UPDATE: February 25, 2014: The RADARduino has received a commodity classification from the Department of Commerce as 6A008.b. Since the design itself will be open source, all of the technology will be freely available. Check back here for more details soon.
It has been some time now since the last post here about the RADARduino. Back in November I wrote an entry on the RADARduino prototype build. Since then a lot has gone on here at Reactance Labs. We have managed to get all 3 main functions of the RADARduino tested out and have successfully produced plots of Doppler shifts from moving objects (cars), a plot of range vs. time for a moving target, and taken data (still working on processing it) for Synthetic Aperture Radar.
Along the way there have been lots of lessons learned and from those I have revised the original prototype board and am now waiting for the next version to come in the mail, with assembly slated to begin in the next couple weeks.
For this test I built up a test housing using a small metal enclosure I bought at the local ham swap meet a few years ago. I machined an antenna mount from aluminum and attached it to the front using a dovetail slide. This allows me to easily swap the antennas to the stepping linear rail carriage described later.
In Doppler mode, the voltage controlled oscillator (VCO) puts out a continuous tone at a fixed frequency. The reflected signal is a frequency shifted version and speed can be determined. We used some modified Python scripts from here. The routine generating the plots isn’t polished yet, but the result makes sense.
In the image below, each red trace corresponds to a passing car. As the car approaches, the measured velocity is at it’s peak (and the correct value). As the car passes, the angle to the car rapidly reduces the effective Doppler speed making it look as though the car is very rapidly decelerating.
Doppler mode offers the most instantly gratifying experience with the RADARduino because you can actually hear the result in real time. Setting the radar on the dash board and plugging the output into a car stereo auxiliary input is pretty interesting. At a standstill you can hear cars passing. When driving, you can hear nearby obstacles as you pass them. It sounds much like a racetrack with high revving race cars passing by.
Another trick with Doppler mode is using it for surveillance. In the very rough demo below, I recorded music by pointing the antennas at a metal cone speaker.
The second function to test is ranging. In this mode, the Radar operates in chirped mode. The VCO output frequency is linearly increased from 5.4 to 6.0 GHz. It does this every 20-40 msec. A sync pulse is generated by the Arduino (which also commands the DAC on the RADARduino) to alert the Python script to the beginning of the chirp. The code then computes the range to each reflection and plots this over time. In the image below, I carried a home made corner reflector and walked away from the radar and then back towards it. The plot shows my distance increase, then decrease.
It should be noted that this graph (while poorly scaled) shows the range of the first prototype RADARduino using the low gain Vivaldi antennas. As the range to target increases to maximum, the signal level diminishes substantially. A circuit change made in revision 2 addresses this issue.
Synthetic Aperture Radar
Unfortunately we have not yet processed the SAR image from this test, due to issues with getting the Python script working right. However, we believe we have good data and are quite close to getting an image. I’ll update this entry as that happens. Our test setup for this experiment uses a motorized rail system that I built. A 9 foot long section of rectangular aluminum tubing serves as the rail, and a carriage riding on small bearings holds the RADARduino and antennas. The carriage is moved by means of a stepper motor directly driving a rubber R/C car tire which presses against the rail.
The setup was done outside my apartment, and we got lots of interesting looks and some questions from the neighbors. Some thought I was building a death ray. I won’t challenge that I stood out in the street holding the corner reflector to act as a really huge target. The radar cross section (RCS) of a corner reflector is ginormous. I will post the SAR image as soon as we have it available!
Updates for Revision 2
A number of items were improved on the second board spin. I am currently waiting for that board to come in, so no promises on what works yet
- The on-board SMT boost converter has been replaced with a single through-hole part. This reduces noise and complexity with a slight increase in cost
- Precision voltage reference for DAC now supplies reference for the IF amp. Dramatic improvement in noise floor (no USB, digital, boost converter noise).
- DC blocking cap between IF port of mixer and IF amp has been increased (error)
- DC blocking cap has been added between IF amp and anti-aliasing filter
- Trim-pot has been added to the IF amp. Gain adjustment was found necessary between Doppler and Chirped modes. Future incarnations may feature digitally adjustable gain or AGC.
- A resistor has been added in series with the sync pulse output to allow user adjustment should it be required.
- The sync pulse inhibit switch has been removed. This can be easily implemented off-board should the user want it. Given the digital control, it seemed outdated.
- A copy of the RF gain stage has been added as a transmit driver. This will increase the output power from ~4 dBm to ~15 dBm which should definitely help increse the range of the unit. The cost is a bit more DC and a few more dollars in components, but will be worth it!
- Other minor changes including more test points and component swaps have been made.
Coming soon, an airborne demo!