On Sun, 11 Sep 2005 23:43:36 +0100, "Highland Ham"
wrote:

The existing filter is for a 7 Mhz receiver, I'd like to have a
similar filter design for 50 to 200 Khz.
===========================
If you are happy with the HF receiver as is, would it not be easier and
more effective to build an LF (50 - 200 kHz) to HF converter. This should
be an easy project and you can select a quiet "HF band" for the conversion.

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.

Paul OH3LWR