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If you use low IF, you will need only one ADC.
If you use zero IF, and your modulation scheme is like an FSK which signal spectrum is not symmetric around carrier, you should use two ADCs and I/Q downconversion.
Deare Circuit_seller :
If the modulator is 64-QAM , The Low-IF nad Zero-IF , is the same . Low-iF need one ADC // Zero-IF using two ADC . And Why ? I know using low-if oe Zero-IF need I/Q . Why need I/Q ? Thanks
Some modulated signals such as FSK have not symmetry around center frequency that is if we downconvert it to baseband without using I/Q downconversion upper half of the spectrum and lower part of it will be aliased(folded).
I know two useful refrences about direct-conversion architecture.
1- Behzad Razavi's "Design Considerations for Direct-Conversion. Receivers".
2- Behzad Razavi's "RF Micro Electronics"
If you want to implement a 64QAM receiver, i recommend you to use other receiver architectures such as superhetrodyne or digital IF receiver.
Two implement an Direct-convesion receiver you have some problems. one big problem is output DC offset of the mixer.
Another problem is the phase differences between LOs and carrier frequency which corrupt signal content aroound DC.
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