On Sat, 09 Sep 2006 15:13:15 -0000, Hamateur
wrote:

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).

First that last is the central description of what an IF should do.
And the word that defines what should or should not be amplified is
selectivity (or bandwidth).

That come from the former use of distributed selectivity in IF stages,
AKA those old IF cans. Since stages were coupled with tuned circuits
it was possible to add both gain and selectivity. However in modern
designs the IF is preceeded by a crystal filter giving lumped
selectivity. In the end the when people talk about an IF, gain, gain
control and selectivity are central parameters of that circuit block.

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

Usually image rejection is perfomed there. Selectivity as in 3khz
bandwidth would be difficult to do at 50mhz!

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).

DC gets its slectivity at baseband using bandpass or peaking filters.
Also if it's not a image reject design it sees images making it's
selectivity effectively twice the bandpass filters width.

Example of DC at 7.1mhz... if the desired signal is 7.1 and
offending signals at 7.101 and 7.099 what do you hear?
That is where selectivity is important.

Drift is a seperate issue and with care very managable.

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".

I'd prefer to not hear that. It muddies the functional description of
what the stage does. It is better to think of RF, Mixer, IF and
detector as distinct systems with functional goals even though the
raw parts used could be very similar.

You use "tuners" in ways that are better described with different and
more specific terms.

For example a tuned circuits at 14.000mhz even with very good Q
will be broad compared to the desired signal. In fact it's barely
narrow enough if the IF is 455khz to suppress the images (lo at
13.545 and image at 13.090). However, at an IF of 455khz with
four tuned circuits of decent Q will give enough selectivity for an
AM signal but marginal for close spaced SSB signals.

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.

Valid and very traditional designs were exactly that. However
consider lumped gain used with crystal filters. Same effect
very different looking. Lumped vs distributed selctivity
and the same for gain.

Old tube designs would have multiple IF stages at moderate
gain with with multiple tuned circuits for selectivity.

Current solid state would use a ceramic or crystal filter
with lumped gain in the form of an IC or two following.

Both could be designed to provide the exact same gain
and slectivity profiles yet their topology is different. In the
we can use the same terms to talk about both as black boxes
but differing terms when discussing the content.


Allison