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.

hello!

radio_rookie wrote:

... IF was used in Superhet transceivers.

well, IF __is__ used in superhet receivers...

My question is why
doesn't anyone use zero IF now a days. ....

In fact, you can very well use zero IF in a receiver: what you get,
doing so, is a "direct conversion", or "synchrodyne", receiver, which
is simple, easy to design and realize, cheap, and yet can offer very
high level performances - as is well known to radio amateurs

But... yes, there's a but, and a price to pay: in a DCR, you put all the
required gain (a hundred of dBs, give or take a few) in the base band
(or audio frequency) chain, and this is bad for noise! Most electronic
devices are sensitive to pressure, and a sound is a pressure wave... you
may end up with something which can be quite a decent microphone!

Moreover, in a DCR it may be rather difficult to implement an effective
AGC, so switching from weak to loud signals may be annoying, and even
dangerous for hearing! Also, it's not easy to obtain a reasonable S-meter...

Last, but not least, a simple DCR is inherently a DSB receiver, lest you
implement it with rather complex mixers - which add much to the circuit
total complexity.

Adding all of this up, a superhet may be a good choice...
--
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

In article ,
i3hev, mario held wrote:

Last, but not least, a simple DCR is inherently a DSB receiver, lest you
implement it with rather complex mixers - which add much to the circuit
total complexity.

As I understand it, in order to do SSB via direct conversion, you must
use a modern version of the old IQ phasing technique. This requires
doing the zero-IF mixdown twice, on two different versions of the
signal (with a uniform 90-degree phase shift being applied to one copy
of the RF or LO signal). The resulting two baseband outputs are then
processed (with a further 90-degree phase shift being applied to one
of them) and carefully mixed. This results in reinforcement of the
desired sideband and suppression of the other.

The circuitry needed to apply the necessary phase shifts is not
trivial (if you want enough accuracy to deliver acceptable
opposite-sideband rejection), and is not necessarily simpler or less
expensive than the filtering and extra stage of mixing done in a
traditional IF-based SSB receiver or transmitter.

These days, of course, you can apply the phase shift by converting the
two baseband signals to digital format, and implementing the final
90-degree phase shift via a digital FIR all-pass filter. This of
course requires your design to have a pair of high-linearity ADCs, a
DSP, and a DAC to reproduce the final (mixed) signal.

The "holy Grail" these days seems to be a direct-from-RF system, in
which the RF signal is _directly_ sampled (at a ferociously-high
sampling rate), and all of the phase shifts and downconversion and
mixing are done digitally. RF-grade ADCs with the necessary linearity
and speed aren't particularly inexpensive, especially if you need your
system to deliver a very high dynamic range which can work properly
even in the presence of strong in-band or near-band interferers.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

On Thu, 07 Sep 2006 21:11:03 -0000, (Dave Platt)
wrote:

In article ,
i3hev, mario held wrote:

Last, but not least, a simple DCR is inherently a DSB receiver, lest you
implement it with rather complex mixers - which add much to the circuit
total complexity.

As I understand it, in order to do SSB via direct conversion, you must
use a modern version of the old IQ phasing technique. This requires
doing the zero-IF mixdown twice, on two different versions of the
signal (with a uniform 90-degree phase shift being applied to one copy
of the RF or LO signal). The resulting two baseband outputs are then
processed (with a further 90-degree phase shift being applied to one
of them) and carefully mixed. This results in reinforcement of the
desired sideband and suppression of the other.

The circuitry needed to apply the necessary phase shifts is not
trivial (if you want enough accuracy to deliver acceptable
opposite-sideband rejection), and is not necessarily simpler or less
expensive than the filtering and extra stage of mixing done in a
traditional IF-based SSB receiver or transmitter.

It is trivial, three stages of all pass using opamps for both the I
and Q paths and sum or difference the result. Using 1% parts and
decent opamps opposite sideband suppression of 45db is easy
and 50db or better attainable. Read Breeds and KK7Bs works.

However to build a good Image reject DCR and a good superhet
is not trivial when you consider shielding and mechanical issues
to get the best results.

These days, of course, you can apply the phase shift by converting the
two baseband signals to digital format, and implementing the final
90-degree phase shift via a digital FIR all-pass filter. This of
course requires your design to have a pair of high-linearity ADCs, a
DSP, and a DAC to reproduce the final (mixed) signal.

This is more reproduceable and works very well.


The "holy Grail" these days seems to be a direct-from-RF system, in
which the RF signal is _directly_ sampled (at a ferociously-high
sampling rate), and all of the phase shifts and downconversion and
mixing are done digitally. RF-grade ADCs with the necessary linearity
and speed aren't particularly inexpensive, especially if you need your
system to deliver a very high dynamic range which can work properly
even in the presence of strong in-band or near-band interferers.

Usually this technique is the "tailend" of a more conventional
Superhet so the RF detection is done at some low IF rather than
baseband. Typically 12-50khz for that IF. Problem is thes end up
being triple (or more) conversion reciever with the need for better
than average shielding or the birdies are prolific.

Allison

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...

--
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

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

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.

"radio_rookie" wrote in message
ps.com...

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?

Actually, it is fairly common -- it is called direct conversion and it is
very popular for simple rigs.

It is not without issues. Converting down to audio means that you cannot
eliminate the "other sideband". Also, since the RF amp needs to be
broadband, you can only get limited gain, so you end up needing a lot of
gain in the audio stage. This is achievable, but it is a little tricky to
manage oscillations, ringing and the like. In addition, getting narrow
bandwidth at audio is also a little dicey, and you can't have it at RF
unless you tune the RF ... also tricky.

Which is why most receivers these days are still superhets. You can choose
an IF that allows you to manage the bandwidth, and even so, multiple
conversion is pretty popular. Many commercial rigs are triple conversion,
typically with IF's around 60 MHz, 10 MHz and 455 kHz, with a very liberal
interpretation of "around" g

...

See URL:
http://www.fas.org/man/dod-101/navy/...perhet.htm#ads

and

http://www.eie.polyu.edu.hk/~ensurya...1/Chapter1.htm

CL


"radio_rookie" wrote in message
ps.com...
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.