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Residual FM and RBW

Question asked by andyluo Employee on Apr 15, 2008
Latest reply on May 7, 2010 by Frank_BR
In AN150, there are the following sentences :
The minimum resolution bandwidth is determined, at least in part, by the
stability of the first LO. Analyzers where no steps are taken to improve upon the inherent residual FM of the YIG oscillators typically have a minimum bandwidth of 1 kHz. However, modern analyzers have dramatically improved residual FM. For example, Agilent PSA Series analyzers have residual FM of 1 to 4 Hz and ESA Series analyzers have 2 to 8 Hz residual FM. This allows bandwidths as low as 1 Hz. So any instability we see on a spectrum analyzer today is due to the incoming signal.

My question is:" since residula FM is larger than 1 Hz, how could we identify the two signal whose difference is 1 Hz.