two stage down conversion receiver

Hi,

I have seen some receivers that has single synthesizer chip from which both LO1 and LO2 have been derived(LO2 have been derived by using dividers), also in some receivers i have seen manufacturer has provided the option of separate LO1 and LO2,I just want to know which option is best ?

can anybody guide me......



Appuarpita

At sliding-IF receiver it is easier to generate LO2 by dividing, but the performance of a double IF receiver where the LO2 is fix much better usually. If the LOs are separated the filtering of the signal can be more effective.

hi,

why it is difficult to filter in sliding IF receiver


APPUARPITA

Even sliding-IF receiver suffers from 1st and 2nd image frequencies. It can reduce the image bandwidth, but cannot filter out the image perfectly.
To reject 1st image it needs image filter in the RF section, which is pain to implement.
To reject 2nd image it needs an IF filter between the mixers. The IF changes and the filter's parameters (low and high cut-off frequencies, quality factor) have to change with the IF. It makes difficult to get higher order filtering. Or an other option, to use a fix bandwidth filter on the whole 1st IF range, but the fix bandwidth will let more noise into the system, which decreases the receiver's noise figure.