why does zero IF receiver mandate quadrature demod

[henrywent]

does anyone know why zero IF receiver mandates quadrature downconvertion? please help, thanks in advance

 

[FvM]

Consider an unmodulated carrier as the most simple case and how the respective IF looks like in an conventional heterodyne versus a homodyne (zero IF) receiver. You'll be unable to detect even the carrier level without quadrature demodulation.

 

[flatulent]

The downconverted signal has positive and negative components. If you do not use the quadrature, the lower and upper sidebands of the signal fold on each other and interfere with each other.

 

[henrywent]

thank you for your kind reply, but i am still confused. If the modulated signal spectrum is symmetry with the carrier frequeny, it will do no harm if the two sidebands overlap each other when it is demodulated directly(i mean without quadrature demodulation). The magnitude of the resulting spectrum will only increase by a factor of 2. So is quadrature downconvertion required by the characteristics of the modulated signal rather than the receiver architecture?please help, i am badly confused

 

[FvM]

I suggest to perform a simulation of a zero IF receiver without quadrature demodulation and study the signals. Try to disprove the above statements.

Just another hint on the overlapping sidebands. They add with arbitray phase. Thus the summed magnitude is any value between zero and doubled single sideband. However, if you manage to phase-lock the demodulator to the carrier, it can work.

 

[eng-a-hesham]

The downconverted signal has positive and negative components. If you do not use the quadrature, the lower and upper sidebands of the signal fold on each other and interfere with each other.

So should i use one or two mixers in Zero-if receivers ?!! ... and if two,what are the differences in design between them ?!!
(i think a phase shift of 90 degrees between the LO signals feeding them is an important difference .. i'm asking for other differences )