On Sat, 09 Sep 2006 19:05:08 -0000, Hamateur
wrote:

wrote:

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.

I would say as long as the desired baseband signal remains within
the received bandwidth, selectivity has been accomplished.

Your misapplying standard terms to describe RF system behavour.

I agree DC receivers tend to have poor resolution, but this

Again, if anything ther is no reolution issue unless you applying it
to the frequency dial/display being used to tune in a signal.
An example of poor resolution would be a dial that reads to the
nearest Khz when you need to read to the nearest .01khz (10 cycles).

cannot be corrected by filtering the baseband signal by sending
it through a parametric audio equalizer. Any selectivity of
basebands has to be accomplished before detection. I'm sure
that's not what you meant- but what you said could be interpreted
that way. If I wanted 7.101 I wouldn't detect first and then
try to filter out 7.101 and 7.099.

Assume a DC RX. Lo at 7.100 for a CW tone of 1khz what frequency is
the recieved signal? It could be 7.101 or 7.099!

I agree some DC receivers seem like Michaelangelo trying
to scuplt David with a sledge hammer. You may still receive
the message but it will be impressionistic and so contain
many other messages.

Not at all and bad example at best.


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.

Q loses meaning when the desired frequency does not lie within
the relevant bandwidth. A tuned component can have a very high
Q and yet be very totally unselective of a desired frequency.

You do not understand what Q means then.

A tuned component can have a very high
Q and yet be very totally unselective of a desired frequency.

Meaningless misstatement!

A tuned component can have a very high
Q and yet be insufficiently selective of a desired frequency.

Would be a correct application.

What you are talking about is not what I would call selectivity.
I would call it "exclusivity" since it is more about excluding
than about selecting. I can acknowledge that if the exclusions
aren't done correctly at any point in the chain then selectivity
could be be lost.

Selectivity is measured in bandwidth and DB. These terms are standard
and meaningful. Exclusivity is marking hype at best and never applied
when refering to selectivity.

For me the difference between selectivity and exclusivity
seem alot like the difference between accuracy and precision.
I can be very precise but inaccurate at the same time,
I can be very accurate but imprecise at the same time.
So I find it easier to think that IF stages are more about
precision (exclusivity) than about accuracy (selectivity).

Get a dictionary.


But I quibble and realize "selectivity" is often used
to mean both accuracy and precision. It just seems to
me that IF stages are more about precision than about
accuracy.

It means neither. Precision is tied to resolution as a concept.
Acccuracy is a matter of calibration or using the same scale.
Selectivity is a matter of what is in or out and the measurements
for radios includes a in or out by how much.

For example a filter with 3khz bandwidth at 6db down with a shape
factor (usually measured at 6 and 60db on the slopes) 2:1 is
6khz wide at -60db. A filter that is 2 khz wide at 6db down with a
shape factor of 3 is also 6khz wide at -60db. However, they will
not sound the same in a given radio nor will the rejection of
undesired signals be the same. This is one of the metrics of
how radios are specified and discussed. To do so any other
way is like specifing the speed of you car in furlongs per fortnight.

A lack of accuracy in language will alway reduce the precision
in the discussion.

Allison