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Bug #5000

closed

SMA placement and count

Added by marwalte about 3 years ago. Updated about 3 years ago.

Status:
Resolved
Priority:
Normal
Assignee:
Target version:
-
Start date:
02/01/2021
Due date:
% Done:

100%

Spec Reference:

Description

Hi,

This is a bug in two parts:

1. The placement of the two SMA connectors along the bottom of the board are very close. The causes issues with terminal-mount antennas, like the Taoglas TG.55.8113W for example:

https://www.taoglas.com/product/white-5g4g-terminal-mount-monopole-antenna/

When affixing these antennas, the larger parts of the antenna interfere with each other. Photo attached.

This can be addressed by spreading the SMA connectors further apart, or by using 90° angled connector along the outside of the board, potentially at 45° angles.

2. 3042 (and larger) cards can support up to 6 MHF-4 connectors (PCIe M.2 spec 2.3.2.1). Especially with 5G cards that have wide Sub-6 band coverage / 4x4 MIMO, up to 6 connectors should be expected. Consider increasing the number of SMA breakouts on the board from 4 to 6.


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sma-connector-proximity.jpg View sma-connector-proximity.jpg 189 KB marwalte, 02/01/2021 06:48 AM
Actions #1

Updated by laforge about 3 years ago

  • Assignee set to mschramm

Thanks for your feedback.

marwalte wrote:

This is a bug in two parts:

I would not neccessarily call those bugs, but let's not talk about formalities.

1. The placement of the two SMA connectors along the bottom of the board are very close. The causes issues with terminal-mount antennas, like the Taoglas TG.55.8113W for example:

The intent wasn't really to directly attach an antenna to those SMA connectors, but to attach antenna cables.

https://www.taoglas.com/product/white-5g4g-terminal-mount-monopole-antenna/

When affixing these antennas, the larger parts of the antenna interfere with each other. Photo attached.

Whether or not they mechanically touch/"interfere", putting the antennas in this proximity will certainly make them interfere electrically. Any conductor in such close proximity of an antenna will inevitably de-tune it and drastically change its characteristics. Hence, antenna cable should be used to achieve a proper minimum distance between antennas.

Furthermore, the SMT-soldered SMA connectors (chosen for their low insertion loss) are unlikely to take a lot of angular force / tear (not sure if that's the correct english word). Attaching such long antennas puts you at risk of ripping off the entire SMA connector together with the top copper layer of the PCB.

This can be addressed by spreading the SMA connectors further apart, or by using 90° angled connector along the outside of the board, potentially at 45° angles.

The circuit board would have to be much larger if you want proper spacing of the antennas in the context of electrical interference. At the same time, we try to keep the PCB as small as possible, as every square centimeter of an impedance-matched circuit board is quite expensive. Using some antenna cable is much lower cost and provides more flexibility.

Furthermore, right-angled SMA connectors are
  • typically more expensive
  • typically exhibit higher insertion loss
  • are typically only available as through-hole

2. 3042 (and larger) cards can support up to 6 MHF-4 connectors (PCIe M.2 spec 2.3.2.1). Especially with 5G cards that have wide Sub-6 band coverage / 4x4 MIMO, up to 6 connectors should be expected. Consider increasing the number of SMA breakouts on the board from 4 to 6.

Possible, but again increases the cost significantly. It's basically a question on whether we want to make the product as accessible in terms of pricing or rather high-end.

Keep in mind that particularly coaxial connectors are often the most expensive part on a BOM...

We could probably compromise by adding more footprints but not always populating all of them? That would still increase the PCB (and hence related cost).

Actions #2

Updated by laforge about 3 years ago

laforge wrote:

Furthermore, the SMT-soldered SMA connectors (chosen for their low insertion loss) are unlikely to take a lot of angular force / tear (not sure if that's the correct english word). Attaching such long antennas puts you at risk of ripping off the entire SMA connector together with the top copper layer of the PCB.

Please ignore that part, I mixed this up with another product/project. WE do use THT connectors in this current design, so switching to horizontal connectors is quite easy and just a question of using a different placement option. This means anyone building this OSHW design can simply use different connectors as they please. Anyone placing a volume order at sysmocom could also easily get a batch that was produced with different connectors.

We could also switch to not soldering any connectors and just sell a kit, but experience shows that many buyers, particularly in acdemia, want a fully assembled product.

Nevertheless, irrespective of the THT/SMT mistake, all other concerns still remain identical (pricing, electrical interference of close antennas).

Actions #3

Updated by marwalte about 3 years ago

Hi laforge,

Thank you for an incredibly detailed response!

This is a bug in two parts:

In retrospect, I think this may actually be more of a feature request. Apologies for that.

The intent wasn't really to directly attach an antenna to those SMA connectors, but to attach antenna cables.

I figured this might be the case - I wasn't sure how dual-purpose this board was meant to be. From the response above, the intent of this board is not to have terminal-mount antennas, especially with later remarks regarding the expense of PCB real estate, minimum distance required for proper antenna performance, etc.

We could probably compromise by adding more footprints but not always populating all of them? That would still increase the PCB (and hence related cost).

I think this is a tough sell. The coaxial connectors aren't strictly required - as far as I can tell from this PCB, the connectors really are converting MHF-4 connectors to SMA, something a cable could do just as well. Converting MHF-4 to SMA is something that can be done using enclosure-mounted adapter cables, or could be offered as a separate board - an MHF-4 to SMA breakout, if you will, that could be mounted somehow next to the main breakout PCB (to ensure strain on the SMA connectors does not transfer to more delicate MHF-4 connections on the modem). Maybe removing the existing connectors provides opportunity to shrink the PCB size? I'm not sure what the cost tradeoff in the PCB would be, between 0, 3, 4 or 6 SMA coaxial connectors. From the response above, it seems like the biggest cost here is the connectors themselves.

Although, the above suggestion is in conflict with:

buyers, particularly in acdemia, want a fully assembled product.

I think drawing a line in the sand somewhere is reasonable. Not having extra SMA connectors does not prohibit one from reasonably using the MHF-4 connectors on the modem. This is unlike, for example, if the SIM2 pins weren't broken out onto a header - using pins 40/42/44/46/48 would be unreasonably harder. With 5G modems, there will be many more connections, 5-6 MHF-4 easily for Sub-6 and GPS. Then there is the topic of mmWave antennas, like the QTM525, that use MHF-7 connectors, plus extra connections for power and enable signals - maybe it's not a one-size-fits-all solution.

Actions #4

Updated by laforge about 3 years ago

  • Status changed from New to Resolved
  • % Done changed from 0 to 100

marwalte wrote:

I think this is a tough sell. The coaxial connectors aren't strictly required - as far as I can tell from this PCB, the connectors really are converting MHF-4 connectors to SMA, something a cable could do just as well. Converting MHF-4 to SMA is something that can be done using enclosure-mounted adapter cables

coaxial cables will also have better performance, both in tems of losses and in terms of isolation. So whenever you can use a direct pigtail, do it. The reason to have them on the PCBA is to offer strain relief in lab situations where you operate a modem without any further enclosure. Once you have an enclosure, for sure you want to use a direct pigtail to avoid additional losses.

I think drawing a line in the sand somewhere is reasonable. Not having extra SMA connectors does not prohibit one from reasonably using the MHF-4 connectors on the modem.

exactly.

We did change from 3 to four connectors in the v3 of the design now, as the PCB size had to be expanded slightly anyway to accomodate for the larger/longer modems.

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