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

"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

...

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!

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.

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

From: radio_rookie on Thurs, Sep 7 2006 1:17 pm

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?

This can be a HUGE subject, but, since this is "homebrew"
we can 'distill' it to a few things: :-)

1. Ever-present random NOISE in the front end. Can't
escape it. Since the amount of noise voltage
reaching the demodulator can be reduced by the
square-root of relative bandwidth, IF bandpass
filtering can cut down that random noise, yield
a constant selectivity regardless of RF input.

2. Direct conversion to baseband is subject to dynamic
range limitations v. the amount of RF input power
and RF input selectivity. i.e., a very strong signal
well out of the desired RF input range might mess
with the sampler causing intermodulation distortion.

3. Lowest RF input level (which determines the
"sensitivity" specification) requires a very low-
noise sampler to equate to a full superhet with an
IF chain. Samplers are not noise-free. Samplers
must compete on the tenths of microvolts (or less)
noise with conventional active mixers of now to
meet high-sensitivity specifications of today.

4 Software (as in an SDR architecture) is NOT simple
to implement, even in a very fast processor. While
it is easy to change demodulation modes, one needs to
understand the math behind the demodulation process.
If you have the TIME and the smarts, go for it; if
not, it may be months before your project works and
then it may not work very well.

5. Not all RF input signals are AM or derivatives of
that (on-off keying, SSB on HF). For FM or
combination AM-PM as in the "modem" fashion, it
might be much easier to implement via a separate IF
plus separate demodulator per mode.

6. In the beginning (1918 and Ed Armstrong in Paris
right after WW1), vacuum tubes were NOT what one
could call the best, noise-free, or even with much
gain. The superhet form allowed the same selectivity
(via the IF bandpass) at any desired RF input
frequency; that did not exist before the superhet.
Since that was a quantum-level improvement at the
time, it had a mystique about it that caused nearly
all designers to follow the IF chain idea with its
diode or tube "detector" (really a rectifier-mixer).
The math of modulation had been published in 1915
(John R. Carson of AT&T) but had yet to spread. It
was not intuitive to the non-mathematical and so
few designers got "into" possible new ways to mix
and demodulate. With better tubes that came after,
the IF and 2nd IF and even 3rd IF as discretes was
easier to design and make. That lasted until
roughly 1980 or about 6 decades, all superhets
having IF chains in a familiar arrangement. It was
"comfortable." More importantly, it worked.

7. If you want selectable bandpass filtering at all
frequencies, the IF with its input bandpass
filters at most any bandwidth you want is the
easiest to design-in and build. That way you
lop off the signals on either side as close to
the antenna as you can get.

8. Heterodyning (mixing) down to one frequency, the
IF, makes it easier to work and debug with a
semi-direct-conversion system. Especially so if
the desired RF inputs have many bands.

9. On the other hand, if portability, light weight,
and low power drain is a requirement (as in military
field receivers), plus all sorts of demodulation
modes, the SDR or Software Designed Radio is the
thing to do, using samplers, A-D conversion and
demodulation in a processor subsystem. Note: You
combine the front end of a conventional IF with
the processor sub-system replacing the IF back end
and 'detectors' to get the best of both.

There isn't any one simple answer. It is all a trade-off
between what is desired and what you can design and make
and how much you have to build plus your budget. Its all
wonderfully complex to decide and I love it. :-)

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?

Thanks.

Andy writes:
This is the eighth response to your question. All of the others
are accurate. HOWEVER, there is one technical problem with
a homodyne that outweighs the previous resonses...

The L.O, has to be EXACTLY the same frequency as the incoming
signal to provide proper demodulation......

With a superhet, depending on the modulation, you can be
many Khz in error ( for AM) or a few dozen off ( for voice SSB).
With homodyne, you may have to process a psuedo-baseband signal which
has frequency components you don't want..... that is a bitch....

In order to achieve the LO to be EXACTLY the same
frequency (phase doesn't matter since you can use I and Q),
it is necessary to achieve frequency lock. That requires a
much higher S/N than a simple superhet with a detector...
So you lose sensitivity. In some systems , you can lock
to a remote carrier, but you aren't really talking about those
methods, I don't. think....

So, the major technical problem is not SIMPLICITY or
BANDWIDTH or NUMBER OF STAGES...,.. it is
how to obtain an LO of the correct frequency.... All the
other problems are simple compared to this.... for most systems...

Andy W4OAH

PS I welcome dissent and would like very much to learn
if anything I have said is in error......

AndyS wrote:

Andy writes:
This is the eighth response to your question. All of the others
are accurate. HOWEVER, there is one technical problem with
a homodyne that outweighs the previous resonses...

The L.O, has to be EXACTLY the same frequency as the incoming
signal to provide proper demodulation......

With a superhet, depending on the modulation, you can be
many Khz in error ( for AM) or a few dozen off ( for voice SSB).
With homodyne, you may have to process a psuedo-baseband signal which
has frequency components you don't want..... that is a bitch....

In order to achieve the LO to be EXACTLY the same
frequency (phase doesn't matter since you can use I and Q),
it is necessary to achieve frequency lock. That requires a
much higher S/N than a simple superhet with a detector...
So you lose sensitivity. In some systems , you can lock
to a remote carrier, but you aren't really talking about those
methods, I don't. think....

So, the major technical problem is not SIMPLICITY or
BANDWIDTH or NUMBER OF STAGES...,.. it is
how to obtain an LO of the correct frequency.... All the
other problems are simple compared to this.... for most systems...

Andy W4OAH

PS I welcome dissent and would like very much to learn
if anything I have said is in error......

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.

I've build direct conversion receivers for many years. Their simplicity
is particularly evident when you compare a transceiver having a direct
conversion receiver with one having a superhet receiver -- to be honest,
be sure to include all the extra filtering necessary with the superhet.
The single biggest disadvantage to DC receivers, in my opinion, is the
difficulty of making a good AGC, particularly in conjunction with narrow
audio bandwidth. And they do have their own set of potential problems,
such as unintended AM demodulation, the effects of LO leakage and
radiation, and the difficulties in making a clean, stable, high gain
audio amplifier. But all can be overcome once one understands the causes
of the problems. All but perhaps the last one will be present in a
digital version, too, so a casually designed and/or built one is likely
to be a poor performer.

Roy Lewallen, W7EL

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