Timeline for Suggestions for possible cause of impedance mismatch between board revisions?
Current License: CC BY-SA 4.0
8 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Jan 6, 2021 at 8:44 | comment | added | citizen | Even though chip manufacturers mean well when they provide reference designs for applications to demonstrate their chipsets etc, often it is not practical to follow a design to the letter. In this case the reference was a 4-layer board with a stack-up suitable for a 1,6mm PCB. Our board had to be 10 layers with 1,6mm thick PCB.... As you can see this would not be "translatable" going from 4 layers to 10 layers etc. was not physically possible and the manufacturers would not accomodate this, ever ... | |
| Jan 6, 2021 at 8:40 | comment | added | citizen | Generally speaking, as one commented already, moving from an existing design, to another board, would normally assume no change in the routing of the design. This was unfortunately not possible, so the routing and stack-up was modified to some extent to accomodate the space constraints. | |
| Jan 6, 2021 at 8:36 | comment | added | citizen | Version A of the board was a version with a microstrip going all the way to the edge of the board that wasn't terminated with the edge-monunt SMA. It was actually the version that led to an additional SMA after ditching the U.FL. So the microstrip track had the original 50 Ohm width. In this version the soldermask was scraped off and an SMA was soldered on directly onto the microstrip, without the SMA pad as this was added in the subsequent revision B. The difference between the two boards was visible by measuring SWR and this was bad for the revised board ... | |
| Jan 6, 2021 at 8:31 | comment | added | citizen | Essentially yes ! The passive components in the photos are followed by a 50 Ohm microstrip, then to an edge-mount SMA with a pad about 40% the length of the visible microstrip, then followed by about 180mm of 50 Ohm coaxial going to an antenna. This SMA pad has about 10 Ohms of impedance based on the substrate thickness, and this lowered the effective overall impedance in an unwanted way... The problem is that from a birdseye view loking at the board all seems tiny and it was unimaginable that the pad could cause this, but simulations confirm it ... | |
| Jan 5, 2021 at 15:28 | comment | added | niko20 | so just an extra wide solder pad caused SWR losses? wow. | |
| Jan 5, 2021 at 13:29 | history | edited | JRE | CC BY-SA 4.0 |
Removed ellipsis from places they don't belong. Periods or commas, almost never ellipsis.
|
| Jan 5, 2021 at 13:19 | vote | accept | citizen | ||
| Jan 5, 2021 at 13:19 | history | answered | citizen | CC BY-SA 4.0 |