negative frequency result in RF mixer

[yefj]

Hello, We we recieve a signal we lower the frequency using a mixer and some of the result is baseband in negative frequency.
I cant understand how its possible to have negative value frequency.
in spectrum analizer wehave only positive frquencies

Thanks.

 

[FvM]

Negative signal frequency is an abstract concept in signal processing. It occurs in complex valued analytic signals. Real signals have only positive frequencies

 

[biff44]

it is "negative" in that the phase vs time goes opposite of what you expect.

a DSP system, properly designed, can sort it all out.
A simpler system would just take those "negative frequencies" and smear them on top of your positive frequencies, messing up any modulation you are trying to receive