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

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

"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

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

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

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

I have to disagree on the MC1350 and way back 30 years to its
predecessor, MC1590. The prototype HF receiver presently on
my workbench has a NF of 5.5 and that hardly rises more than
that with AGC current applied to the AGC pin.

BTW, that receiver, single-conversion with one IF at 21.4 MHz,
uses only MC1350s up to the detector, including the one mixer
stage. [ LO is a separate PLL board ]

On 19 Aug 2006 20:19:19 -0700, "
wrote:


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

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

I have to disagree on the MC1350 and way back 30 years to its
predecessor, MC1590. The prototype HF receiver presently on
my workbench has a NF of 5.5 and that hardly rises more than
that with AGC current applied to the AGC pin.

Read EMRFD page 6.16 (ARRL press) they tested the 1350 and at the
point where the gain cell has equal conduction on both legs the noise
rises significantly. I duplicated the test fixture and yes, it's
noisy, from around 6db to around 11db in my fixture when gain is
reduced by 10db and that was at 16mhz. In a reciever that used
it I went to two cascode stages using JFETs and the difference noise
was notable for weak signals just into the agc range. I restrict the
1590/1350/ca3028 for lower perfomance recievers now.

I also verified that the 1590 does same and also the CA3028
wired as differential AGC. Even tried three 2n3904s and
same result. The agc range was good and at full gain the
noise was ok but the noise increase at partial agc was surprizing.

BTW, that receiver, single-conversion with one IF at 21.4 MHz,
uses only MC1350s up to the detector, including the one mixer
stage. [ LO is a separate PLL board ]

I do most of my RX experimentation at 6/ 2M and 70cm SSB so
noise and overload perfomance are important to me. Images
are also a big problem as I'm near a lot of VHF/hf broadcast.

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