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. :-)