Negative frequencies on spectrum analyzers

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

Reply to
Joel Koltner
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By golly; you have just solved the economic crisis! As we speak, congress will be asking the FCC to auction all of the negative frequency spectrum! They will have to wait until after the ongoing auction of the blue light electromagnetic spectrum. Top bidders are Motorola, IBM and KMart.

--
Joe Leikhim K4SAT
"The RFI-EMI-GUY"©
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Reply to
RFI-EMI-GUY

All your negative frequencies are belong to us!

Reply to
Joel Koltner

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Reply to
Charles

Fine, but the question was... does anyone ever ask their spectrum analyzer to use a negative center frequency and, if so, why? A spectrum analyzer displays a real input signal, so most of them (that just have magnitude detectors) are going to display the exact same thing at -f as at f, and even those with phase detectors will just display the same thing but 180 degrees out of phase.

Reply to
Joel Koltner

A real-valued signal has a spectrum which is Hermitian--i.e. G(-f) = (G(f))*.

Your common-or-garden spectrum analyzer displays the power spectrum, which is obviously symmetrical about 0, so there isn't any point in tweaking it up to show negative frequencies--even if it could tell the difference.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

We make multichannel frequency synthesizers. If you set up, say, a

3-phase, 400 Hz waveform set, and consider the phase rotation to be clockwise, you can then set the frequency of the set to be -400, and it rotates ccw.

John

Reply to
John Larkin

Yes, negative frequencies can have very "real" physical meanings. But apparently no one around here ever enters "-1MHz" on their spectrum analyzers... which is good to know. :-) (I kept thinking some old timer would tell me that, e.g., people will look at 1MHz and then at -1MHz to see how symmetrical their AM sidebands are or something...)

Reply to
Joel Koltner

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