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]

Performance is variable at the moment. The first prototype was ok, second is a bit more flakey. There's been some changes in the design but I'm not sure how to explain the difference. However, it was tested in a different environment so maybe there were interference issues. For the next field test I be doing some channel scanning and better selection of channels (previously I just had the option of 2 different presets in my firmware).

I have also now got a receiver with a better antenna so that will help. Performance still is an issue in my workshop if I hold the enclosed sensor behind my back but I'm not sure how relevant that is.

Thanks, Joe