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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