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" ?

[Hung Bui]

Actually it's simpler than you thought. You can simply make the length of the antenna longer than calculated and then cut it off gradually with a knife. But I agree, it's harder if you don't have a network analyzer to check. But you can use RSSI and number of packet missing to verify the strength.

We can also do radio tuning for you for free. However, you need to come up with a final design, and send us all the final product with housing.

Human body can really affect the RF performance as 2.4GHz is absorbed greatly by water.