The filter from a GRC-106's RT unit will do, but it's too big to fit in the '47. The '47's filter will fit inside a '106's RT and give it LSB though. http://www.co.missoula.mt.us/acs/radios/so%20now%20you%20have%20a%20prc-47.htm There was a guy in Japan $elling a dual filter, 1.75MHz USB LSB for the GRC106, I imagine it would work in the '47 also, but I can't find the web page.