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

I think the main issue with the contemporary transceivers is covering DC to
daylight and trying to make the thing work. A single band design seems to
be a lot easier to make bulletproof.

Modern QRP design has advanced a lot since the DeMaw days, even over the
venerable 75s4. Take a look at designs by K8IQY or AD6A or N7VE and you
will find receivers that will be head and shoulders beyond anything that
W1FB ever dreamed of.

The front end overload issues on simple receivers seems to be largely
related to the use of the 602/612 mixer. The passive mixer, or Tayloe
mixer, designs seem to fare much better. The Elecraft K1 is the exception
that proves the rule; I suspect careful gain management may have something
to do with that.

KK7B has also written a LOT on receiver design, and his designs have a good
rep, but I've never used them so I can't comment from experience.

Doug's designs are fun, and sort of nostalgic, but if you are looking for
21st century performance, it isn't the place to look.

...

wrote in message
ups.com...
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

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.

Try Drake R4B and Softrock tuned to the first IF 5645 (?) you will get best
of both worlds. Switching before and after filters gives more options.

Yuri, K3BU

wrote in message
ups.com...
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

) writes:
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

But then you're just recreating something that is open to problems.

That sort of scheme was used to get a constant tuning range over each band,
and because synthesis wasn't easily applied.

But on anything other than the core band, you've got double conversion that
moves the selectiving past two mixers. And unlike current up-conversion
schemes, there is not even a crystal filter of some sort at the output of
the first mixer.

Switch to an IF in the HF range, and you immediately eliminate most image
problems. No fussing over the front end about that, and of course, you
won't have to have the front end tracking the tuning oscillator.

Remember, a whole wave of amateur transceivers and receivers went
to that sort of scheme. And unlike general coverage receivers, you
don't have to worry about any problems due to the IF being in the tuning
range, and you can think up various schemes to do the tuning since you
don't have to cover the 30MHz.

YOu can stick with the modular theme, and thus build only for the
bands you want or even build a band at a time. But instead of a whole
converter, you'd have the preselector circuitry (and maybe an RF stage) and
the variable oscillator for each band. Or build a good receiver up till
the input of the mixer, and then figure out what comes next.

IN the sixties, that wave of single conversion to IFs in the MHz range
used various schemes to deal with the local oscillator. Obviously some
bands needed a frequency range that could easily be supplied by a variable
oscillator without drift (and some of the rigs took that to the extreme
and used the variable oscillator directly on 10metres). The problem
with bandswitching the oscillator would of course be the issue of
getting it to tune only 500KHz or so on each band (and any stability
issues caused by the switching of the LC circuits).

Other rigs used pre-mixing, so the variable oscillator would always
tune a fixed range, but it would be converted to the needed frequency
with a mixer and crystal oscillator. One does have to watch out
for spurs on the output, but it gets the extra mixer out of the signal
path, and given a relative handful of 500KHz ham bands in the shortwave
region, the cost of the crystals wasn't out of range (though maybe
today..).

Then later, some rigs used PLLs. I can't remember if the Signal One
used one, but certainly in the seventies they came along. Same basic
idea as the premixer, but the PLL was the filter so the VCO directly
fed the receiver's first mixer.

Rigs like the TS830S used a PLL for that same purpose, though they
came up with a pretty fancy scheme to limit the number of crystals
needed.

One of the things about receiver design is that the trends have
often reflected limitations of the times. There may be a good
reasons for doing things a certain way (such as adding a third
conversion to a receiver so the BFO is on a different frequency
from the one where the main gain is), but it may also mean
they couldn't do anything better at the time.

Michael VE2BVW

Then later, some rigs used PLLs. I can't remember if the Signal One
used one, but certainly in the seventies they came along. Same basic
idea as the premixer, but the PLL was the filter so the VCO directly
fed the receiver's first mixer.

Michael VE2BVW


Well the CX-7 didn't use a PLL, but the CX-11 did. It was however only for
generating a comb frequency to be used with premixing. The analog PTO's
still tuned 3.1 to 4.1 MHz.

The main problem with up conversion is the hit you take on oscillator phase
noise. These days, with the advent of the "H" mode mixer, oscillator phase
noise and birdies are THE limiting factor in receiver compromise.

W4ZCB

Harold E. Johnson wrote:
Then later, some rigs used PLLs. I can't remember if the Signal One
used one, but certainly in the seventies they came along. Same basic
idea as the premixer, but the PLL was the filter so the VCO directly
fed the receiver's first mixer.

Michael VE2BVW


Well the CX-7 didn't use a PLL, but the CX-11 did. It was however only for
generating a comb frequency to be used with premixing. The analog PTO's
still tuned 3.1 to 4.1 MHz.

The main problem with up conversion is the hit you take on oscillator phase
noise. These days, with the advent of the "H" mode mixer, oscillator phase
noise and birdies are THE limiting factor in receiver compromise.

W4ZCB


I've heard that DDS units do NOT have the phase noise problem that
conventional oscillators do. DDS does generate spurs, but if you can
find a DDS that will allow for a clock rate many times the desired
output frequency the spurs a far out of band. The Analog devices AD9954
family of DDS chips have a max clock rate of 400 mhz. If you do NOT use
the on board PLL multiplier and clock it externally from a good low
noise clock, you can overclock these chips to as much as 600 mhz!
So if you up convert to 70mhz you'd need a 100 mhz local oscillator at
10 meters. Thats 1/4 the clock (or 1/6 if over clocked).
With a 45 mhz first IF even better.

BTW I had an idea for a rig up converting to 6 meters as the first IF.
This would be dual conversion on the HF bands, though a crystal filter
at 6 meters for a fixed first IF would not be impossible here.

wrote:
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?

The DeMaw designs are very nice but with their band-wide converters
they also suffer from having selectivity downstream rather than at the
front. Preselection at the front helps with any of these.

The modern choice of a 45MHz or so first IF is really pretty nifty for
a general coverage receiver from the past couple of decades. Lack of
preselection can be largely cleared up by bolting something before the
front end, and indeed many of the receivers you see that cost as much
as a car have tracking preselectors.

But if you don't want to upconvert, as I see it you have two choices:

1. Don't even try bandswitching. Coil sets for each band. You end up
with the HBR-16, a very elegant and homebrewable receiver.

2. Do bandswitching but with elaborate tracking front end (and if
necessary a tracking IF, but with modern synthesizers you probably
wouldn't), probably with many switching sections to handle the required
octaves. You end up with a R-390A.

Tim.

"Tim Shoppa" ) writes:

1. Don't even try bandswitching. Coil sets for each band. You end up
with the HBR-16, a very elegant and homebrewable receiver.

One reason there was that phase with a separate converter per band ahead
of a receiver tuning a fixed band was to avoid switching tuned circuits.
By that time, the semiconductors cost so little that it was easier to
duplicate them for each band, and then the bandswitching becomes so much
easier. No fussing about getting close to the tuned circuits, you simply
switch the "B+" and the input and/or outputs. Or, make those
converters plug in, and then no switches required at all.

I sort of alluded to this in an earlier post. Make a good receiver,
minus the variable oscillator and the front end tuning. Maybe even
put the mixer in the "plugins". Then you end up with a good receiver
that is quite flexible, because the things that you may want to play
with and may give trouble are in a separate box or plugins.

YOu can even play with things like tuneable frontends versus something
that is broadband across a ham band. Some bands might interest the
builder more than others, so they could build a really good plugin
front end for that band, and lesser front ends for other bands, or
leave off the bands they aren't interested in (but those bands
can easily be added later, unlike a bandswitched rig).

IN the sixties, there was a guy who had a whole slew of receivers
described in CQ. Virtually all of them were single bands, and he
made the point that it left off bandswitching, and of course he
could choose an IF that better matches the tuning range. Good points,
but an awful lot of duplication. Build a good receiver first,
and then play with the frontends endlessly.

Michael VE2BVW

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?

I'm partial to KK7B's stuff, since the less you mess with the
signal turning it from RF to audio, the better it sounds.
And if you *want* to mess with it, you've got a better
place to start.

My best shack receiver is a Harris RF-590. A genuine
professional military/industrial/spook radio, rack mounted,
weighs a ton, megabucks new, but reasonable if you can
get one surplus. If you can't hear it on this radio, you don't
need to hear it.

Laura Halliday VE7LDH "Que les nuages soient notre
Grid: CN89mg pied a terre..."
ICBM: 49 16.05 N 122 56.92 W - Hospital/Shafte