Digital capacitors and NRF24L01+

[Joe Wright]

Hi, possibly a bit of a stupid question but here we go:

I'm developing a prototype system, which includes my own design of sensor modules that transmit their data via nrf24l01+'s to a base station. One is located in a racket handle and, while is generally working, sometimes suffers from radio link problems. I read that having your device in an enclosure, and interaction of the body (hand in this case) can throw the tuning off.

When I've finished my prototyping I will take along what I've done to some professionals to make it market ready, including antenna and matching network tuning, as I don't have the knowledge or equipment to do a really good job. However, I was wondering if I could do a better prototype for field testing, using digitally tunable capacitors to try improving the link. I was inspired by reading pages like:

www.psemi.com/.../digitally-tunable-capacitors-dtc

electronicdesign.com/.../back-basics-impedance-matching-part-3

My thoughts would be to arrange the sensor/racket in various positions, chuck a load of data out to test link reliability (and counting number of retries, lost packets, etc...), with automation to test different DTC settings.

So my questions are:

1. Would this work/be worth doing?

2. If yes, is there any reference circuit designs I could look at that would be suitable for using the DTCs?

I'm thinking of including an RFX2401C for PA/LNA, and currently I'm using a chip antenna (Wurth 7488940245). In the racket sensor I could use a much longer antenna but I don't know what would be suitable.

So far I've been following as much as possible the Nordic reference designs.

Thanks for any help, Joe

[Hung Bui]

Hi Joe,

1. It would possibly work. But I guess you are using a PI tuning network here ? What if the tuning require one of the component be a inductor ?

2. No we don't have one. I would suggest you to try using a PCB antenna instead, because it's easier to tune by simply use 1 shunt component and the length of the antenna as the 2nd factor to tune. We provided a tuning guide here.

Side question, how is the radio performance currently on your board ? How bad is the "radio link problems" ?

[Joe Wright]

Thanks for the reply.

1. Yes I'm using a pi network. I spotted the issue of the inductors as well but wondered if another network could be used. It seems the dtc's are being promoted for tuning so I wondered if it could be done.

2. Wouldn't length of PCB antenna tuning require getting more PCBs manufactured? I get my PCBs from China so it would be a lengthy process. Also I don't have network analyser. Really all I can do is digital testing by sending a load of packets and measure number of retries, etc... If I had a digital method of altering the tuning and matching then I could automate this process. Otherwise, I guess I'll have to wait until I can hand over to someone to do the final tuning/matching for me....