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

[Andrew S.]

Great! Thank you for taking a look. If I add a Pi network to the layout as in Figure 12 of the antenna tuning guide (two components in parallel on either side of a third component), won't the center component's length affect the path length from the matching network to the antenna? Is that path length critical to obtaining good performance?

Also, to be sure I understand the antenna tuning procedure, I should do this:

1. Assemble a board with all components, including the chip antenna, excluding the nRF24L01+, matching network, and pi network.

2. Short circuit the pads of the series components leading from the nRF24L01+ to the antenna.

3. Calibrate VNA with SMA cable length being used.

4. Solder center conductor of SMA cable to the pad leading from the nRF24L01+ ANT2 pin, solder mesh to the ground plane.

5. In a single port measurement, use VNA to find power/phase/impedance reflected back from antenna from 2 - 3 GHz and especially at 2.4, 2.45, and 2.5 GHz.

6. Calculate pi network components to tune antenna to 50 ohm, solder to board, and repeat measurements and change components until desired impedance is reached.

I was also wondering--the chip antenna in this design, according to the datasheet, claims that it does not need a matching network and is already tuned to 50 ohm impedance. Is the idea that the nRF24L01+ matching network provided does not actually deliver 50 ohm impedance in real-world applications and the pi network is necessary to do further matching? Otherwise, isn't additional tuning unnecessary?

Thank you!

All the best, Andrew

[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