Antenna and Filter Chain

[travisgoodspeed]

After the GoodWatch10 is verified, we'll need to extend the design with a proper antenna and filter chain in order to make the radio functional. We'll use this issue to track the design, then close it when parts have been selected and the filter chain has been integrated.

Self-assigning this issue, but you two are welcome to join.

[travisgoodspeed]

We need a chip antenna for something in the 430-440 range. (America's range is larger, of course, but we'd like it to operate in all ITU regions.)

Europe has no 33cm allocation, so 900MHz is not an option. I'd prefer not to be stuck on 433MHz because of interference, but will accept it as a last resort.

From Digikey's search engine, I'm afraid that these three antennas are our only options. Check the boxes as we measure the fit of each part, or edit this comment to strike out the offending part entirely.

712-1568-1-ND from Johansen Tech

The 712-1568-1-ND from Johansen Tech is 1.2mm tall, the thinnest of all available on Digikey in this band. It covers 423MHz to 443MHz, and is in stock. Its biggest drawback is the length, which at 25mm is nearly the entire length of the LCD.

25x5x1.2mm

https://www.digikey.com/product-detail/en/johanson-technology-inc/0433AT62A0020E/712-1568-1-ND/4770717

• [ ] 712-1568-1-ND Fits (God help us if it doesn't!)

311-1570-1-ND from Yageo

The second-thinnest chip antenna, the 311-1570-1-ND covers 419MHz to 447MHz, nearly the entire North American band. It is less than half the length of 712-1568-1-ND, comfortably fitting within the length of the watch.

12.3x4x1.6mm

https://www.digikey.com/product-detail/en/yageo/ANT1204F002R0433A/311-1570-1-ND/3927232

• [x] 712-1568-1-ND Fits

ANT-433-USP from Microsplatch

The ANT-433-USP looks like a good option, but it is out of stock at Digikey and past due for delivery. It covers 430-437MHz, which is three quarters of the desired band. Height (2.62mm) would be an issue with this part, and it might not fit at all.

https://www.digikey.com/product-detail/en/linx-technologies-inc/ANT-433-USP/ANT-433-USP-ND/3830738

Although the main part is out of stock, chips are available through the eval kit.

https://www.digikey.com/product-detail/en/linx-technologies-inc/AEK-433-USP/AEK-433-USP-ND/3830744

• [ ] ANT-433-USP Fits
• [ ] ANT-433-USP Available

[travisgoodspeed]

A proper reference design in two layers for the CC1101 in 434 and 315 MHz can be found as SWRR046A, then adapted for the CC430. While we wouldn't use it in a watch, a uFL connector might be handy for profiling and testing.

[travisgoodspeed]

Rather than the discrete implementation of a matching circuit, we'll be using an all-in-one matcher from JTI in datasheets/easymatching.pdf. Our antenna will be the Yageos chip antenna documented in datasheets/yageos432.pdf.

This combination will allow us 430 to 435 MHz with a minimal part count.

[travisgoodspeed]

Settled on the 311-1570-1-ND from Yageo as an antenna, with the Johansen 0433BM15A0001 as a balun to minimize part count. Will choose a crystal in the morning.

The layout will be a very tight fit. I'm hoping that the antenna can fit beneath the LCD without plastic mods, but we'll see how it goes.

[travisgoodspeed]

XT2 will be a 26.0MHz FA-128 crystal from Epson, digikey p/n SER4186CT-ND.

https://www.digikey.com/scripts/DkSearch/dksus.dll?Detail&itemSeq=240044490

[travisgoodspeed]

The 26.0 MHz FA-128 crystal expects a load capacitance of 12pF. Using the equations on page 88 of SLAS554H and assuming a parasitic capacitance of 2.5 pF, the loading caps for that crystal ought to each be 19pF.

(%i7) solve(1/(1/x+1/x)+2.5=12,x);

rat: replaced -9.5 by -19/2 = -9.5
(%o7)                              [x = 19]
(%i8) 

[travisgoodspeed]

I'm sure this still violates some best practices, but it'll suffice to get the watch on the air. Closing this issue, and I hope to order boards tonight or tomorrow.

--Travis