Difficulty Tuning nRF24L01+ Matching Network

[Andrew S.]

Hi there,

After prototyping a device with modules and on a breadboard, I transitioned to a custom PCB with nRF24L01+, matching network from the spec sheet, and 2.45 GHz 50-ohm chip antenna. Other components on the board include an ATMega328P, 3.3V regulator, LiPo charger, USB-micro-B connector, and a 400mAh LiPo battery glued to the back of the board. The device is to be part of a battery-powered remote sensor network.

Unfortunately, I have not been able achieve good performance with this system. Using a spectrum analyzer with cable soldered to the board, at best I can achieve -12 dBm power at the 50 ohm point of the matching network in TX mode at 2.4GHz (channel 0). At channel 127, output power is significantly worse (-24 dBm). A single E12 step in either direction for inductor L1 (8.2 nH in the reference network) reduces output power to -16 dBm.

Admittedly, my layout does not match the reference design and I am using 0805 components, including wirewound inductors. Is it possible to achieve good performance in this configuration? Should I change the value of components other than L1 to try to boost output power? Should I re-design the board to use 0603 or 0402 components and more closely match the reference design before doing anything else? Will adding a Pi network at the 50-ohm point help?

Thanks so much! Andrew

[Øyvind Karlsen]

Hi again, Andrew,

I have added a picture with illustrations of comments and improvements.

You want to measure at the boundary between your matching network and the pi network, so solder center and ground as shown in the picture(blue).

The chip is reliant on good grounding, so I would suggest adding vias(marked as circles) to get better grounding. The long thin wires to ground can cause inductance.

The sketched area(green) can be removed to improve antenna performance.

Even though the datasheet for the antenna says 50 ohm, the antenna itself will not be 50 ohm in a use-case scenario. As a demonstration you can hook up a WLAN-antenna from your router to the VNA and see how the impedance changes when you curl your hand around the antenna. The antenna characteristics change a lot with ground plane, this is why we need a pi network.

[Andrew S.]

Hi Oyvind,

Thank you again for all your help and suggestions. I've had some limited success tuning the matching network and have increased the range of the devices to around 45 feet, which is acceptable for this use case, but not ideal. Here's what I did:

1. Ordered new revision of PCB following your suggestions, though I used 0603 components rather than 0402 because they are just too time consuming to solder given that I will have to manually assemble a number of these devices.

The EagleCAD files for this revision are here:

www.dropbox.com/.../AABi8F5Ng7ZkzUcc4L2nH6rra

2. Using a network analyzer and calibration kit, performed a 1-port calibration using short/open/50-ohm load standards.

3. Cut a short length of SMA cable and used it to calibration the cable length (port extensions) of the network analyzer using its automated length calibration process where you provide the cable open and short circuited and it does the math.

4. Soldered the SMA cable to the pads indicated in your last post. Here's a microscope image:

nixt.org/.../soldered_pads.jpg

5. Did an S11 reflection measurement on the board with markers at 2.4/2.45/2.525 GHz which resulted in these impedances:

nixt.org/.../initial_impedances.jpg

6. To tune the matching network to the middle of the range (2.45 GHz) I picked the impedance 62 + j8 ohms and calculated that a series inductor of 1.65 nH and shunt capacitor of 0.65 pF on the antenna side would get me to 50 + j0 ohms. The closest values in my 0603 design kit were 1.5 nH and 0.5 pF, which I soldered to the board and repeated the measurement, below:

nixt.org/.../matching_impedances.jpg nixt.org/.../matching_swr.jpg

7. The matching components brought me to 52.4 - j0.1 ohms which I reasoned was a good match, and so I assembled several boards to test. I did not have access to a spectrum analyzer so I could not determine the power output, but the range at 2.4 GHz was approximately 45 feet compared to <5 feet previously.

Is there anything else I can do to increase the performance at this point? Can I tune the Nordic-provided matching network to more closely match the antenna? Should I order more components to get a better match?

Thanks so much!

All the best, Andrew

[Øyvind Karlsen]

Hi Andrew,

Great job matching the antenna!

At this point I would recommend checking your design with our support team engineers for further verification and improvements, you can get in touch with them through the mypage support portal. They also offer assistance with tuning.