Sunday, June 30, 2013

DJI NAZA GPS - PepsiCola's SAW filter mod

<WARNING>

I no longer recommend using DJI flight controllers or equipment. There have been too many flyaways, too many crashes, and DJI has done little to look into the problems or fix them. I'm now pretty certain there is a serious hardware or software bug in their flight controllers, and although it doesn't strike often (perhaps 1 in 100 or so flights), when it does strike, you will have no control over your aircraft, even in manual mode.

Given all that, I will no longer be supporting the SAW filter mod for the NAZA GPS. If you decide to use DJI stuff, you are on your own.

Instead, I recommend using one of the many excellent alternative flight controllers:
  • The Pixhawk, PX4 or APM from 3D Robotics: http://store.3drobotics.com/. These excellent flight controllers get better every year. This is what I fly now (I have a PX4 with PX4IO, and the old V1 GPS). I use the free, open source ArduCopter software, which is very actively maintained and gets updated frequently. There's a large community of helpful users and developers using and building this system. It's also far more powerful than the NAZA, offering great features like waypoints. It has an "autotune" feature to take the hassle out of gain tuning. Soon it will have a "hybrid" mode that flies just like NAZA's own GPS mode, perhaps better.
  • AutoQuad: http://autoquad.org/. Expensive, but very good.
  • OpenPilot CC3D or Revo (not quite ready for prime time yet).

</WARNING>

This is the official guide to my SAW filter mod to the NAZA GPS. This mod works for all the NAZA GPS units (including the Wookong). It was originally developed and tested on the NAZA-M GPS and might need some small changes for the other units.

Note that at least some of the DJI Phantom GPS modules seem to already have input filtering circuitry, so if you have one, open it up and check your circuitry before you order your filters.

This mod solves the problem many of us have been having with these GPS units: we don't get satellite lock when the unit is close to our VTx or noisy electronics like the GoPro. With this mod, I have my GPS directly next to the antenna of my 1W 1.28GHz VTx, and I still get more than 7 satellites solidly locked throughout my flights. :-)

Total cost: $2.08 + shipping

Time required: 1-2 hours

Warning: this mod requires excellent soldering skills - the filter is a very small and temperature sensitive surface-mount chip. You'll need a magnifying glass and one of those "helping hand" tools. I spent a full hour just on the soldering.

Background

Civilian GPS operates on 1575.42MHz ("L1") with a bandwidth of a few MHz. Most modern GPS receivers incorporate a SAW filter ("surface acoustic wave") and an LNA ("low noise amplifier") between the antenna and the main GPS chip. These filter out out-of-band radio frequencies and boost the signal-to-noise ratio of the signal.

Unfortunately, DJI decided not to add any filtering in the NAZA-M GPS, even though it is intended to be operated in an environment where there is strong radio interference present. As such, the antenna is connected directly to the RF_IN pin on the GPS chip, which is a uBlox NEO-6. The antenna does not have good frequency selectivity and readily picks up other frequencies too - and if those other frequencies have high power, they totally overwhelm the RF input on the NEO-6 and the chip can't lock on to any satellites. See this post for more info, and check out this screenshot.

Indoor satellite reception with my 1.28GHz 1W Lawmate VTx turned on 30cm from the GPS unit, before the SAW filter mod. The unit is so overwhelmed by noise, it thinks it is getting signals from every satellite at once, and of course none of them are any good.
Parts

Almost any SAW filter designed for GPS frequencies will work. I happened to use the EPCOS B39162B9080L310, which is available from Digi-Key for $2.08 plus shipping. I got two just in case.

Digi-Key link: http://www.digikey.com/product-detai...9-1-ND/1858965

Datasheet: http://www.epcos.com/inf/40/ds/mc/B9080.pdf

This filter has 55dB+ of attenuation below 1.4GHz, 50dB at 2.4GHz and 15dB at 5.4GHz, and has excellent low insertion attenuation of 1.2dB. To put this in perspective, 50dB attenuation makes a signal from a 1W TX look equivalent to the signal from a 0.1mW TX.

Instructions

These instructions are for the NAZA-M GPS and might need to be modified slightly for the other GPS units.

Note that the uBlox chip and SAW filter are both ESD and heat sensitive. Use a grounding wrist strap (or frequently touch something grounded), and keep the soldering iron contact time to a minimum.
  • Remove the screws and pry open the GPS unit carefully using a screwdriver or your nails. There is little or no glue holding it together, but the two halves do click together, so it'll take some pressure.
  • Carefully cut the trace between the antenna solder blob and the RF_IN pin of the uBlox (pin 11, second from the bottom). Make sure you don't accidentally short the RF_IN pin to ground when cutting the trace - use a knife to remove any stray copper and tidy things up carefully. Use a multimeter to check that there is no connection between the antenna and the RF_IN pin or the RF_IN pin and ground.
  • Find a place to put the SAW filter. The case is connected to its ground pins, so make sure it's sitting either on the ground plane or on something insulated; I used a tiny bit of double-sided foam tape to help hold it in place. Place it with the pins facing towards you.
  • Connect the antenna solder blob to pin 1 of the SAW filter (look for cut corner on bottom of SMD package). Use a very short length of wire to do this - the impedance and capacitance of the circuit matters a lot, and the shorter the wires the better. I used 2-4mm lengths of stripped wire wrapping wire.
  • Connect the diagonally opposite pin of the SAW filter (pin 3) to the RF_IN pin of the uBlox.
  • Connect either of the two remaining pins of the SAW filter (pins 2 and 4) to ground. I scraped off the green from the ground plane next to the filter to provide a nearby ground that I could get to without a long wire, but you could also connect to either of the two pins adjacent to the RF_IN pin on the uBlox since both are grounded.
  • Double check your connections using a multimeter, and inspect your handiwork carefully with a magnifying glass.
  • Place some hot glue or liquid electrical tape over and around the SAW filter and your wiring to hold everything together. The copper pads on the SAW filter aren't very well bonded and will easily come off otherwise (I'm speaking from experience!).
  • Put the plastic cover back on the GPS unit and replace screws.
  • Note: I don't recommend adding any copper shielding inside the GPS unit when doing this mod - it can easily short out a connection. If you do add shielding outside the unit, which should be unnecessary anyway, make sure it's connected to ground, otherwise it won't be effective. Also, only add shielding below the GPS unit, otherwise you'll block the antenna.
Testing

If all is well, you should still be able to get a full 7 satellites locked with your NAZA, when outdoors, within 30-60 seconds. If you want to be certain everything is working, you'll need to hook up an FTDI serial cable as per ehx's instructions and download the (very good) u-Center software to check the signal quality. Look for the SVINFO message under the UBX protocol node in the messages view. You should see several (more than 7) satellite locks with C/N0s in the 30-50dB range and quality scores above 5, with hopefully more than a few 7s.

If you still get three red flashes after more than a few minutes, check for loose connections.

Indoor satellite reception with the VTx turned off, after the SAW filter mod.

Indoor satellite reception with my 1.28GHz 1W Lawmate VTx turned on, after the SAW filter mod. Woohoo - there's virtually no interference whatsoever!

Video

Wrice4@RCGroups made this nice little video showing how to do the mod. Thanks Wrice4! Note: I definitely don't recommend adding copper shielding inside the GPS unit, so if you watch this video, ignore the shiny stuff inside the plastic cover.



Notes

After adding mod, it is not necessary to put any copper shielding around the GPS unit. However, it still makes sense to avoid mounting it close to noisy electronics or to metal, if only for the sake of the compass.

I'm guessing performing this mod will void the warranty you have on your GPS, so be careful!

To DJI

If any folks from DJI are reading this, I would humbly ask that you add a SAW filter and LNA to your next generation GPS units, as per the recommendations in the uBlox hardware integration manual. They're cheap and effective, and your competitors are ahead of you in this (e.g. 3D Robotics).

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