I realize that historically swept spectrum analyzers could have negative center frequencies due to signal detection being performed at some IF, wherein back at the mixer if your VCO went low enough you'd start tracing out the shape of your RBW filters, eventually hit DC, and then just alias right back up to some frequency which -- mathematically -- is "negative." But is there any particular *use* for having a negative frequency? Or was it just an artifact of the hardware design that no one actually cares about other than as a curiosity? I've noticed that even some contemporary digitizing/microprocessor controlled SAs are more than happy to let you
*enter* negative center frequencies from their keypads!---Joel