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Active GNSS Antenna

A project log for Asset Tracker

STM32L433-based board with CAM M8Q concurrent GNSS, EM7180+MPU9250+MS5637 for absolute orientation, and an ESP8285 for wifi connectivity.

kris-winerKris Winer 10/25/2017 at 18:024 Comments

The CAM M8Q is the heart of the Asset Tracker and what a great GNSS engine it is. Even though I am using the minimum recommended pcb board dimensions the chip antenna on the CAM M8Q is really superb. I have been able to track cars around town with little trouble and even get 3D/DGNSS fixes at my workstation indoors, albeit after a ~20 minute wait.

Still, there are many applications where good enough really isn't, like keeping track of vehicles in a crowded construction site, or when the Asset Tracker is inside of a container, etc. Even in my tests so far, having to wait twenty minutes for a fix is a real power waster. It is best to be able to keep the CAM M8Q on for as short a time as possible to get a fix, and to ask for a fix a very few tmes a day to have any hope of keeping the Asset Tracker going for a year or more on a LiPo battery, which is the project goal.

For these more practical cases, I need to have a means to increase the sensitivity and what better way than an active antenna. The active antennas themselves are cheap and easy to use, the hard part for this project was figuring out how to allow either use of the chip antenna or an active antenna without having to solder 0 Ohm jumpers or otherwise fuss with the board. With the help of the u-blox forum I found an elegant and effective solution, which I will now describe.

The solution requires use of an RF switch which also has an embedded LNA (low-noise amplifier) that allows automatic detection and RF path switching when an active antenna is attached but otherwise routes the chip antenna output to the CAM M8Q RF input. There are a few of these switches around, some embedded in SMA connectors; I chose the MAX2674 (actually recommended by u-blox) because it is small, cheap, and easy to use. It is a 2 x 3 WLCSP (SMD wafer-level chipscale package) so some surface mounting experience is required to make use of it. Here is the schematic for the MAX2674 that I used (basically a copy of the reference design, of course):

There are 100 pF DC blocking capacitors at the CAM M8Q ANT and RFIN pins; in addition, there is a matching 88 nH inductor in series with the DC blocking capacitor for the CAM M8Q chip antenna. I could only find 91 nH inductors but these work just as well. I am not sure how sensitive this choice is but it is a little concerning Maxim chose to specify 88 and not 80 - 100 nH, for example. I will try to find an 88 nH inductor. I have noticed it is often the case that passives are specified in reference designs that can't be found by mere mortals!

The MAX2674 power pin is bypassed by a 10 pF capacitor. The RF switch and LNA require power, but the MAX2674 also powers the active antenna which can draw quite a few mA, so this adds to the average power usage and somewhat offsets the shorter time to acquire a fix. Lastly, there is a shut down function that can cut power to the MAX2674 to 10 uA. In this design, I have connected the shutdown pin (via a 25 K resistor and 39 pF DC blocking capacitor) to the CAM M8Q LNA_EN pin. The LNA_EN pin is set to HIGH (3V3) when the CAM M8Q is in active mode and is set to LOW (0 V, I added a 100 K pulldown on LNA_EN to be sure) when the CAM M8Q is in power down mode. This ensures the MAX2674 will be powered off (and use 10 uA) even when an active antenna is attached as long as the CAM M8Q itself is sleeping.

On the Asset Tracker, board space is tight, but this solution requires a very small footprint:

It's not great to have 90-degree turns in an RF feed but this is the only reasonable way I could find to make this solution work. At the bottom of the image is the CAM M8Q. Left edge in the middle is the shut down passives and LNA_EN connection. The MAX2674 power pin bypass capacitor is between the CAM M8Q ANT and RFIN traces. The active antenna connects to the uFL connector using an uFl-to-SMA cable adapter that can be mounted on the side of the container for the Asset Tracker, providing a clean and compact solution.

How does it work?

Well, with just the chip antenna, it takes ~20 minutes from cold start to get a 3D fix when the Asset Tracker is connected to my laptop with a USB cable and an FTDI connector and sitting between the laptop and the window next to my workstation (indoors, of course!). This is great performance for a 20 x 45 mm pcb.

With the active antenna connected, it takes ~2 minutes from cold start to get a 3D fix and <4 minutes to get a 3D/DGNSS fix with HDOP < 1 meter.  Both solutions see about the same number of satellites, it's just that the signal strength without the active antenna is about 10 dB lower overall.

So adding an active antenna makes a very significant improvement for just a few dollars extra BOM cost. It is really great to have the option now depending on the use case, but I can see I will get spoiled pretty quickly with this kind of performance!

Discussions

Kris Winer wrote 04/02/2018 at 21:46 point

I can sell you ten for $4 each plus shipping. If interested please send me an e-mail to tleracorp@gmail.com

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Kris Winer wrote 04/02/2018 at 18:16 point

I bought them from Maxim. How many are you looking for?

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Sindre wrote 04/02/2018 at 21:32 point

Ah ok, I see that minimum order qty is 2500 from Maxim. 10 at first. 

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Sindre wrote 04/02/2018 at 18:14 point

Hi! Where did you buy the MAX2674 ? I usually buy my parts at Mouser or Digi-key but they are sold out... Seems not many distributors sell this component! Great sharing btw :-) BR, Sindre

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