This is a quick update post. The road I took with this little project is slowly getting to a dead end due to what it seems to be a lack of available crystals for 144.8 (like 48.26 or 28.96 MHz, for using the third or fifth harmonic). I believe that, currently, all APRS modules are done with some sort of DDS, which should be much easier to do. Due to my lack of expertise in this area (in the end is a hobby) there might also be some other methods to get — by analogic means — the carrier for APRS (144.8 MHz). I will dig further, but for now it seems that the DDS path is much more convenient. Although this looks like a dead end I don’t mind, I’ve learned a lot. Including how to use my tools. I made some minor modifications today by adding a rudimentary input for modulation. It works, but the modulation is very weak (or the output power is too low). Boosting (+ 6 dB) the input through a preamp does not help. On the contrary, some nasty artefacts occur. Back to the drawing board.
This should be the starting point for improvements. Now the prototype looks like this:
I glued 🙂 an extra PCB board (for the preamp part) and stick some ABS strips beneath the rest of the board, for a better prop and grip. These also helped me fixing the board in a vice, in vertical position for an easier connection of various cables and attaching the power supply (via two crocodiles). The latest version of the schematic is this (I will probably get rid of the preamp and add an additional RF amplification stage):
Some comments about the filter. I did some simulation today and the simulated profile seems to fit the findings in the real world:
The simulated response is better than the calculated (-50 dB @ 108 MHz simulated vs. -45.3 dB calculated etc), however the actual measurements I took (see here) are with one order or magnitude poorer. However, next step will be to create a proper PCB (etched) and get rid of this dead bug layout. I suspect that many artefacts and poorer performance than expected are also due to this method of laying out the components.
Not much to add for now. The main problem I face is to find an appropriate crystal to get 144.8 MHz carrier for APRS. It seems there is no such crystal as 48.26 (third harmonic) or 28.96 MHz (to use the fifth harmonic). DDS approach will be a parallel development for sure, I still did not gave up to my original idea of using only discrete, analogue components. That’s it for today.