RONJA

RONJA (Reasonable Optical Near Joint Access) is a Free Technology (like Free Software) project consisting of an optical point-to-point Free Space Optics data link. The design is released under the GNU General Public License (you get all the necessary documentation and construction guides for free). The current range is 1.4km and the current communication speed is 10 Mbit/s, full duplex. Ronja is a project of Twibright Labs.

Models

Ronja 10M Metropolis: Provides 10 Mbit/s full duplex on a full duplex AUI interface. The distance is 1.4 km with 130 mm lenses and 700 m with 90 mm lenses. The transmitter is a 625 nm 70 mA LED diode and the optics are made from loupes. Material for one Ronja 10M Metropolis device costs 2000 CZK (about 85 USD) and building the device takes 70 hours.
Ronja Tetrapolis: Like metropolis but instead on AUI connects on TP (UTP, RJ-45 etc.) network card or switch.
Ronja Inferno: Like Tetrapolis, but infrared (invisible).
Ronja Benchpress: A measurement device for developers for physical measurement of lens/LED combination gain and calculation of range from that

Innovations

Technical

Preamplifier stage

Usual approach in FSO preamplifiers is to employ a transimpedance amplifier. Transimpedance amplifier is a very sensitive broadband high-speed device featuring feedback loop. This fact makes layout plagued with stability problems and special compensation of PIN diode capacitance must be performed, therefore doesn't allow selection of wide range of cheap PIN photodiodes with varying capacitances.

Ronja is using feedbackless design where the PIN has a high working resistance (100 kiloohms) which together with total input capacitance (roughly 7 pF, 5 pF PIN and 2 pF input MOSFET cascode) makes the device to operate with passband on a 6 dB/oct. slope of low pass formed by PIN working resistance and total input capacitance. Then the signal is immediately amplified to be removed from the danger of contamination by signal noise, and then a compensation of the 6 dB/oct. slope is done by derivator element on programming pins of NE592 video amplifier. A surprisingly flat characteristic is obtained. If the PIN diode is equipped with 3 kΩ working resistor to operate in flat band mode, the range is reduced to about 30% due to thermal noise from the 3 kΩ resistor.

Nebulus infrared LED driver

The HSDL4220 infrared LED is originally unsuitable for 10 Mbit/s operation. It has bandwidth of 9MHz, where 10 Mbit/s Manchester-modulated system needs bandwidth around 16 MHz. Operation in usual circuit with current drive would lead to substantial signal corruption and range reduction. Therefore Twibright Labs developed special driving technique consisting of driving the LED directly with 15-fold 74AC04 gate output in parallel without any current limitation. As the voltage to keep the nominal LED average current (100mA) varies with temperature and other component characteristic, an AC-bypassed current sense resistor is put in series with LED. A feedback loop measures voltage on this resistor and keeps it on preset level by varying supply voltage of the 74AC04 gates. Therefore the 74AC04 is operating as structured power CMOS switch completely in analog mode.

This way the LED junction is flooded and cleared of carriers as quickly as possible, basically by short circuit discharge. This pushes the speed of LED to maximum, which makes the output optical signal fast enough so the range/power ratio is the same as with faster red HPWT-BD00-F4000 LED. Side effect of this brutal driving technique are two: 1) the LD overshoots at beginning of longer (5MHz/1MHz) impulses to about 2x brightness. This was measured to have no adverse effect on range. 2) A blocking ceramic capacitor bank backing up the 74AC04 switching array is crucial for correct operation, because charging and discharging the LED is done by short circuit. Underdimensioning this bank causes the leading and trailing edges of the optical output to be reduced.

Organizational

The whole toolchain is strictly built upon free tools and the source files are provided for free under GPL. This allows anyone to enter the development, start manufacture or invest into the technology without entry costs. Such costs normally can include software licence costs, time investment into resolution of compatibility issues between proprietary applications, or costs of intellectual property licence negotiations. Decision to conceive the project this way was inspired by observed organizational efficiency of Free Software.

Ronja became on the Christmas 2001 the world's first 10 Mbit/s Free space optics device with free sources.

Examples of tools:

gEDA gschem (Schematic capture) ^*
QCad
BRL-CAD
The PCB program ^*
Sodipodi for Vector graphics

External links

ronja.twibright.com — Ronja Homepage
Gallery of Registered Installations

Wireless communications | Free software