Measurement for Noise figure accurately

Hi, Here is a problem,
In a system where RF --->IF with AGC then to ADC then digital AGC then demodulator are the basic blocks. filters are present at each stage to handle the rejection issues and let say neglect them for simplicity.
Is there any method to find the NF accurately?
I posted a question for C/N but got no reply. So please......

Are you asking how to measure or how to calculate noise figure? If you would like to measure it, for that entire chain, I suspect that you'll want to disable AGC and back out a new figure from a Bit error rate measurement. This would include the demodulator performance as well, so it would be pretty complete.

If you'd like to calculate it, is there any reason you can't use the Friis equation and cascade the noise figure through the system?

Dave
www.keystoneradio.com

Measuring noise figure refers to the linear part of a receiver. Detectors, AGC and ADC must be removed to measure noise figure.
Any linear receiver can comprise a (low-noise) preamplifier, a down-converting mixer, filters, IF amplifiers. To measure noise figure, IF or RF output power is measured for at least two input noise temperatures introduced into RF input. One can use the Y-factor method or "twice the output noise" method described in textbooks.

Following the Friis' formula, the most significant noise contribution is from the first stage, often a LNA. If a mixer is used (often at microwaves), it is usually lossy, then the first IF stage defines the system noise figure.

The above rules are often misunderstood and IF detector is wrongly used to evaluate the output power; its response is rarely proportional to power. AGC, ADC and other non-linear circuits must be excluded from any NF test- noise figure is a linear parameter of a receiving system.