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ab0wr · December 26th 06, 02:50 PM posted to rec.radio.amateur.homebrew

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

Jim,

I recently went through what you are now doing. Get a copy of EMRFD and read
through it.

I built up a DDS vfo that I could use from a few khz up to about 20mhz. I
put it in a well shielded enclosure to minimize spur pickup. The one I
built also has an lcd readout and keeping that in the shielded enclosure
keeps noise down in the rest of the receiver.

I then built up front end bandpass filters for just the ham bands and worked
with them till I was happy with their performance.

I then picked up surplus crystals off of ebay. Get 100 of the same kind if
you can. The hc49 units I got seem to work well. Then build up the crystal
filters you need and get them working.

I then built up an IF using a 1496 ic and the crystal filter.

Then I built up a mini-circuit sbl1 mixer and started hooking things
together. The mixer needs a good diplexer arrangement to do its best. I
spent some time getting that built up. I learned quite a bit about
interstage impedance matching. I used iron core toroid transformers to do
most of that (e.g. btwn the mixer and filter and btwn the filter and the IF
amp.

Add in a simple product detector and a *good* audio stage and you are all
set.

The receiver hears at least as well as my icom 751a. It doesn't have
passband tuning (and likely won't) or a notch filter (I have plans to add
an audio notch filter) so it isn't quite as good in a crowded situation.

But it *is* simple and easy to work on.

Oh, I almost forgot. I wound up building up an RF amplifier to enhance the
receivers sensitivity and added a switchable 20db pad to help with overload
situations. I also used a set of latch relays to control everything with a
whole bunch of pushbutton switches on the front panel. The relays were
expensive but work really well.

Anyway, I think you can build a single conversion receiver that will be
quite adequate even compared to todays equipment. Give it a try!

tim ab0wr