I am designing a bluetooth low energy device. It basically reads a sensor over I2C and collect data. You can check data in your phone whenever you want. I have two questions, one related with antennas and another one related with battery.
first of all, this is my layout so far
It is based in this schematic
IC2 is B1 from schematic, IC3 is X1. The left part is my specific sensor.
My questions are: First, the one related with antennas. Since this is a homemade project I do not have a vector network analyzer so I can't tune the antenna. However, I found this antenna from Texas Instruments:
http://www.ti.com/lit/an/swru120b/swru120b.pdf
They claim its impedance is 50 ohm, so could I just attach its 50 ohm feed point to my 50 ohm output from the balun? with a extremly short wire so I don't add extra resistance or inductance (or as low as possible)
Second question is I need it to be battery powered and small. I went for a CR2032 battery. However I checked its current specifications and it says 4mA maximum discharge current. I need around 20mA in some moments when the radio is working. I thought I could add a large capacitor (C14) so it can provide higher current for a short time and then it will be charged again when the system is sleeping. It is possible?
Thank you.
[EDIT]
I added a pi-network just in case. My layout is this now
and this is the schematic
I'll use a 0 ohm resistor as R3. R4 and R5 are not mounted initially. This would allow to eventually tune the antenna, but it won't be easy since I don't have lab equipment. I am wondering if this would work without tuning?
Best Answer
First, referring to your existing design, the important thing about a PCB antenna is to have no ground plane under the entire antenna, and to follow the layout shown in TI's application note exactly. It's not a bad idea to include the Pi network, it may be useful for during RF testing.
You should be able to continue the 50 ohm feed for a little ways, as long as it is surrounded on both sides by a ground place as shown in the app note.
Re the 2032 battery, they are designed for a pulse drain of 15 mA, although they can go higher, say 30 mA, with a minor (9%) drop in capacity. So you shouldn't need a large cap (although it wouldn't hurt).
However I suggest switching over to a BLE module that has a built-in antenna, like the BLE113 from Bluegiga. It includes an 8051 which you can use to run your own application code, and has SPI, I2C, UART, and 12-bit ADC interfaces.
Even though the maximum transmit power is 14.3 mA, this is for a very brief time, and the BLE113 is specifically designed to work with a CR2032 battery -- in fact their development board includes a holder for a CR2023 for testing that configuration.