Michael Black wrote:

In that case though, you should be worried about that nearby strong signal
dropping the gain of the receiver so you can't hear the weak signal...

may be you are not thinking of cw...

Provided the SNR is good enough, you can filter and post-amplify your
weak signal, e.g. with a good AF filter, or a dsp.

Of course, if the IF stages are (reasonably) linear in response, you can
disable the AGC, but this would be no answer to the original question

--
73 es 51 de i3hev, op. mario

Il vero Radioamatore si riconosce... dal call in firma!
- Campagna 2005 "Sono un Radioamatore e me ne vanto"

it.hobby.radioamatori.moderato
http://digilander.libero.it/hamweb
http://digilander.libero.it/esperantovenezia

On Fri, 18 Aug 2006 19:54:00 +0200, "i3hev, mario held"
wrote:

Michael Black wrote:

In that case though, you should be worried about that nearby strong signal
dropping the gain of the receiver so you can't hear the weak signal...

may be you are not thinking of cw...

Provided the SNR is good enough, you can filter and post-amplify your
weak signal, e.g. with a good AF filter, or a dsp.

Of course, if the IF stages are (reasonably) linear in response, you can
disable the AGC, but this would be no answer to the original question

Since the SNR is established by the frontend the IF system can have a
more relaxed SNR.

However be wary of ICs like the MC1350 as the gain reduction occurs
the internal noise is bad. I've built several recievers using this
part and at ~10db gain reduction the noise jumps way up. I've gone
to cascode JFETs as the noise is more predictable and generally
lower. The device used does make a difference.


Allison

On Fri, 18 Aug 2006 16:33:25 +0200, "i3hev, mario held"
wrote:

tim gorman wrote:

You might also ask yourself whether it really matters or not. If your signal
is strong enough to begin driving the AGC to limit the system gain do you
really care what the noise level actually is? ....

But you may very well care for the SNR on an interesting weak signal
which you are trying to listen to, while a strong nearby signal
activates your AGC...

In theory the system bandwidth should not allow that strong signal to
hit the AGC. Of course practical systems this may not be true.

However, manual gain control helps if the stronger signal is not
overloading the front end causing gain compression and
intermodulation.

Allison

Regardless of an amplifier's gain, the signal to noise ratio at its
output remains the same as the signal to noise ratio at its input.

Obviously - if its a linear amplifier!
----
Reg.

Reg Edwards wrote:

Regardless of an amplifier's gain, the signal to noise ratio at its
output remains the same as the signal to noise ratio at its input.

Obviously - if its a linear amplifier!

I'm afraid this is quite wrong

Your statement would be a correct one if and only if the amplifier is a
non-noisy one - which, alas, is not a real case...

--
73 es 51 de i3hev, op. mario

Il vero Radioamatore si riconosce... dal call in firma!
- Campagna 2005 "Sono un Radioamatore e me ne vanto"

it.hobby.radioamatori.moderato
http://digilander.libero.it/hamweb
http://digilander.libero.it/esperantovenezia

"i3hev, mario held" wrote
Reg Edwards wrote:

Regardless of an amplifier's gain, the signal to noise ratio at
its
output remains the same as the signal to noise ratio at its input.

Obviously - if its a linear amplifier!

I'm afraid this is quite wrong

Your statement would be a correct one if and only if the amplifier
is a
non-noisy one - which, alas, is not a real case...

===================================

If an amplifier incorporates a noise generator or also behaves as a
filter, then it is no longer just an amplifier. My statement is quite
correct.
----
Reg.

Reg Edwards wrote:

If an amplifier incorporates a noise generator...
... then it is no longer just an amplifier. ....

.... so, we must conclude that amplifiers do not exist!

of course, there is no real amplifying device which does not generate
noise; or, more precisely, there is no non-noisy real device at all

If you choose not to call "amplifier" a device which amplifies signals
if it has a non-unity noise figure, please feel free to do so - we live
in a (at least partially) free world... but you will be alone! )

Regards!
--
73 es 51 de i3hev, op. mario

it.hobby.radioamatori.moderato
http://digilander.libero.it/hamweb
http://digilander.libero.it/esperantovenezia