A VLF/LF converter

Up conversion is at times used for reception of very low-frequency and low-frequency radio (9 kHz to 300 kHz). The VLF or LF input is mixed with the LO to provide an output which falls within the range of a shortwave receiver, over the range 3.509 MHz to 3.800 MHz. A block diagram of the up converter for VLF/LF is shown in the figure given below. The LO frequency is 3.500 MHz.

This VLF/LF converter generates mirror-image output duplicates, one above 3.509 MHz and from 3.200 to 3.491 MHz.When you tune from 3.509 to 3.800 MHz, hence hearing VLF/LF signals from 9 to 300 kHz, you are listening to little slices of the USB of a wideband AM signal.

                                                                             Figure-- A passive mixer.