How to distribute receiver noise among various circuits?

[sriniv1422]

Receiver noise figure

Hi all

After determining the noise figure of a receiver from the system level specs , how should we plan to distribute the noise figure among various circuits like LNA ,Mixer , filter etc . Any ideas and hints on how to proceed with this would be greatly appreciated.

thanks

 

[aboozar.hamidipoor]

Re: Receiver noise figure

Hi:

I think the most important parameter which you should consider for distribute the noise figure between networks is the SNR .
as you know the Niose figure (F) define as the ratio of (SNR) at the input of network to (SNR) at the output of it .
another important parameter is the Ferris formula :

(total noise figure)=F1-1+(F2-1)/g1+(F3-1)/(g1g2)+(F4-1)/(g1g2g3)+...

hope to help you.

 

[flatulent]

Re: Receiver noise figure

The best way is to start at the mixer. It will have a noise figure and a gain or loss. The first IF amplifier will have a noise figure. From this you can determine the gain of an ideal amplifier preceding it to make the system noise figure.

This is my general starting point. Then you adjust the real RF amplifier gain to get the noise figure you need.

Do not forget to include the losses from the antenna to the RF amplifier.

Here are some rules to keep in mind:

The system noise figure can never be lower than the loss from the antenna to the RF amplifier.

The system gain should be as low as possible before each frequency constriction. This reduces IMD problems. What this means is that the RF bandwidth is restricted at the input antenna connector. The IF bandwidth is restricted before the gain stages. Passive mixers, in order to have high IP3, must have all three ports terminated in the design impedance over all frequencies for GHZ in all directions. This means that you will need diplexers on all three ports.

If management complains about the cost of the above then they are trying to fool Mother Nature. The specifications must be relaxed to meet the cost goals. Hay is cheaper after passing through the horse. You cannot make a silk purse out of a sow's ear.

You cannot have both low noise and low IMD at the same time. Selecting the antenna pattern will remove sources of signals that produce IMD. Selecting the location of the receiver helps a lot. One example is the space probe tracking stations that are located in rural areas with no people or electrical devices for tens or hundreds of miles around.