Tuesday, June 17, 2008

Homemade GPS Antenna

I needed a very small, lightweight passive GPS antenna for an ET-301 USGlobalSat GPS module, but didn't have anything to use on-hand. I started some research to make one but information on this topic is very limited... so I figured a good option would be to make my own. As far as passive antennas go, there doesn't look to be very much too them. I looked at lots of pictures of antennas online and reverse engineered them as best as I could. The result was an antenna made out of a thin piece of metal that is 24mm x24mm located 2mm over a 30mm x 30mm ground plane.

Now i'm not an RF engineer, and basic antenna math fails me on this antenna design (1/4 wavelength at a gps frequency on the L1 band of ~1.5Ghz would be 50mm) which is much larger than the antenna I designed mine after... but upon power up I had GPS lock.

So it works... which really shows how sensitive modern GPS receivers are. This is really amazing since this module is sitting inside, on my bench, located about 6 feet away from the nearest window... and I made the antenna by copying antenna designs I found online. The NMEA $GPGGA string shows that there are only 4 satellites in view (which is expected since I am indoors) so I am really looking forward to testing it outside.

At some point I hope to find a good resource for GPS antenna designs to really make a proper antenna the right way, but for now it works... and I am still impressed by the sensitivity of this receiver.

Tuesday, June 10, 2008

Cooperative multitasking

Sitting down this past weekend for a large amount of time to begin a new project (details will be provided in the next post)... I discovered an amazing feature with Rabbit Semiconductors microcontrollers. Cooperative multitasking. I have actually always known about the multitasking capability of these processors, but never actually used them in any real form.

Cooperative multitasking unlike preemptive multitasking has many benefits. For one, variables can be easily shared among different tasks. This simplifies the necessary code needed immensely as you don't need to take any necessary precautions while sharing variables in a typical interrupt driven preemptive environment. Cooperative multitasking also allows many tasks to be run at once (as they only appear to... time slices are actually given to each function running just like in any modern multitasking OS). The microcontrollers also take advantage of the natural delays that occur in most code execution to provide cpu cycles to other tasks.

Playing with this multitasking environment I was able to update information on an LCD via an array of switches in real-time, without taking any time away from any other running timers or processes. It essentially allows me to run several tasks at the same time while providing data input without the necessity of any type of interrupt. This is extremely powerful.

I have nearly finished the code for the current project tonight, I hope to have the project finished by the end of the week as an update to what this project is will be given upon completion.

Sunday, June 1, 2008

Clean Bench :)

There hasn't been an update for awhile, but I have some projects in the works. This weekend I finally had time to clean my bench so there will be some updates soon...