Absolutely, and the closer you can get the analog to
digital conversion to the antenna input the less
chance there is to introduce all that mixer noise. The
key design challenges at that point are to manage the
noise in the A/D converter itself and to have good
dynamic range, neither are trivial I'm sure.
Ultimately the goal is to reach the holy grail of no
mixers at all by directly digitizing all RX signals.
It will be somewhat ironic to see radio technology
come all the way around to its origins by abandoning
the super heterodyne approach. Mr. Armstrong's basic
RX design idea has indeed had along run as the
dominant RX design, but I do get a growing sense it
will begin to fall out of favor.
As I've already pointed out in some of my previous
posts, I am partial to a direct conversion into DSP
design approach. There have been many reasons already
given to support the DC/DSP methodology instead of
using numerous IF stages with their cumulative noise
contributions. And we won?t even talk about the
convoluted schemes done to achieve passband tuning by
analog means. Also don?t forget all those pesky
ringing, ripple, and group delay kinds of problems
with aggressive analog filtering techniques. While
DC/DSP isn't the truly ideal direct digitization of
the RX signal, it is the next closest thing there is
with the technology that we could reasonably expect to
be able to afford for our ham shacks.
Duane
N9DG
--- cherry@getnet.net wrote:
> That is why a IF DSP receiver has the potential to
> be very low noise.
> The IF DSP receiver does not need narrow ringing
> filters or complex
> circuits to get passband tuning. There is no noise
> introduced by the
> product detector since it has no product detector.
> Multiple notches
> and noise cancelling can be added with no additional
> circuitry.
> Jim W7ANF
>
> Quoting n4lq@iglou.com:
>
> > In my unprofessional opinion.....Every time we run
> that little signal
> > through a non-active component such as a
> transformer, diode, coil,
> > capacitor or filter of any type, we loose some of
> it. Then we must boost
> > if back up with amplifiers. We worry about images
> so we use a high
> > frequency IF. Doing this adds more noise. To get
> the 14mhz signal to the
> > IF frequency we must add a
> mixer/oscillator....more noise.
> > Then we say...whoops..The IF frequency is too high
> for good selectivity!
> > So we must reduce it and install a filter. Slap
> another few mosey,
> > unproductive stages in there...another mixer,
> another few transformers
> > and some crystal filters....Oh..Crank up the gain
> boys, we're starting to
> > loose our patient! But wait...We need passband
> tuning..Slap in another
> > pair of mixers and oscillator and more
> transformers and BOOST HER UP
> > AGAIN! By the time we're done, our poor, wimpy
> little signal is almost
> > buried in all that racket.
> >
> > So why do we have all this extra baggage?
> >
> > A lot of it started when AGC became a standard
> fixture in our cw
> > receivers. With it, we need enough gain to produce
> a bias voltage and
> > make our S-meters flop around.
> >
> > Then came the general coverage receiver craze.
> Suddenly, the receiver
> > must cover 100khz to 30mhz....Not a bad feature in
> itself but various
> > compromises are needed now.
> >
> > Finally, along came the quest for super
> selectivity and passband tuning
> > the results of which we are all familiar.
> >
> > The QRP boys believe that most of the gain and
> selectivity should be done
> > at the audio level. Hum and microphonics are
> design problems that have
> > been overcome in recent years. A tight front end,
> one conversion stage, a
> > moderate gain IF with a few crystals in "ladder"
> arrangement, BFO/Mixer
> > and high gain audio amp blended with DSP would
> sure make a wonderful
> > sounding cw radio wouldn't it? :*)
> > LQ
>
>
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