Your intrepid, no expenses spared blogger wanted to try an SDR on 500kHz. Being a big spender and having some PayPal bonus vouchers to redeem I splashed out and purchased a Softrock Lite II 80m Kit, it cost me £4 that !!
Jim Moritz published the details for converting the old Sotfrock Lite to 500kHz and 136kHz, there is not a lot of difference so I reckon that it should be OK.
The main difference is the lack of divide by 8 so a 2MHz Xtal will be needed for 500kHz, as for 136kHz we must experiment.
First step for me was to build and test on 80m, my other SDR is 40m TX/RX so 80m would be a nice band to keep available. Hmm, I suppose I could spend another £4.
I must say it performs very nicely on 80m although FULL band coverage still needs one more Xtal than the 4 I have.
Well I spent the other PayPal voucher on a 9m glass fibre pole so I followed my original idea of making the Softrock Lite II 80m easily changeable to 500k and 136k.
First of course we need changeable, plug in Xtals, no problem. The Oscillator/Buffer has a bias change for the lower frequencies, 2 resistors need to be removed and another added. I mounted those on spigots for ease of removal. The other changes in this department are added capacitance, this can be added to the rear of the board.
The front end is the other change, we must built an LPF which will be too large for the board so this will be separate, this feeds the input transformer which must replace the 80m component.
To achieve this easily I have provided tabs to mount L1 and T1 for ease of swapping.
It won’t be a luxury band switch job but at least it’s not butchery. Now I need to find a source of 75 material toroid cores.