Receiver SNR question!!SOS

[liberal]

receiver snr

I have a receiver,whose signal flow is --LNA-----RF-----IF----AGC----
I measure the SNR at RF port ,it is 10dB,then the SNR at AGC output it is 5dB.why depreciated by 5dB,it should be equal,is't???
could the AGC depreciate the SNR??

 

[Element_115]

snr and agc

It doesn't look wrong.
If the RF block = 15dB gain, ~3.5dB NF
and the IF Block = ~-12 dB Gain (Conversion gain/loss), ~x dB NF
And the AGC = -ydB attenuated [y=a(Gain) - b (Attenuation)]
Then the NF could go to 5dB, so:

10dB(Sin/Nin) - 5dB(NF) = 5dB(Sout/Nout)
10 - 5 = 5

 

[kspalla]

rf snr nf

The above calculations will be correct if the AGC is not compressing the gain.

 

[vfone]

nf measure agc

Always the SNR at the output of an amplifier is smaller than SNR at its input. With or without AGC.

 

[kspalla]

snr at receiver

Vfone:
I agree with you that SNR Is smaller at the output with or without AGC

What about NF calculations under AGC?
I suppose SNR is worse under AGC and gives you wrong NF.
Am I correct?

 

[vfone]

snr receiver

Cannot say the NF measurement is wrong. Can say that is different.

For example the high-frequency VGA manufacturers usually specify the NF of the controlled amplifier at maximum gain, and sometimes providing graphs for NF vs Gain.
Doing an evaluation of the controlled amplifier for NF vs Gain will give an input for your system analysis.