Scouting around (plagiarising?) published designs for HR RXs and sundry
boatanchors, it seems that where there are tuned RF stages, then 2 tank
circuits seems to be the norm, and then only 3 ccts if the RX IF
is ooo 455kHz when the coverage needs to extend to 30MHz.

Looking at the rate of progress here (or lack of it!!) I am trying to
conceive of a scheme to use up all the tuning condensers (capacitors
only came in AFTER some of them were manufactured :-) ) in
the junk box in one final big push to releve the executors of my will
of difficult decisions :-)

On Fri, 14 Feb 2014, gareth wrote:

Scouting around (plagiarising?) published designs for HR RXs and sundry
boatanchors, it seems that where there are tuned RF stages, then 2 tank
circuits seems to be the norm, and then only 3 ccts if the RX IF
is ooo 455kHz when the coverage needs to extend to 30MHz.

Looking at the rate of progress here (or lack of it!!) I am trying to
conceive of a scheme to use up all the tuning condensers (capacitors
only came in AFTER some of them were manufactured :-) ) in
the junk box in one final big push to releve the executors of my will
of difficult decisions :-)

One reason for the superheterodyne receiver was to get away from the TRF
reciver, which was some stages of tuned amplification on the signal
frequency. Lots of problems once you started getting enough
amplification.

With the superhet, the RF stage before the mixer is there to offset the
noisy mixer (which for most of the HF range isn't a problem) and to get
rid of any images.

The cheap ham and shortwave receivers of the old days would tend to have
only one RF stage, and you'd see reviews for the commercial receivers with
things like "the image was as strong as the desired signal". Some
military receivers improved on that, and the HRO receiver was well known
for better image rejection at the higher frequencies despite having a
455KHz IF. It had not only two stages of RF amplification, but apparently
well designed tuned circuits.

Double conversion was a better way to get rid of images. The first
double conversion receivers tended to be to a low HF frequency, say about
2MHz, with a tuned circuit or two, then a drop to 455KHz for selectivity.
That wave of receiver generally didn't have a lot of selectivity at the
first IF frequency. Indeed, in a number of receivers, the mixer stage
became another 455KHz IF amplifier on the lower frequencies where the
double conversion didn't kick in. (Or to look at it in reverse, one of
the 455KHz IF stages became a mixer on the higher frequencies when double
conversion kicked in.)

The other method at the time, if you had the money, was like the Collins
75A series receivers, which in effect were a tuneable receiver converting
to 455KHz, with a crystal controlled converter ahead of that tuneable
receiver. Those were all the same basic scheme, though some tuned a ham
band directly, while others (like the Collins) tuned some other segment
with the converter needed for all ham bands).

But both double conversion schemes were as good as the tuned circuits.

It was only when HF crystal filters came along that image rejection became
much simpler. Pick a 9MHz filter, and the image is 18MHz away, not much
needed to reject the image. At that point, you could do away with the RF
stage, or at least keep it to a minimum, maybe just enough gain to offset
the front end circuits.

It's better to kick in amplification when needed than keep it in the
circuit all the time, which is how more recent designs have gone.

Once filters above the shortwave spectrum were viable, that really put the
image frequency far away from the signal frequency. Some of the early up
converting receivers just used a low pass filter ahead of the mixer (or
had that as one of the options, I think the Racal receiver with the Wadley
loop had this option, but I may be misremembering).

With such a high IF, it was simpler to get rid of images.

INdeed, one could basically build a receiver and leave off the front end
selectivity. Build the receiver well to that point, then play with what
comes between the antenna and the mixer. Got a really tough RF
environment? Maybe a crystal filter at the signal frequency is the
answer, so long as you only need a very small band of the spectrum. Put in
simpler filtering for the bands that you don't care about, make it better
for the one you do care most about. Or live with a low pass filter for
general coverage. For a band that you might care about most, have a good
filter that can be peaked on the signal, for some other band (or where you
need to tune for a converter for the VHF/UHF bands) you might want a
broadband filter that covers the 500KHz of the band, not requiring any
tuning within that band.

The point is that once you make the image rejection easier, you aren't
fussing with image rejection, you are working on other things that is more
important.

Michael

Michael is right. For the entire HF spectrum, you don't need RF stages in order to conceal mixer's noise: noise picked up by the antenna is stronger. Wes Hayward W7ZOI has been publishing articles on this since the 80s.
And you don't need active stages to get selectivity against image frequencies either, a double or triple tuned passband can be placed right begore the mixer.
On the other side, adding RF stages before the mixer decreases the receiver's dynamic range: makes it easier to be overloaded by strong signals some kHz or tens of kHz apart from the desired one.

You know, ISTR that during WWII, some military receiver specs called for two, well isolated, RF stages to reduce the possibility of local oscillator signal from reaching the antenna terminal. Posing the possibility of enemy radiolocating devices pinpointing location. A low level signal can be radiated for all to hear.
In the late 1940's a QST "Hints and Kinks" article described using an all band receiver as a signal generator. If you knew the IF frequency (single conversion was King in those days), it was simple to calculate (and use) the local oscillator's output for a variety of purposes. (Think modern transceivers)
Some of us, probably many of us, used that technique as a pretty stable and fairly accurate signal source working on homebrew (and surplus modification) projects.

Old Chief Lynn, W7LTQ

"lw1ecp" wrote in message ...
Michael is right. For the entire HF spectrum, you don't need RF stages in order to conceal mixer's noise: noise picked up by the antenna is stronger. Wes Hayward W7ZOI has been publishing articles on this since the 80s.
And you don't need active stages to get selectivity against image frequencies either, a double or triple tuned passband can be placed right begore the mixer.
On the other side, adding RF stages before the mixer decreases the receiver's dynamic range: makes it easier to be overloaded by strong signals some kHz or tens of kHz apart from the desired one.

On Sun, 23 Feb 2014, coffelt2 wrote:

**** You know, ISTR that during WWII, some military* receiver specs called
for two, well isolated, RF stages to reduce the possibility of local
oscillator signal from reaching the antenna terminal. Posing the possibility
of enemy radiolocating devices*pinpointing location.*A low level signal can
be*radiated*for all to hear.

And generally, they were an extra stage, but which didn't actually
amplify. I'm sure the receivers with two RF stages were better than the
ones with one, but definitely, there were some receivers with a stage
there just to isolate the antenna from the oscillator. You'd see
converstion articles, "take out this stage, it doesn't do a bit of good",
and I want to say it was the BC348, but I may not be remembering properly.

**** In the late 1940's a QST "Hints and Kinks" article described using an
all band receiver*as a signal generator. If you knew the IF frequency
(single conversion was King in those days), it was simple to calculate (and
use) the local oscillator's output for a variety of purposes. (Think modern
transceivers)

There were also articles that turned a receiver into a piece of test
equipment. Put a jack at the front of the audio chain, you've got a
general purpose audio amplifier, or a signal tracer. Put a jack at the
output of the IF, and you have a signal at the IF (so often 455KHz), again
useful for injecting a signal into another receiver's IF. I forget all
that was suggested, though even in the TV days there were articles
suggesting the same thing with a TV set.

For a decade, I had an SP-600, the model that was set up for diversity.
So the oscillators were already brought out (and I assume buffered in some
way). I know I used the oscillator output for some testing.

They still suggest that, but generally just to use a digitally tuned
receiver to pickup the oscillator, or as a reliable signal. Though most
have first IFs above the signal frequency. It's a wonder, you can get for
under a hundred dollar something with a readout that was only reached in
the old days by the most expensive or receivers.

That SP-600 was great as a piece of test equipment. It tuned to 54MHz, so
I'd put it on the highest band (30 to 54MHz) and spin the dial, it had a
great flywheel. When I tuned past a harmonic of an oscillator I was
playing with, I'd notice, so it was a simple matter of slowly tuning
backwards from where the dial stopped. Then I'd get a rough frequency,
and by going through the bands, find where something was actually
oscillating at.

A general coverage receiver is a great thing to have, though I suppose now
it's taken for granted since most recent transceivers have general
coverage receivers. But in the old days, you had a general coverage
receiver if you bought one before becoming a ham, only to discover how
little space on the dial the ham bands took up, or if you had money to
burn so you could get a general coverage receiver in addition to a ham
band receiver.

Someone I knew just died last week, when I met him forty-two years ago, he
had a KWM-2 (and the 6 and 2 metre transverter), but he had an R388 and an
SP-600 (the one I had the use of for a decade, actually). I guess he was
better off than some. ANd even there, he had both receivers, the SP-600
for rapid band scanning, the R388 for more precise work.

Michael

**** Some of us, probably many of us, used that technique as a pretty stable
and fairly accurate signal source working*on homebrew (and surplus
modification) projects.
*
**** Old Chief Lynn, W7LTQ
**** *
"lw1ecp" wrote in message
...
Michael is right. For the entire HF spectrum, you don't need RF stages
in order to conceal mixer's noise: noise picked up by the antenna is
stronger. Wes Hayward W7ZOI has been publishing articles on this since
the 80s.
And you don't need active stages to get selectivity against image
frequencies either, a double or triple tuned passband can be placed
right begore the mixer.
On the other side, adding RF stages before the mixer decreases the
receiver's dynamic range: makes it easier to be overloaded by strong
signals some kHz or tens of kHz apart from the desired one.