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PCB: first try

A project log for Robot Radar Module

60 GHz radar distance sensor for your robot

Ted YapoTed Yapo 04/27/2018 at 01:152 Comments

I designed a 32x32mm breakout PCB for the A111 sensor today.  The datasheet says you need a solid ground plane on the top layer.  I did the best I could using the OSH Park 2-layer rules.  The reference PCB sold as a development module has a really unbroken ground plane - they must be using via-in-pad, which isn't an option here. 

The datasheet states that the plane is required as a reflector for the antennas.  I suspect this plane shields the signal traces from picking up the 60GHz signal, too.  60GHz has a wavelength of 5mm in air, so a resonant 1/4-wave antenna is only 1.25 mm long.  That's shorter than the length of a via (1.6mm) on this board!

There are a few traces on the top side.  They all look like great antennas at 60 GHz, but I'm hoping I can get away with it.  Even the 2mm spacing between stitching vias at the board edge is way too huge for 60 GHz - they're 2/5 of a wavelength apart.  They're really there for the lower-frequency signals on the board.

There are also three places where I had to cheat on design rules - 6/6 isn't small enough to bring one of the traces out of the BGA.  I ended up with a short segment violating the width and spacing on both sides, but I'm hoping that I'll still get a workable board.

Only the radar module itself is on the top; the rest of the components are on the bottom.

The circuit is a basic breakout board.  A 1.8V regulator supplies power to the A111, while 3x 74LVC2T45's translate the digital interface from 1.8 to external logic levels between 1.8-5V for maximum flexibility.  I'll almost certainly just use it at 3.3V, but it's nice to have the option so early on.

This board really is just a quick test so I can get some mileage on the part, find any issues quickly, and start playing with software.

For the final sensor module, it looks like it makes sense to integrate a cheap MCU on the board, but the details have yet to be worked out.

Now I wait for parts and PCB to arrive.

Discussions

deʃhipu wrote 04/27/2018 at 07:56 point

I wonder if the thickness of the PCB will have an impact here too. Do you plan to use the standard 1.6mm?

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Ted Yapo wrote 04/27/2018 at 12:46 point

The first proto is on 1.6mm mainly because I can have it in my hand in about a week.

If the top plane were really continuous, the circuit on top couldn't tell how thick the PCB was.  One of the speakers at ESC noted that there's around 200dB loss through a solid copper ground plane.  Since there are some holes, it may make a difference, but I have no idea how much.  Hopefully, the chip itself is designed right - there's little I can really control about 60GHz signals on a 6/6 FR4 PCB.

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