Note specifically I said input referred noise.

If you don't put gain in the preamplifier (i.e. you do the so-called
impedance only function), then the AGC of the radio will amplify the
noise of the preamp. Since there is no gain in the preamp, you have
the combination of the noise from the preamp and the amplifier (i.e.
AGC) of the radio. If the preamp has some gain, then the AGC will do
less amplification, thus the noise will be dominated by the preamp.




David wrote:
On 11 Feb 2006 10:51:04 -0800, wrote:

It seems to me that to keep the input referred noise as small as
possible, you would want gain.

An amplifier can't tell ''noise'' from ''desired''. S:N is unchanged
by an amplifier.

On 11 Feb 2006 16:54:54 -0800, wrote:

Note specifically I said input referred noise.

If you don't put gain in the preamplifier (i.e. you do the so-called
impedance only function), then the AGC of the radio will amplify the
noise of the preamp. Since there is no gain in the preamp, you have
the combination of the noise from the preamp and the amplifier (i.e.
AGC) of the radio. If the preamp has some gain, then the AGC will do
less amplification, thus the noise will be dominated by the preamp.



IP3

Your point?

David wrote:
On 11 Feb 2006 16:54:54 -0800, wrote:

Note specifically I said input referred noise.

If you don't put gain in the preamplifier (i.e. you do the so-called
impedance only function), then the AGC of the radio will amplify the
noise of the preamp. Since there is no gain in the preamp, you have
the combination of the noise from the preamp and the amplifier (i.e.
AGC) of the radio. If the preamp has some gain, then the AGC will do
less amplification, thus the noise will be dominated by the preamp.



IP3

On 11 Feb 2006 20:38:10 -0800, wrote:

Your point?

David wrote:
On 11 Feb 2006 16:54:54 -0800, wrote:

Note specifically I said input referred noise.

If you don't put gain in the preamplifier (i.e. you do the so-called
impedance only function), then the AGC of the radio will amplify the
noise of the preamp. Since there is no gain in the preamp, you have
the combination of the noise from the preamp and the amplifier (i.e.
AGC) of the radio. If the preamp has some gain, then the AGC will do
less amplification, thus the noise will be dominated by the preamp.



IP3

A well designed input stage can handle the difficulty you described.