RONJA (Reasonable Optical Near Joint Access) is an Free Technology (like Free Software) project of 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 free). The construction costs are minimal; it s probably the cheapest wireless system ever. Current range is 1.4km and current communication speed 10 Mbit/s full duplex. Ronja is a project of Twibright Labs.

= Models =

= Innovations =

== Technical ==

=== Preamplifier stage ===

Usual approach in Free Space Optics 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 (100kohm) which together with total input capacitance (roughly 7pF, 5pF PIN and 2pF input MOSFET cascode) makes the device to operate with passband on a 6dB/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 6db/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 3kOhm working resistor to operate in flat band mode, the range is reduced to about 30% due to thermal noise from the 3kOhm resistor.

=== Nebulus infrared LED driver ===

The HSDL4220 infrared Light Emitting Diode is originally unsuitable for 10Mbps operation. It has bandwidth of 9MHz, where 10Mbps Manchester encoding-modulated system needs bandwidth around 16MHz. 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 semiconductor junction is flooded and cleared of charge carrier 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 GNU General Public License. This allows anyone to enter the development, start manufacture or invest into the technology without barriers to entry. 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 10Mbps Free space optics device with free sources.

Examples of tools:

= External links =