I have some doubts, and maybe someone can help me.

In a superheterodyne receiver design, I want to make a Noise Figure analysis by Spectrasys Simulation from mixer and for our intermediate frequency (3100Mhz). The graphic shows normals values for all components except for the amplifiers. The Noise Figure in these cases depends of the gain(the value showed is: NF value expecificated in the model parameters + gain expecificated in the model parameters) why it is happening?

(In the case of the amplifiers before baxic mixer the values are ok)

Thanks!

In a superheterodyne receiver design, I want to make a Noise Figure analysis by Spectrasys Simulation from mixer and for our intermediate frequency (3100Mhz). The graphic shows normals values for all components except for the amplifiers. The Noise Figure in these cases depends of the gain(the value showed is: NF value expecificated in the model parameters + gain expecificated in the model parameters) why it is happening?

(In the case of the amplifiers before baxic mixer the values are ok)

Thanks!

Here is some information about the effects of system noise figure when individual circuits are cascaded together. In a receiver design the overall system noise figure will be dominated by the noise figure of the first device. This is why in many cases a low noise figure amplifier is used first in a receiver system.

Let’s assume we have 3 amplifier stages with the following parameters:

· Stage 1 has a noise figure of 5 dB and a Gain of 20 dB

· Stage 2 has a noise figure of 10 dB and a Gain of 20 dB

· Stage 3 has a noise figure of 15 dB and a Gain of 20 dB

When cascading noise figure elements use the equation:

F=F1 + F2-1/Ga1 +F3-1/(Ga1)(Ga2)

Where F is the Noise Factor, which is linear units so for stage 1, 5 dB of noise figure F1= 5 dB/10 =0.5; 100.5 = 3.162

F2= 10 dB/10 = 1; 101 = 10

F3 = 15 dB/10 = 1.5; 101.5 = 31.6

Using the above equation F=F1 + F2-1/Ga1 +F3-1/(Ga1)(Ga2)

F=3.162 + 10-1/100 + 31.6-1/(100)(100)

F=3.162 + 0.09 + 0.00306

Noise factor=3.25506

Converting to noise figure = 10log3.25506

Noise figure system = 5.1256 dB

You can see how the first stage noise figure of 5 dB is going to dominate the overall noise figure of the system. The gain values really don’t affect the system noise figure, but the general rule of thumb is you want your 1st amplifier stage in a receiver to have the same amount of gain as the total loss of the system and not more.