Dear all,
here is an update of this project. After mulling over the modulator design I decided to build both the VGA to composite video converter and the A/V modulator onto a single PCB. The design is SMD-based an will fit in an aluminium box roughly the size of a cigarette pack. Power options will be either USB or VGA, although I believe the latter will not work on most PCs. Here is the circuit diagram some values might still change slightly when I build the circuit:
I have included a low pass filter at the output in order to prevent hamonics from spilling into Band III where I have all my 625 channels. Band I will be used for exotic signals such as this one.
I have found a company that still offers to make custom crystals in small quantities, at 30€ each not a cheap option though. I have ordered two vision and two sound carrier crystals each for the French F3 Channel i.e. 56.15Mc vision, and 67.3Mc sound. They are scheduled to arrive in early July.
Today I finished the first revision of the PCB. This will work as a single layer, but I will use a double layer one where the top layer will be reserved for ground. Since this is an RF circuit it is always a good idea to have a large ground plane:
Weather permitting i.e. if it rains
over the weekend I will assemble a prototype and post results here.
Cheers
Semir