AGC signal/noise question...
 

Hello.
I'm wondering about a thing...

When the AGC reduces the gain of an amplifier, composed by FETs (DG or SG)
or BJT, the SNR remains constant?

Or the performance of the amplifier may degrade/upgrade?

I'm researching about the matter and I just read that, in a BJT for instance,
emitter current is inversely proportional to the noise. So, if AGC reduces
the gain (so current), SNR degrade?

The question arises from a thing I just noted with a HF receiver... disabling
AGC reduces (slightly) the noise (at least in FM reception)...
Maybe just be a side effect, like noisy gain control signal...

Ciao,
AB

.... Andrea Baldoni, 2002: messaggio non protetto da copyright.

AGC signal/noise question...
 

From: Andrea Baldoni on Thurs, Aug 17 2006 1:34 pm


Hello.
I'm wondering about a thing...

When the AGC reduces the gain of an amplifier, composed by FETs (DG or SG)
or BJT, the SNR remains constant?

Maybe, maybe not...

Or the performance of the amplifier may degrade/upgrade?

Maybe, maybe not...

I'm researching about the matter and I just read that, in a BJT for instance,
emitter current is inversely proportional to the noise. So, if AGC reduces
the gain (so current), SNR degrade?

Not necessarily true. Noise, true natural noise, in a bipolar
junction transistor is dependent on MANY things besides the
emitter current. Foremost of all is the source impedance
of the BJT's driving source: SNR will increase on both sides
of the measured least-SNR current in some transistors. If you
look at the characteristics/datasheet information on several
hundred different transistors you will begin to get the idea
of how many factors go into noise generation within the
transistor. :-)

Most complex Gilbert-cell ICs (Motorola MC1590, MC1350 as
examples) will control gain by "stealing" current of a
differential-input, common-emitter stage. While the example
ICs can control gain over a 60 db dynamic range, the worst
noise generated inside those bipolar-transistor architecture
ICs occurs at minimum gain control input (maximum
amplification).

At higher and higher input signal levels, there would be more
AGC control action but the SNR would also be higher and higher.

The question arises from a thing I just noted with a HF receiver... disabling
AGC reduces (slightly) the noise (at least in FM reception)...

There isn't much FM on HF. What there is would be in
narrow-band Data mode signals. Some of that Data is a
combination of AM and PM similar to a wireline modem's
modulation.

Disabling the AGC would put the overall gain of the receiver
at maximum. Naturally there would APPEAR to be "more noise"
since the receiver would be picking up ALL noise available at
the antenna input.

Maybe just be a side effect, like noisy gain control signal...

That's also possible, but unlikely there in my opinion.

What is needed in an investigation of this is a reasonably-
well-calibrated signal generator with a calibrated attenuator.
On AM you could simply increase the signal generator output
until the total signal plus noise reaches a certain level at
the detector, usually 10 db above input of just noise alone
(common definition of SNR).

AGC signal/noise question...
 

Andrea Baldoni wrote:



Hello.
I'm wondering about a thing...

When the AGC reduces the gain of an amplifier, composed by FETs (DG or SG)
or BJT, the SNR remains constant?

Or the performance of the amplifier may degrade/upgrade?

I'm researching about the matter and I just read that, in a BJT for
instance, emitter current is inversely proportional to the noise. So, if
AGC reduces the gain (so current), SNR degrade?

The question arises from a thing I just noted with a HF receiver...
disabling AGC reduces (slightly) the noise (at least in FM reception)...
Maybe just be a side effect, like noisy gain control signal...

Ciao,
AB

... Andrea Baldoni, 2002: messaggio non protetto da copyright.

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? Once the AGC threshhold is
reached, if the signal strength goes up faster than the noise contribution
from the system goes up do you really care what the noise level actually
is?

tim ab0wr

AGC signal/noise question...
 

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... :)

--
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

AGC signal/noise question...
 

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.

AGC signal/noise question...
 

"i3hev, mario held" ) writes:
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 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,
the fact that the noise level may increase won't matter because the
lower gain will make the signal unreceivable anyway.

Michael VE2BVW

AGC signal/noise question...
 

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

AGC signal/noise question...
 

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

AGC signal/noise question...
 

"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.

AGC signal/noise question...
 

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