So today i thought i might be fun to whack together a VXO for 80m and see just where i landed me. Now i know that pulling crystals very far is not an easy thing to actually do, especially when they are on the lower end of the spectrum. So with this in mind, i set my initial goal as 1khz pull. I figure if i can get that, its kind of usable in other projects.
First job is to get on google and find a simple schematic, with that out of the way, I can get to putting the ugly back into ugly style home brewing and slap something together on some scrap board.
And slapping together is just what I did, a pair of 3.6864 xtals, with 10uH series inductance and 170pF of capacitance in the polyvaricon.
Give it some power and connect up the scope and freq counter and am greeted with this ugly waveform, harmonics, bias, its a mix of everything that is wrong in the world. So let see how far we can pull these bad boys.
High freq is 3.6867
Low freq is 3.6863 HAHA we managed a whole 400hz agility, as good as useless, but it was rock stable, pun intended. So i did a but of goggle foo and found that I can get much better pulling power with more inductance, 10uH is orders of magnitude not enough, so i added another 200uH in series and got a total pulling power of 2khz. 2khz is not much in the scheme of things, but, for a cw 80m rig, 2khz is a nice slice of spectrum to be frequency agile on.
So, whats next? rip out the xtals and drop in a ceramic resonator and adjust the series inductance to suit. Should see a good pull range with that.