What would this solve ? You still need some selectivity in front of
converter.

I would also question the need for a bandpass filter, but a good low
pass filter would definitively required in any case. I would suggest a
low pass filter below 150 kHz in Europe, Africa and Middle-East and
below 500 kHz in the rest of the world to get the very strong LW/MW
broadcast band signals out of the mixer. If 455 kHz IF is used, the
LPF would have to be below 400 kHz in the rest of the world.

I have seen designs with a SBL-1 mixer

SBL-1 is specified for 1-500 MHz on the RF and LO port, so not really
suitable for this band. However, the SBL-3 goes from 25 kHz to 200
MHz. The SRA-6H goes from 10 kHz to 50 MHz and should be able to
handle up to +10 dBm signals.

but also a number with the NE612
osc/mixer.

I have used the Datong LF converter, which uses the Siemens S042
mixer/osc IC similar to the NE602/612 and it definitively needs a
preselector in front of it to get away with spurious responses all
over the LF band from broadcast stations.


Thanks Paul, and yes....you're correct. Building a conventional
converter would still require a passband filter, so little is to be
gained, except that perhaps someone else has already done the
design::

Also, this receiver design has no mixer, it is simply a detector and a
very linear one to boot. No mixing byproducts are present because
there is no non-linear mixer. In effect, this design is already a
converter....except that it converts to audio directly from the rf
frequency input.

The "spurious responses all over the LF band from broadcast stations"
probably don't exist in this type of receiver, which is one of the
attractions for VLF use of this technology.

Take a look at the link to the design in the original message and you
will learn how it operates without mixers and without non-linear
detectors.

It's WHY I so interested in this particular method of reception and
WHY I want to make a front end for vlf for it. The concept is
explained in a QEX article in greater detail, Im happy to send the url
to anyone who wants to learn more about these high performance direct
conversion receivers.

Regards and again, Thanks,

T