Radiated Emissions failure (receiver spurious emissions test)

RE: Radiated Emissions failure (receiver spurious emissions test)

FormerMember — Wed, 04 Jul 2018 10:29:49 GMT

When you have finished the new layout, you can upload it here for a review.

RE: Radiated Emissions failure (receiver spurious emissions test)

EclectronicDesign — Tue, 03 Jul 2018 13:39:14 GMT

Thank you for your additional comments, I will refer back to the reference layout.

RE: Radiated Emissions failure (receiver spurious emissions test)

EclectronicDesign — Tue, 03 Jul 2018 13:38:30 GMT

Hi,

Thank you for taking the trouble to review the layout. Your comments have been taken on board. The ground via on the antenna was a good pick up, it was not intended to be like that. I will go back to the reference layout and see what improvements can be made.

RE: Radiated Emissions failure (receiver spurious emissions test)

FormerMember — Tue, 03 Jul 2018 08:31:59 GMT

@ambystomalabs has a lot of good points.

I would like to add: Regarding P0.25 and P0.26:

When adding 12 pF capacitors, they have to be added as close as possible to the pins, see the refence layout.
When you grounded P0.25 and P0.26, how did you do it? The very best is to ground them to the center pad.

In addition, the routing of the RF part has to be an exact copy of our reference layout, see this tutorial, section "Matching networks"

Regarding the antenna: the ground plane has to be extended so that the stub can be grounded.

RE: Radiated Emissions failure (receiver spurious emissions test)

AmbystomaLabs — Mon, 02 Jul 2018 23:49:37 GMT

There are significant issues with your layout. I will just go through the items I see wrong. Also, oddly you show layer one mirrored left/right. I guess your cad app likes to think the top is the bottom and the bottom is the top? Normally layer 1 (top) is dead on normal. But either works as long as your board vendor is happy.

1. Insufficient ground vias for nRF. On our own products we use a 5x5 grid of 8 mil vias for the ground slug on the nRF. I think Nordic uses a 4x4 grid. 2x2 is just insufficient as it forces ground currents to go a long, long way to get back to the origin. This can easily cause a variety of RF noise and even RF instability.

2. I assume that is meant to be a shorted stub tuner on your meander F-hybrid, otherwise there would be no point in the via. But you didn't connect the via to a suitable ground. You should double check this against your antenna design. Maybe the via keeps the layout app happy for open stub tuners.

3. For some reason you chose to float the flood on inner 2. For this style of layout all the inner layers should be ground. Only if you were trying to do power planes would this not be ground, but in general power planes invite more problems than they solve.

4. Besides your antenna probably being well tuned to 4.8GHz due to the incorrect stub tuner a big mistake is the missing ground connection at pin 32 and the 0.8pF cap that is part of the output match/harmonic filter. Due to the poor grounding overall, it is likely the 0.8pF cap isn't doing anything at all. Thus 4.8GHz is just passing through network.

5. And, it looks suspiciously like you intended to copy the shunt C, series L output match of the nRF52832 reference design. But the inductor is too far away. Changes of as little as 1mm influence the effect a reactance has in a circuit at these frequencies. So, you should double check your design with regard to the component placement.

Hard to say what the biggest problem is for your compliance issue. Mixers really don't like a poorly matched output and in RX mode likely there are few gain stages and more coupling with what is likely a poor match at the output. This can cause the second harmonic rejection of the network to be worse.

RE: Radiated Emissions failure (receiver spurious emissions test)

EclectronicDesign — Mon, 02 Jul 2018 22:45:09 GMT

devzone.nordicsemi.com/.../Layer-stack.pdf

Hell Kristin,

I have uploaded the layer stack from top to bottom. Looking forward to your advice.

RE: Radiated Emissions failure (receiver spurious emissions test)

FormerMember — Mon, 02 Jul 2018 12:48:03 GMT

Okay, I see. Could you upload an image of the ground layer as well? Which layer is the ground layer, layer 2 (inner layer 1)?

RE: Radiated Emissions failure (receiver spurious emissions test)

EclectronicDesign — Mon, 02 Jul 2018 11:32:53 GMT

Hello Kristin,

We tested the solution today and the result was unsuccessful. Both pins P0.25 and P0.26 are currently allocated in our design, so we put 12pf caps from these pins to ground and saw no change in the 4.8GHz emission. We then connected these pins to ground and saw no improvement to the emission.

We used a near field sniffer probe for the test and it was hard to pinpoint where the source was. We disconnected the antenna by removing the series L and that made no difference, so I presume is not coming from the antenna.

I have pasted an image of the top layer of our four later board for your reference.

This product is going into mass production in a few months time so we are really needing help to fix this issue. Any advice you can offer is appreciated.

Regards

Mike

RE: Radiated Emissions failure (receiver spurious emissions test)

EclectronicDesign — Thu, 28 Jun 2018 12:14:30 GMT

Thank you very much for your help. It sounds like this problem is already known, so we will try it and let you know the result.

Unlucky for us we have assigned both of these pins, but that is something that we can handle.

Thanks

Mike

RE: Radiated Emissions failure (receiver spurious emissions test)

FormerMember — Thu, 28 Jun 2018 08:39:01 GMT

QFAX: Unfortunately, it is no sufficient to configure the pins as GPIO and set internal pull-down, the pins has to physically be grounded or "grounded" via a capacitor.

CIAA: This is normally not a problem for the CSP variant. What is the case number of the private case?

RE: Radiated Emissions failure (receiver spurious emissions test)

WoutZenso — Wed, 27 Jun 2018 13:57:31 GMT

Hi Kristin,

We encoutered the same issue (I made a private ticket where I shared my design files). We use the CIAA package and there is no updated reference layout for this one, only for QFAX. The P0.25 and P0.26 are unused in the final product, but by coincidence we used these pins for the UART connection for DTM mode. I was wondering, since we don't use these pins and I would like to find a fix which does not mean another iteration on the PCB, if it is sufficient to ground the pins by setting an internal pull-down? Or is it required to have a direct ground connection?

Best regards,

Wout

RE: Radiated Emissions failure (receiver spurious emissions test)

FormerMember — Wed, 27 Jun 2018 12:55:54 GMT

Yes, that emission is from the RX LO, and most likely, it's related to errata 138. The fix is to either ground P0.25 and P0.26 if the pins are not used, or place a 12 pF capacitor near the pin to ground on both pins. (See the reference layout.)