wrote:
I don't much like the receivers I've used in contemporary tranceivers
-- the general coverage synthisized open front end ones. (I hasten to
add I haven't used any of the $4000 rigs; can't afford them). But the
ones I have used seem plagued with near-signal desensitization, front
end overload, etc., and I suppose all that comes from putting the
selectivity so far downstream.

I'm almost tempted to get an old 75s4 and shut up, but I really don't
need another room heater, so, instead, I'm thinking of building my own
receiver along the lines laid down by the late Doug DeMaw in his _QRP
Notebook_. Single conversion 160m superhet with Collins mechanical
filters in the IF and a series of down-converters for the other bands.
Anybody got any experience with the DeMaw Design?

Jim, K5YUT

There are many variations on this theme. There are two camps here,
the Collins version was a 160 meter first IF with a 455 khz second if.
The Drake version was an 80 meter first IF with a second if of 455 khz,
and a third if of 50 khz. So you have the 75S series vs the 2B series.

Lafayette radio's famous HA350 used the 2B method as did a home brew rig
in the '67 handbook (both without the 50 khz if, they used a Collins
filter.)

I've been thinking of building something along these lines to make use
of some 85 khz ARC-5 IF cans in the junk box. You'll get somewhat
better image rejection above 20 meters if you use an 80 meter IF instead
of 160, but otherwise either IF scheme is ok.

Yet another idea is a single conversion with a 9mhz IF. Since I have a
bunch of surplus 9mhz filters (they are 8 pole units with 3.2 khz
bandwidth) I was also thinking of a rig with these. True the filters
are a bit wider than common today, but if I put THREE of them in cascade
(between IF stages) they should do a good job. I would use a DDS VFO,
but would also used a tuned (not broadband) front end. I have enough
toroids and multi section variable caps in the junk box for that. I
also have some old tv turret tuners that would make a good band switch
for the front end (put the toroids on the tuner strips). I have lots of
J310 fets, so I'd use them in the front end as well.

On Sat, 23 Dec 2006 11:32:04 -0500, ken scharf
wrote:

Yet another idea is a single conversion with a 9mhz IF. Since I have a
bunch of surplus 9mhz filters (they are 8 pole units with 3.2 khz
bandwidth) I was also thinking of a rig with these. True the filters
are a bit wider than common today, but if I put THREE of them in cascade
(between IF stages) they should do a good job.

A single 9 MHz IF filter unit with I/Q detection (to handle the
opposite sideband) could be an other alternative and do the rest of
the filtering in audio stages.

I would use a DDS VFO,
but would also used a tuned (not broadband) front end. I have enough
toroids and multi section variable caps in the junk box for that.

The _unloaded_ Q values shown by toroid manufacturers are not very
spectacular (in the 200-300 range at most). If you aim for a filter
loaded Q of 100, there are going to be a considerable loss (several
dB), so placing the filter before the first RF amplifier stage will
deteriorate the noise figure quite badly, which can be a bad thing on
upper HF bands.

With a preselector loaded Q in the 50-100 range would still cover an
entire WARC band without tuning and with wider bands and tunable front
end filters, a 100-500 kHz segment would still be present at the mixer
input at full amplitude. Thus, the mixer would still need to be strong
to handle all those signals in that range.

A preselector filter will most definitively help in keeping out strong
broadcast band signals (e.g. the strong 49 m BC band in Europe) from
the mixer, but it does not help much against strong amateur signals in
the same amateur band.

Paul OH3LWR