What is the advantage of having the extra stage of IF, extra local oscillator and extra filtering stage and amplification. It just seems like more work and circuitry. Surely the direct conversion to baseband and the intermediate conversion to baseband are both going to go through the same passband filter at the end of it that isolates the original frequency range?
I see answers such as 'it allows common circuitry to be used rather than separate circuitry for each frequency', basically:
Without using an IF, all the complicated filters and detectors in a radio or television would have to be tuned in unison each time the frequency was changed, as was necessary in the early tuned radio frequency receivers
But I don't understand because they could all be tuned to the baseband signal for instance rather than the IF, eliminating the problem described.
I also see:
So a narrower bandwidth and more selectivity can be achieved by converting the signal to a lower IF and performing the filtering at that frequency
But why not perform the filtering only at the baseband frequency IF=0. That is a lower IF, right. Does the addition of the IF passband stage improve the sharpness of final passband as opposed to doing it directly?
The only disadvantage I can understand is the leaking local oscillator and doesn't this happen more at higher frequencies? An IF could allow the first LO to oscillate much slower.