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.