip packs six connections into a microscopic little package. I was probably just lucky that it reflowed without any bridges.

Johanson 2450AT18A100 Antenna.pdf

RE: Review of nRF 8001 schematic with Antennae

Ole Morten — Thu, 20 Feb 2014 10:10:08 GMT

This is really not a related question at all, and would hence be better off as a separate question (or a support case).

However, as wilhelmsen says, when choosing a balun, it's important to pick one that's specifically been measured and verified with the chip in question, and using the balun you've linked to with an nRF51822 is hence not something we'd recommend. Also, as I said in my other reply here, following the reference design is especially important when using a balun, since there isn't any components to tune with.

I'd recommend you to open a support case with your full design as schematics and Gerbers/Altium files, and request a proper review.

RE: Review of nRF 8001 schematic with Antennae

Ole Morten — Thu, 20 Feb 2014 10:07:17 GMT

Hardware reviews are in general better done as regular support cases, since they will often conclude with hardware being sent in for tuning.

To answer your specific questions Manish, the values of a pi-network can't be decided beforehand, and must be determined by measuring on a prototype. This kind of measuring is something we can help with, but it requires that the design has been reviewed in a regular support case, and all our comments taken into consideration.

Your schematic doesn't show what kind of balun you use, but in general, you need to make sure that the balun is made for exactly the chip that you're using. For the nRF8001, you can use the Johanson balun linked above for example. When using a balun it's also extremely important to follow the reference layout of the balun manufacturer exactly, and place the balun at exactly the correct distance from the chip, and making sure the grounding of the balun is as good as possible.

As for antenna tuning components, this is dependent on the antenna type used. If you use a straight PCB antenna, it's usually sufficient to include one shunt capacitor, and making the antenna a little longer than should be needed. The antenna can then be tuned by adjusting the capacitor and the length of it.

However, for meander, inverted F or chip antennas, it isn't really possible to tune by adjusting the length, and a full pi-network is hence recommended. This should be placed between the balun/ matching network and the antenna, exactly as you've done. You can never combine components for antenna tuning with components of the matching network or the recommended circuit for the balun, so make sure to include all 3 footprints separately. It could very well be that not all of them will actually be needed, but initially, the footprints should be put in place to allow maximum freedom when tuning.

As mentioned, it is not possible to determine the length of the antenna or values for the shunt capacitor or pi-network without measuring on the design, which is one reason for creating a regular support case and requesting help with this.

If you want to do tuning yourself, we do however have a whitepaper on tuning the matching network and a separate one on antenna tuning, which may be useful to refer to anyway.

RE: Review of nRF 8001 schematic with Antennae

wlgrd — Tue, 18 Feb 2014 18:27:07 GMT

I personally don't know at all, just wanted to make sure you had considered it :)

RE: Review of nRF 8001 schematic with Antennae

thomas — Tue, 18 Feb 2014 18:25:13 GMT

RF part on nrf518001 and 51822 are very similar and i've vhosen this ballun because it's bigger than the recomanded BAL-NRF02D3 but yes, maybe it was an error...

RE: Review of nRF 8001 schematic with Antennae

wlgrd — Tue, 18 Feb 2014 18:06:32 GMT

Are you sure it will work okay with that balun? I just saw that the sheet says "Optimized for Nordic's Chipset nRF24L01, nRF24L01+, nRF24LE1, nRF24LU1, nRF24AP2, and nRF8001".

RE: Review of nRF 8001 schematic with Antennae

thomas — Tue, 18 Feb 2014 17:54:19 GMT

Hello, I'm interested in an answer too. RF part on my prototype (nrf51822) doesn't work. I have chosen this ballun and this ceramic antenna with a 50ohm line. I'm not RF engineer, did I forget something?

Attached, my shematic and my layouts