radio_rookie wrote:
Hello,
I want to know the importance of intermediate frequency in any
receivers. IF was used in Superhet transceivers. My question is why
doesn't anyone use zero IF now a days. What is the problem of brining
the RF signal directly to baseband?

Thanks.

Andy writes:
This is the eighth response to your question. All of the others
are accurate. HOWEVER, there is one technical problem with
a homodyne that outweighs the previous resonses...

The L.O, has to be EXACTLY the same frequency as the incoming
signal to provide proper demodulation......

With a superhet, depending on the modulation, you can be
many Khz in error ( for AM) or a few dozen off ( for voice SSB).
With homodyne, you may have to process a psuedo-baseband signal which
has frequency components you don't want..... that is a bitch....

In order to achieve the LO to be EXACTLY the same
frequency (phase doesn't matter since you can use I and Q),
it is necessary to achieve frequency lock. That requires a
much higher S/N than a simple superhet with a detector...
So you lose sensitivity. In some systems , you can lock
to a remote carrier, but you aren't really talking about those
methods, I don't. think....

So, the major technical problem is not SIMPLICITY or
BANDWIDTH or NUMBER OF STAGES...,.. it is
how to obtain an LO of the correct frequency.... All the
other problems are simple compared to this.... for most systems...

Andy W4OAH

PS I welcome dissent and would like very much to learn
if anything I have said is in error......