Hello,
I have a 4 layer PCB with a roughly 16mm feed to a Bluetooth chip antenna on TOP.
I tried to design it to be 50 ohms, but when I started testing the antenna performance it was wildly different from what I was expecting, so I started investigating and it seems to me that the first problem is that my transmission line isn't even close to 50 ohms.
I would be very appreciative if more experienced people could help me understand where I went wrong (if it's at the design stage, or the measurement stage, or something else altogether that I haven't considered).
The PCB : 4 layers, 1mm thick, designed for JLCPCB's impedance controlled stackup
JLC7828.
JLC's [url=https://cart.jlcpcb.com/impedanceCalculation]calculator gives 11.55mil /
0.293mm line width for these specifications.
Saturn PCB gives a slightly different answer:
- Microstrip, 18+18um copper, FR-4 with Er=4,6 (the dielectric constant for prereg 7628 according to JLC's page), conductor height = 0.2mm, F=2400 MHz ==> about
0.35mm width to get 50ohms
- Coplanar wave, FR-4 with Er=4,6 (the dielectric constant for prereg 7628 according to JLC's page), conductor height = 0.2mm, Gap = 0.18mm ==> about
0.34mm width to get 50ohms
I settled with 0.3mm for the first PCB prototype, since even according to Saturn PCB it gives about 53ohms, not wildly far off, and I would adjust for the next iteration.
That test PCB is aimed at testing the antenna and the effect of the parts around, some which have significant metal pieces (piezo buzzer, speaker), so I added two 0402 footprints to connect the feed + antenna to either where the Bluetooth chip is, or to a U.FL connector.
I realized a little late that the U.FL footprint might not be great with the thermal spokes so small, and on my test board I added a large solder blob on each side of the connector, but that's not the main point I think, since my big question is about the trace impedance.
So I will spare you most of the confusion of the initial tests and focus on the trace impedance.
After an OSL calibration with a cheap U.FL calibration kit (one on a RF demo kit like the ones people often get with a nanoVNA or similar) on my Siglent SVA1032X (CF=2441MHz, span=390MHz), I tried a sanity check : I cut and removed the trace at the place where it leaves the ground plane to go connect to the antenna (top right), populated the 0402 footprint to connect the U.FL and antenna feed with a 0R (thin film), and soldered a 49.9ohms 1% 0402 resistor (green on the picture below). I was expecting something not too far from the center of the smith chart but to my surprise it wasn't anyway near (arc on a circle that crossed the horizontal line around 70 ohms, though I did not record that part exactly).
I tried the techniques from this link (
https://chemandy.com/technical-articles/measuring-track-characteristic-impedance/measuring-track-characteristic-impedance-article3.htm ) and this video (
) to measure the impedance, and both gave roughly 40 ohms.
I cut the side of the PCB to verify the material thickness and measured 0.19 mm, a difference from the 0.2 that should make the impedance greater, not smaller, and only by a couple ohms.
Gnd plane is also well connected (I carved the PCB to make sure vias connected to it).
Any idea why my trace isn't the 50 ohms it should be ? or am I going in a completely wrong direction ?
Thanks in advance.