DATV-Express Project – August update report

[KenW6HHC]

Art WA8RMC had a busy month on the project. He identified the +/- 1.34 MHz switching-power-supply spurs in the RF were actually AM modulation of the RF buffer amplifier due to small switching ripple in the 5.5V power supply. The addition of a ferrite filter at the RF Buffer and some addition bypass caps on 5.5V bus cured the spurs. Art has also spent a full month trying to get the software to work correctly with his board. It took two weeks of frustration to identify a bad USB cable that would work OK with other devices and OS. Then the DATV-Express software was loading and running and transmitting, but no video??? Art has tried a dozen things including buying a new notebook computer to resolve the issue. The team is now narrowing the problem resolution down to the need for configuring the DATV-Express software to use the video1 device, not video0 on Art’s computer??

Tom WB6P has been kept very busy during the month of August. First the team did a design review of the final list of changes that needed to be done to the etch for pre-production. The changes included:
• rotating the DAC chip 90 degrees to minimize detected IQ phase shift in the IQ modulator
• adding some additional filtering to the power source for RF buffer amplifier
• moving back some inner etch from the edge of the board
• adding the CE Mark to silkscreen to declare flammability compliance for Europe.
The next step was to have a team design review for changes that were implemented in the schematic-capture file. Finally, Tom began to make the etch changes to the etch layout file. Tom completed the entire PCB/PCBA package for review this past week.

Etch Layout Tool showing I etch and Q etch being made equal trace lengths after rotating the DAC chip by 90 degrees

Charles G4GUO ran a number of RF power output measurements on different ham bands to assure that the hardware design would support the full range of the DATV-Express capabilities. The results below measure the carrier power output:
• 70 MHz 16.54 dBm
• 144 MHz 17.70 dBm
• 420 MHz 17.97 dBm
• 900 MHz 17.65 dBm
• 1320 MHz 17.62 dBm
• 2450 MHz 15.20 dBm
Charles also determined that an occasional board power-up issue was caused by the I2C communications occasionally hanging. So the FPGA code was tweaked to improve the I2C communications and also to improve the filter math (added extra interpolators) that is in the FPGA. This was put into the Debian v1.03 package. Finally, he added two indicators to the GUI screen that show whether video and audio elementary streams are being captured by the capture device.

Ken W6HHC had an opportunity to provide a presentation and a demo of the prototype DATV-Express transmitter at the local club, Orange County Amateur Radio Club. Then he used the DATV-Express to transmit a DVB-S signal that ended up being Skyped to Australia for uplink during the VK3RTV (D)ATV QSO Party. After the QSO Party was over, Ken then installed the Debian v1.03 package at his QTH to assure there were no unusual problems…and eliminate Debian v1.03 as somehow being the source of Art’s video problems.

The team is now focusing on the design review of the new layout package and releasing the design files to pre-production (maybe ten boards or so).

“full speed ahead”…de Ken W6HHC