Most of authorities allow transmission somewhere above 500kHz. In The Netherlands the permitted range is 501-505kHz. In the future, depending on the decision of the WRC12, this will possibly change to 472 to 480kHz. The U.S.of A. proposed the following ranges 461-469 and 471-478 kHz.
Lets look at the (inexpensive) option the box73 SDR. The 80m version of this receiver uses a 14.000MHz oscillator. Operation on the 600m band can be achieved by changing the front-end filter and the SDR-LO.
Considering 48kbps sampling, the LO-frequencies would be the following
- QRG: 470kHz - LO: 1.843MHz
- QRG: 500kHz - LO: 2.000MHz
- 460.8 -/+ 24 = 436.8 .. 484.8
- 500.0 -/+ 24 = 476.0 .. 524.0