Thursday, May 23, 2013

GPS Noise Isolation Part 2

Experimental opto-isolator board on Data Bus
I'm now using a opto-isolators on my GPS to mbed connection, a much better solution than the chokes I used to stamp out RF interference previously.

Opto-isolators, for those that don't know, are an IR LED and IR Phototransistor (or similar) housed in a single package, in this case a 4-DIP. They appear to provide good noise isolation among other benefits.

The inputs and outputs are on opposite sides of the board with nothing connecting them. I added filtering caps to further filter the power supply.

Simple opto-isolator circuit
Opto-isolators have fast rise/fall times, so they can support high data rates. I think the chokes may have been interfering with communication from the mbed to the GPS.

'a' at 38400 bps
Also, the board is much lighter and smaller than the two giant chokes that kept flopping around all over and pulling themselves loose from the GPS module.

As I write this, the uBlox is tracking 9 satellites in my basement with a horizontal dilution of precision (HDOP) of 1.0, that is, really good. The absolute accuracy is several meters. Not bad for sitting in the basement.

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