The Iris Transmitter project was originally inspired by the fantastic work of a number of members of the #highaltitude IRC channel on Freenode. In particular, in thanks to the inspiration and consultation with KT5TK.
The goal of the project was to design and construct a rather small and lightweight transmitter capable of sending APRS formatted AX.25 Packet over amateur radio, primarily on the 2m band, but frequency agility being a plus.
Revision one was of course, the simplest in design, and least in ambition, however, still met the goals. The processor is an ATmega328P, which talks to a ADF7012 PLL synthesizer to generate the 144.39MHz signal. The signal is modulated by means of a VCXO acting as the reference for the PLL. Total output power was brought up by means of an RF Fet amplifier stage, to a level of approximately 250-300mW. The board required a power supply of a regulated 5V input, and included a linear regulator onboard to supply 3.3V to the ADF7012.
Revision 1.1 built upon the successes of Revision 1, and improved on some of the weak points. Revision 1.1 included a boost regulator to generate the 5V needed for many aspects of the board, operating from 2 AA batteries, RF output filtering was improved, and total board size was reduced.
Revision 1.2 was somewhat more of a re-architechture from the 1 and 1.1 revisions. Revision 1.2 brings an entirely new radio chip, the SI4464, replacing the ADF7012, which was prone to output power variations based on which batch the chip was from. The main processor is now operated from 3.3V instead of 5V, which allows for the boost regulator to deliver 3.3V and provide more efficent power regulation. A uBlox MAX-6 GPS device and chip antenna are added to the board to remove the need for an external GPS to be attached for positioning. The overall board size was again reduced, and to save power, the extra RF amplifier stage was removed. The board now runs happily from one AA battery for more than 18 hours of flight tracking.
Plans for Revision 1.3
Revision 1.3 is planned to bring radio receive capability to allow remote commands to be sent. It is also planned to include a switchable RF amplifier stage to allow for more power to be used when necessary, but to bypass and save power when not required. This will require refactoring the boost regulator to be able to support the larger power draw of the amplifier. Additional reductions in board size would be welcome.