RF

Homodyne/Zero IF/Direct Conversion   

Homodyne, Zero IF and Direct Conversion are all referring to a same thing. In this method, the signal get converted directly from RF to baseband and baseband to RF. That's why it is called 'Direct Conversion'. There used to be technical issues (e.g, DC offset or noise removal) and it is hard to do direct conversion when the RF frequency is very high. But the benefit for this system would be obvious. Less component, less cost and probably/hopefully less trouble for tuning components.

Possible Issues and Solutions :

i) Since signal frequency and LO frequency is same, engergy from LO can interfere with received or transmitted signal.

--> Possible solution : Increase LO frequency twice as signal frequency and use frequency divider (1/2) between LO and mixer.

 

ii) Even a small differnece between signal frequency and LO frequecy can directly influence the downconverted signal (baseband signal) which would cause BER.

--> Use AFC (Automatic Frequency Control) to dynamically tune the LO frequency to catch up signal frequency.

 

iii) Downconverted signal may contain baseband DC component (DC offset) which may de-sensitize the baseband demodulator.

--> Use DC Notch filter after the mix.