I need to do more tests with the 27MHz VCXO and divide by two flip-flop configuration, though there is a few issues that needs to be look into:
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the VCXO is specified for 3.3V power supply, it seems to withstand 5V and work as expected, however the nominal frequency still requires 3.3V/2 at the VC pin even with 5V supply. Having the pull-up/pull-down resistor modification as discussed in this thread Cadet RGB Encoder + BMD Mini Analog->SDI = :-( - #10 by syntonie does help with BM Analog to SDI picking up the signal, though it messes up a bit with the genlock. A switch as @rempesm did in the mentioned thread and @creatorlars suggested above would work better, as using the phase pulse out of the PLL as I did isn’t ideal.
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The addon board is mounted using a 2x3 smd header, so it asks to solder it to the Cadet board in place of the 13.5MHz VCXO and then solder the addon board on top of it, which makes it hard to remove if there is an issue.
I was mostly thinking of a way for DIYers who already ordered some C1 boards to be able to use it with a 27MHz VCXO, however, I’m not sure if there is a real need for this addon board, and going for a redesign of the board as @reverselandfill suggested would make more sense. Here is my C1 with the two addon boards:
I guess there is a few different options as Lars summed up:
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Keeping the existing MCU and code and replacing the 13.5MHz VCXO with the 27MHz one, adding the divide by two flip flop, and pull-up/pull-down resistors (either manually switched in/out or using an active switch controlled by the PLL as I did). I’ll get back to my modified C1 soon and see if I can tweak the pull-up/pull-down resistors in order to keep the right resting frequency without messing too much with the genlock. Would also ask to use a few SMD parts to keep the board at a reasonable size
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Replacing the MCU/re-write the code and run it with the 27MHz clock: I guess going from an AVR to an ARM would allow to work with a clock higher than 20MHz, though it would ask to re-write the ASM code accordingly, haven’t worked much with MCU beside a few simple Arduino project, and don’t know about ASM either
Then, I’m might be wrong, but ARM chips only exist in SMD package (?) -
Going for an FPGA based sync gen, with iCE40/iCE5, though once again, those only exist in SMD packages, the outer pins of the QFN package the iCE40 comes into can be soldered by hand with a thin iron tip, a good amount of flux and some patience, however the central ground pad will definitely require some hot air and solder paste.
The sync generator board I did for VU007B (LZX Show & Tell, lzxpcb-fpga12 / ESG3 review - #13 by syntonie) could be used, however it is a fairly dense board and even if I soldered the first prototype boards by hand, it is far from being DIY friendly. I could also look into providing the pre-assembled sync generator board, though might be going against the DIY spirit of reducing the assembly cost by doing it yourself.
I think C1 ingenuity is the fact that it is made of through hole components only (except for the VCXO), making it easy to DIY, so not exactly sure what would be the best course of action considering this.
I suppose it would also be possible to source the 13.5MHz VCXO by requesting it from the supplier, but that probably means ordering a 1000 or so, and not sure if there would be enough interest to justify it. Also kind of move away from the open source DIY nature of the project where DIYers can source the components themselves without relying on a third party.
Anyway, open to any suggestions and ready to help keeping C1 alive!