"jawod" wrote in message ...

Your advice is taken. But, look thru the thread: there was reasonable
consensus and I recognized some "big hitters".

Yes, this particular thread seemed to gather surprisingly few space cadets

...

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.

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

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.

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

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

Henry Kiefer wrote:

You mentioned FM. I have a general question about the Tayloe mixer. Is it
possible to receive NBFM or FSK from VHF with it? I'm a little lost in the
question how to use the for outgoing phases or I/Q to demodulate FM. All I
found on the Net was doing shortwave SSB demodulation.

Provided you implement the Tayloe mixer with sufficient baseband
bandwidth (probably about 20 KHz for NBFM), you can mix, say, a 2 meter
FM signal to baseband and demodulate it with an audio discriminator
(some sort of audio frequency to voltage conversion scheme). Mix the
signal to be either exclusively in the upper or lower sideband of the
output. So, for example, a NBFM sig at 146.000 MHz mix with the Tayloe
mixer set to 145.990 MHz and select the upper sideband. The upper or
lower sidebands are, as you probably know, selected by phase shifting
and summing circuits following the Tayloe mixer (implemented in
software in SDRs). Note that SDRs like the Flex-Radio SDR 1000 do not
process incoming signals near 0 Hz anyway, but mix the desired signal
centered around about 11 KHz (I think).


Are there other analog switches going higher than the mentioned 70MHz for
FST3253 or 74HC4066? Perhaps video switches? What is better: Tayloe 4-phases
or a switched mixer with one output only? In the latter case I would need a
conventional IF filter after the switching mixer?


The Tayloe mixer is a passive mixer terminated in large capacitors, and
similar performance can be obtained at VHF, UHF and microwaves with two
FET ring mixers driven with quadrature signals and also terminated with
capacitors (that is, no wideband transformer on the mixer outputs, but
capacitors followed by HiZ input differential audio amps). I've done
this with the Peregrine Semiconductor FET mixers.

If I need a DSP at the baseband doing math with the phases or I/Q I would
think of an www.wavefrontsemi.com DSP AL3101/2CG DSP-1K will suffice? Wolud
it suffice? It is a little simple DSP mainly for doing FIR - but FAST and
with 24-bits including audio AD converters ip to 50KHz. The DSP runs with
50MHz up to 1000 instructions long until it repeats the prog. The nice think
is a very low pin-count package and cheap too.

There is maybe a middle way with a device like the www.cypress.com PSoC
family of mixed-mode Microcontroller with programmable analog cells. There
even exists a PSoC app note describing a heterodyne FSK receiver for 130KHz.

Maybe a www.microchip.com dsPIC is better?

Only familiar with the dsPIC, though the others sound okay. The dsPIC
would work, though its 12 bit A/D doesn't give a lot of dynamic range.
Plenty for NBFM though, especially since you can run the signal(s)
through a limiter first. For NBFM sixteen bit DSP is sufficient.

Regards,
Glenn

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

i3hev, mario held wrote:
Dave Platt wrote:

As I understand it, in order to do SSB via direct conversion, you must
use a modern version of the old IQ phasing technique. ...

that's correct
the problem here is that you need a phasing filter offering a constant
(and precise) 90 degrees phase shift all over the receiver band, and
that can be tricky to do...

These days, of course, you can apply the phase shift by converting the
two baseband signals to digital format...

well, you're opening doors to digital radio
But, AFAIK, the actually attainable dynamics does
not seem to incite enthusiastic greetings...

IQ phasing detection (AKA image reject mixers) are only necessary
if you want to build a single signal receiver (a good idea).
Many modern DDS chips provide a way to generate perfect quaditure
outputs and by using DSP you can combine the resulting quaditure
af outputs into a single signal. One way to do this on the cheap
is to have a pc sound card sample the two signals.