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Receiver basics question



Greetings all...

I know this may be off topic since my question has to do with HF receiver
design but I thought this may be the best place to ask since we sat - ops
are interested in weak signal work.

If one was to build a receiver strictly for detecting weak signals, and
adjacent signal interference was nonexistent,  that is, under ideal
conditions would there be any reason to use double or quad conversion
designs?  Wouldn't a single conversion receiver offer the least amount of
loss and a lower noise floor?

I recently compared three HF receivers for the ability to discern signals
just above the noise floor and would like some suggestions or comments from
the experts.

I found the following while testing double and quad conversion rigs. The
signal level to each rig was identical
and just above the noise floor.    All comments and obvservations are with
minimum signal detection in mind.

1:  The double conversion rig seemd to have a slight edge in sensitivity and
a slightly lower noise floor.
      This seemed to hold true for both double conversion rigs.

2:  The quad conversion rig seemd to have more noise and hiss at the noise
floor which I'm assuming is due
      to the additional stages that make up the quadruple conversion design.

3: While monitoring the signal just above the noise floor, I switched the
cascaded 500hz cw filters in and
     found that the insertion loss from the filters dropped the signal to
almost nil.  Switching the 1st I.F. filter
     in alone seemd to have less loss than using both at the same time.

4: Although the quad conversion rig showd slightly higher hiss and noise in
the headphones it seemd to
    present the cleanest tones, CW dot's and dashes seemd seperate and
clean, even though the double
    conversion rigs presented a better signal to noise ratio, the dot's and
dashes seemd to run together.

5: despite the differences in design and price, the weak signal detection of
all three rigs was nearly
     identical under ideal conditions.. no adjacent signals to bump the AGC
etc..

To sum it all up, the quad conversion rig with cascaded filtering would of
course win hands down on a
crowded cw contest band or stone wall unwanted adjacent signals.  The
slightly higher hiss and noise levels
seem to be inherent in quad conversion receiver design and a trade off for
the better crowded band performance.  It is odd though to know that in terms
of absolute minimum signal detection under ideal conditions, a $4000 dollar
rig will not hear better than the modern day $1000 HF mobile.

Thanks for any suggestions or comments.

Tony/AB2CJ




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