On Fri, 08 Sep 2006 19:05:49 +0200, "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...

This is not that hard and there are tricks to do this over very wide
ranges. The easiest is create a fixed 90 degree reference and down
convert it.

Another is use one of the I&Q output DDS chips.

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...

The problem is finding fast A/D with enough bits (more than 16)
and using 32bit processors to grind the data. Those two things
are not easy or cheap.. yet. But we are getting there.

Allison

wrote:

This is not that hard and there are tricks to do this over
very wide ranges. The easiest is create a fixed 90 degree
reference and down convert it.

that's true but, imho, there's no meaning in removing the (moderate)
complexities of the typical superhet receiver just to put (trickier)
complexities in quadrature downconverting... apart from part count,
controlling the reciprocal phase in two "identical" converters is in
itself not an easy task, since just a single degree of dephasing is
enough to degrade unwanted sideband rejection.

... Those two things
are not easy or cheap.. yet. But we are getting there.

I'm sure we will get there soon enough, and it may
well be that just a few years will suffice; but, as
for now... they are not easy or cheap!

--
73 es 51 de i3hev, op. mario

Il vero Radioamatore si riconosce... dal call in firma!
- Campagna 2005 "Sono un Radioamatore e me ne vanto"

it.hobby.radioamatori.moderato
http://digilander.libero.it/hamweb
http://digilander.libero.it/esperantovenezia

Roy Lewallen wrote:

I believe that what you said is true, but only for AM. What other kind
of modulation requires better frequency accuracy from a direct
conversion receiver LO than from a superhet LO? I believe that only a
small fraction of today's amateurs are interested in AM reception, but
of course it's the bread and butter of the SWL and BCL.

Roy Lewallen, W7EL

Andy responds

I don't disagree with anything you've said. However , i've
found it a lot easier to make the final frequency adjustments
in a superhet since it can be done at a lower frequency.
Trying to sync up at 915 is more challenging than at 455 khz,
obviously. And for systems like FSK and SSB, you gotta
do it somewhere..... And with FM, since the beat formed
by the carrier depends on the modulation index of the
received signal, getting rid of it can be squirrelly.....

Like youself, I've built both types , both as home projects,
and as commercial products, for a long long time. In the
DC versions, if I use I/Q and combine them to form the
audio, the AGC is the same as with superhet SSB. And I generally
use PIN diodes before the front end for the first AGC stage...
But, that's just my own preference.....

Andy W4OAH

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.

Hey radio rookie,

Thanks for posting this excellent question.
The answers you received have taught me more than a few textbooks have.
Some very good minds have weighed in on your query.

Keep up the good work and welcome to ham radio.

To the respondents, I say "Thanks!" for approaching ideal NG activity!

Makes one proud to be a ham.

John
AB8O

On Fri, 08 Sep 2006 22:04:06 +0200, "i3hev, mario held"
wrote:

wrote:

This is not that hard and there are tricks to do this over
very wide ranges. The easiest is create a fixed 90 degree
reference and down convert it.

that's true but, imho, there's no meaning in removing the (moderate)
complexities of the typical superhet receiver just to put (trickier)
complexities in quadrature downconverting... apart from part count,
controlling the reciprocal phase in two "identical" converters is in
itself not an easy task, since just a single degree of dephasing is
enough to degrade unwanted sideband rejection.

I disagree with trickier vs moderate. For a given level of
performance the two are comparable. The only context is
one may be familiar vs different as in new.

The phase of two converters is pretty easy to maintain, having
done it. However, creative part was down converting the LO
rather than the more critical small signal paths. Since the LO
would only at most require low pass (VFO at 70-124mhz with
fixed IQ generation at 70mhz would give a simple dc-54mhz LO)
to prevent the 70mhz and higher components. If passive DBMs
are used phase shifts from gain and added LCs can be avoided.
Also it's possible to put inline measurement of the resulting LOs
and feed back any phase error (found unneeded) for correction.

To build a superhet (ignore selectivity and overload performance for
the moment) to cover dc-54mhz you need upconversion and then down
conversion to IF and lots of filters along the way with switching.
Then you have the problem of various LOs and carrier osc (SSB/CW)
getting back into previous stages causing undesired tones. The more
conversions the greater the problem and the better shielding must be
to avoid it.

Having built both each has challenges and to do it as a high
performing reciever (or TX) requires a very similar effort even if
the areas where the effort need be applied differ.

The one big difference is that the image reject system is less
hardware between antenna and the baseband and that hardware
is more frequency agile. The best available analog answer is the
KK7B MiniR2 (or R2-PRO), base design uses SBL-1 mixers good
from 1-500mhz making generation of an 90degree LO the only real
work. As you go up in frequency a quadrature splitter will have
both phase and level bandwidth that increases making a
tuneable VHF or UHF IF practical. (FYI: accepable (under 1degree
and under 1db level variation) bandwidth at 14mhz is 1mhz.)

Of course the image reject detector system could exist at any fixed
frequency such as 66 mhz with a fixed LO. Then the upconverter only
needs a conventional LO of 66 to 120mhz still yeilds an RX that tunes
from DC to 54mhz and has few if any adjustments. This would yeild a
Single conversion RX with a VHF IF that is remarkably simple
and needs few filters.

If you take that to the next step using DSP at baseband where it's
cheap (using PS and soundcard) you have a very straightforward
high performing multimode radio. This is the current SDR approach.
When you consider a mainboard with ram, sound and other needed bits
running at 1+ ghz is dirt cheap (found free sometimes) and small this
is achieveable. Especially considering the software is already out
there. Also unused CPU cycles could also do the "glass front pannel"
and manage things like tuning the DDS or PLL.

... Those two things
are not easy or cheap.. yet. But we are getting there.

I'm sure we will get there soon enough, and it may
well be that just a few years will suffice; but, as
for now... they are not easy or cheap!

Actually it's progressing faster than you'd notice. The number of Ham
radios with IF DSP as the detection is considerable. Most of the
upper models of Tentec, Yaesu, Kenwood and others are already
doing it. Also the vendors of the DSP processors do have boards
for proto work to venture into this realm. With more people doing it
and plenty of alreay published work it may be more assembly
of components (be they hardware or software) than
construction.

While this wanders far from the IF question posted it's an example fo
how you can have IFs from baseband through UHF or higher.

Allison

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 don't disagree with anything you've said. However , i've
found it a lot easier to make the final frequency adjustments
in a superhet since it can be done at a lower frequency.
Trying to sync up at 915 is more challenging than at 455 khz,
obviously. And for systems like FSK and SSB, you gotta
do it somewhere..... And with FM, since the beat formed
by the carrier depends on the modulation index of the
received signal, getting rid of it can be squirrelly.....

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.
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?

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?

Would it better having the IF not at zero but at a usually higher IF, say
25KHz (remember the NFBM!)?

regards -
Henry

"john" wrote in message
...
radio_rookie wrote:

Some very good minds have weighed in on your query.

I hate to sound "college professor-y", but you really need to be familiar
with the literature.

One of the challenges with anything on the Internet is that anyone can say
anything. That is one of the great advantages, too! The problem is,
especially in a technical forum like this, you will get responses from folks
who don't have a clue but want to sound knowledgable. What is perhaps
surprising, is that sometimes, especially on this NG, some of the truly
giant minds in the field will also weigh in. Unless you are familiar with
who is who, you will have a tough time assessing the responses.

But if you review the books and magazine articles on the subject, and browse
the archives of various other forums, you will see that some of the leading
experts have posted here. Recognize that they might not sound too different
than someone who is simply guessing, and be critical in your analysis of the
responses you have received.

...

xpyttl wrote:
"john" wrote in message
...

radio_rookie wrote:


Some very good minds have weighed in on your query.


I hate to sound "college professor-y", but you really need to be familiar
with the literature.

One of the challenges with anything on the Internet is that anyone can say
anything. That is one of the great advantages, too! The problem is,
especially in a technical forum like this, you will get responses from folks
who don't have a clue but want to sound knowledgable. What is perhaps
surprising, is that sometimes, especially on this NG, some of the truly
giant minds in the field will also weigh in. Unless you are familiar with
who is who, you will have a tough time assessing the responses.

But if you review the books and magazine articles on the subject, and browse
the archives of various other forums, you will see that some of the leading
experts have posted here. Recognize that they might not sound too different
than someone who is simply guessing, and be critical in your analysis of the
responses you have received.

..


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

I didn't mean to suggest that one learns best on the NG. Rather, my
intent was to support and give kudos to those who bring the NG up to a
reasonably high standard: no flames, insults, diversions...you know, the
usual stuff.

Here are some of my "favorite" NG remarks
"Get help"
"You have serious issues"
"What's YOUR mensa number?"
"I spent xx years at xx and xx years building xx and have xx degrees, so
I know what I'm talking about"

As one monitors these NG's, one begins to "smell" the diff between
the knowledgeable and the blow-hards. The only problem comes when they
are one and the same. Luckily, this is uncommon.

I appreciate your advice, "Professor"

John
AB8O

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.