Model System Noise Figure  RF receivers amplify signals and
shift them to lower frequencies. The receiver itself introduces
noise that degrades the received signal. The signal-to-noise
ratio (SNR) at the receiver output ultimately determines the
usability of the receiver.  The preceding figure illustrates the
effect of the receiver on the signal. The receiver amplifies a
low-power RF signal at the carrier fRF with a high SNR and
downconverts the signal to fIF. The noise figure (NF) of the
system determines the difference between the SNR at the output
and the SNR at the input:   where the difference is calculated
in decibels. Excessive noise figure in the system causes the
noise to overwhelm the signal, making the signal unrecoverable.
Create a Low-IF Receiver Model  The model ex_simrf_snr simulates
a simplified IF receiver architecture. A Sinusoid block and a
Noise block model a two-tone input centered at fRF and low-level
thermal noise. The RF system amplifies the signal and mixes it
with the local oscillat