measure SNR

i'd like to know how do you measure SNR using spectrum analyzer. my first thought was to measure the level of S+N, convert to mW, then remove the S and measure the worst case N or harmonic, convert to mW. Then convert the difference to dBm and take the difference between S and N. Is this correct? Any issue in the RBW setting with respect to the RF or IF bandwidth? Is there any other method / computation using spectrum analyzer?

One pitfall to avoid is the very bad noise figures of spectrum analyzers. You need to put a low noise, high gain amplifier on the input of the analyzer. Check for the noise floor raising a large amount when you connect your circuit to the low noise amplifier. Otherwise the analyzer noise will make errors in your calculations.