2
\$\begingroup\$

Following up on this question, I am using an RN2903 module with a U.FL connector (with an additional option for a through-hole wire antenna). My PCB looks like this:

enter image description here

I used this coplanar waveguide calculator to set the impedance to 50 ohms, using a 30 mil trace width and 6 mil trace gap (with Er=4.8). I have done some preliminary signal strength tests, and for LoRa 900 MHz, it seems my own PCB is about 2 units lower in SNR than the Microchip RN2903 Pictail (which as the same track characteristics, but has a longer, curved trace as shown in my previous question).

My main concern is that my trace design is hindering the performance of the LoRa module. I have already attempted filling the wire antenna hole with solder, thinking this might improve performance, but improvements were only very slight. Additionally, I have seen these types of designs, such as this Adafruit module that has space for both an SMA connector and a through-hole wire antenna.

Do you see any definite NO-NOs in my design?

EDIT: Here is a picture of the PCB with the wire antenna hole filled with solder.

enter image description here

\$\endgroup\$

1 Answer 1

1
\$\begingroup\$

I don't see very strange things but:

  1. You use evenly spaced GND via to connect your top and bot GND planes. It's better to put them randomly. Your repetitive pattern "selects" some frequencies that will resonate through your planes. But, I don't think this is your issue here. Just to mention.

  2. I think your impedance controlled track is not at 50Ohm.

Usually, when you want to have a controlled impedance track on your PCB, you ask the PCB shop to do it for you.

They will give you an approximate width to put on your design. And they will ask you to ensure that none of the other tracks has the same width. When processing your file prior to manufacturing, they will search for all the tracks with that width and replace them with the correct width according to their process.

The adjustment is very small, it's almost the same width they provided you with.

They also include an area that contains a "test track" of that width somewhere on the PCB panel. They will measure the effective impedance of that track after manufacturing to ensure that the tracks are really 50Ohm with the given dimensions.

What can you do ?

  • You can use a PCB workshop that can provide controlled impedance tracks.

or

  • You can design such that a small mismatch is not a big deal.

or

  • You can choose a kind of controlled impedance track that is less sensitive to the manufacturing tolerances. Use your online calculator and try to add 1 mil to every dimensions (clearances, width, etc) and see which kind of design (microstrip, stripline etc ) is less sensitive to that error.
\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.