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Hamateur · September 9th 06, 04:13 PM posted to rec.radio.amateur.homebrew

wrote:

On 7 Sep 2006 13:17:19 -0700, "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?

Images of the same signal may be a source of interference.

Does the IF stage conditions the incoming signal?

Yes, in many ways. The most significant are amplification
and selectivity.

What are the advantages of the IF stage?

Less amplification needed at the recieved frequency.
Gain at a frequency removed from the recieved frequency.
Selectivity is easier to obtain at lower frequencies.
Gain control can be applied if needed.

Allison

I can easily agree that an IF amp's job is to cleanly
and efficiently amplify a specific, modulated, carrier frequency
and to allow for gain control feedback.

But I don't see how "selectivity" should be considered a
function of an IF amp (other than they're not amplifying
what they shouldn't amplify).

It seems easier to think of "selectivity" as a property of
a tuner or several tuner stages.

It's hard for me to think of IF "stages" as improving tuner selectivity
when my homebrew 40m DC recvr seems to be selective enough
so that when I listen to CW the pitch will not change audibly.
The frequency may fluctuate a little, but certainly not enough to
loose a signal, and it does not drift monotonically enough to
worry about (except perhaps as a matter of pride).

The superhet's conversion mixers/filters/amps seem to be
considered sub-steps of "IF stages", but I find it easier to
think of the mixer/filter steps as "stages of tuners interlaced
with IF amplifier stages".

Since the final conversion step may represent a detection,
the idea of "selectivity" as being interlaced with IF amps
has a more tidy representation in my mind.

Comments, criticisms, corrections, caveats - are always welcome.

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