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

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.

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).

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).
this is not selectivity, this is stability. selectivity is filtering
nearby strong signals, which direct conversion has more trouble with,
especially as they get closer.
the IF allows cheap, narrow, lower freq filters, which will have great
side skirts to remove the nearby
strong signals.

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".
ok, then.

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.

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).

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).

this is not selectivity, this is stability. selectivity is filtering
nearby strong signals, which direct conversion has more trouble with,
especially as they get closer.
the IF allows cheap, narrow, lower freq filters, which will have great
side skirts to remove the nearby
strong signals.

I tend to think of "stability" more in terms of random fluctuations.
Instability may or may not effect selectivity. As long as my desired
selection remains decipherable, I would say that selectivity has
been accomplished regardless of whether there's any kind of instabilty.

Filtering strong nearby signals seems more about "exclusivity"
than "selectivity". I would rather say that IF stages maintain
selectivity while they are excluding undesired mixing products
and other signals.

As long as a recvr includes my desired frequency and it
does not drift out of my receiver's bandwidth requiring
me to retune, then I would say that the recvr is maintaining
selectivity even if I get *more* than what I want.



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".
ok, then.

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.

ok, call these things whatever you want, but you are making up new names
for existing specs.
I have never seen an 'exclusivity' db rating for a radio, and your
desired signal remaining in the passband has little to do with selectivity:
try listening to a weak signal next to a SW broadcaster, then you find
selectivity. It is invisible until you need it.

end

Hamateur wrote:
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).

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).
this is not selectivity, this is stability. selectivity is filtering
nearby strong signals, which direct conversion has more trouble with,
especially as they get closer.
the IF allows cheap, narrow, lower freq filters, which will have great
side skirts to remove the nearby
strong signals.

I tend to think of "stability" more in terms of random fluctuations.
Instability may or may not effect selectivity. As long as my desired
selection remains decipherable, I would say that selectivity has
been accomplished regardless of whether there's any kind of instabilty.

Filtering strong nearby signals seems more about "exclusivity"
than "selectivity". I would rather say that IF stages maintain
selectivity while they are excluding undesired mixing products
and other signals.

As long as a recvr includes my desired frequency and it
does not drift out of my receiver's bandwidth requiring
me to retune, then I would say that the recvr is maintaining
selectivity even if I get *more* than what I want.


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".
ok, then.
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.

Hamateur wrote:
. . .
I tend to think of "stability" more in terms of random fluctuations.
Instability may or may not effect selectivity. As long as my desired
selection remains decipherable, I would say that selectivity has
been accomplished regardless of whether there's any kind of instabilty.

Filtering strong nearby signals seems more about "exclusivity"
than "selectivity". I would rather say that IF stages maintain
selectivity while they are excluding undesired mixing products
and other signals.
. . .

You're certainly free to make up interpretations of words any way you
choose. But if you want to communicate with others, that is, to have
them understand what you're saying and for you to understand what
they're saying, it's necessary to use common terms in the way they're
widely understood to mean.

In this context, "selectivity" is universally understood to mean the
ability to pass some signals and reject others, on the basis of their
frequencies, and is quite independent of stability. If this isn't what
you mean by "selectivity", you should use some other word or make up a
new one and define it, if your objective is to understand and be understood.

In a superhet receiver, most of the selectivity is achieved in the IF
stages, for a number of good reasons. One of the reasons is that it
prevents off-frequency signals from being amplified to a high level
where they can cause intermodulation and other problems. In a direct
conversion receiver, all the selectivity (other than relatively broad
selectivity from any bandpass filtering ahead of the mixer) is achieved
by audio filtering. Properly done, this filtering is near the input of
the high gain audio amplifier. Neither is inherently better than the
other at the basic job of providing selectivity.

Roy Lewallen, W7EL

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

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.

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

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.


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.

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.

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).

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.


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

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

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? Does the IF stage conditions the
incoming signal? What are the advantages of the IF stage?

Just confused. Can anyone throw some light on this?

Thanks.

Direct conversion is used in nearly all modern mobile phones, because it is
cheaper (no IF filters), and because the baseband amplifiers use less
current than the old IF amplifiers used to. The RF performance is not
necessarily as good as a well designed superhet.

It is difficult to make the receiver immune to "AM detection" which is a
problem caused by receiving a strong interfering signal with amplitude
modulation on it, at a frequency other than the one that you are trying to
receive, but which somehow gets turned into a baseband frequency signal
coming out of the mixer, even though it shouldn't. There are plenty of
reasons why this can happen, such as second-order nonlinearity in the
mixer, meaning that a strong interferer coming into the RF port of the
receiver can mix with itself in your mixer and end up on top of the wanted
signal. Another cause could be if there is coupling between the LO
generation circuit and the RF input (in either direction, both are bad.)
Also it is common to get large DC offsets coming out of direct conversion
receivers, and for some modulation formats where you're interested in
frequencies down to DC, that can be a pain. People have pretty much solved
these problems in phones, after a lot of work.

Chris