Many thanks to SWLing Post contributor, Ron, who is a fan of longwave DX and notes that HRO has a Heros Technology VLF/LF up-converter on consignment/used. Ron mentions that the $140 price tag is quite a bargain for customers in the US market. Ron notes::
Here is a Link to the User Manual with pix of inside and some very cool videos of it in action:
https://www.herostechnology.co.uk/pages/VLF_LF_Converter.html
https://vimeo.com/119645116
And here is a Link to the one at Ham Radio Outlet:
http://www.hamradio.com/detail.cfm?pid=91-004203New ones are $235 delivered to your door from London-no U.S. source.
Thanks, Ron!
https://www.herostechnology.co.uk/media/BandConverters/Heros_Technology_VLF-LF_Converter_Architecture_Diagramx1K.jpg
https://www.herostechnology.co.uk/pages/VLF-LF-MF_Up-Converter.html#LFPICT
I was going to say that $235 was a lot of money for such a simple device, but having looked at the block diagram it is evident that the designers have taken a lot of trouble to keep unwanted signals at bay, even filtering the local oscillator to stop any chance of harmonic mixing. They have even taken care to keep the ring mixer happy by fitting a diplexer. A pretty nifty design, I’d say.