Topband: 160m Receivers - the argument for true DSP (long)

[jbattin]

----- Original Message -----
From: "Steve Ireland" <sire at iinet.net.au>
To: <topband at contesting.com>
Sent: Friday, October 17, 2003 11:20 PM
Subject: Topband: 160m Receivers - the argument for true DSP (long)


>
>
> In terms of noise if we build a simple DC receiver and use a PC sound card
> to sample the quadrature output at 44kHz then we have 88kHz and tens of
> milliseconds to look at that band and work out what is noise and what it
> signal. Such a wide bandwidth means that the noise pulses are not slurred
by
> passing through crystal filters, which simply make matters worse. Once in
the
> digital domain we can then subtract the noise, not just blank it or cancel
> it. Software that can perform this sampling is available for free on the
> Internet - the effect of such
> noise reduction is staggering, especially if your current experience is
> limited to IF LMS noise reduction or add-on AF DSP systems on radios such
> as the FT1000MP.
>
  Impulse noise usually requires a group of pulses to have enough energy to
be a problem. Even a spark plug generates thousands of individual pulses,
and if a noise blanker is to be effective, it must be able to blank these
pulses individually and allow the recovery of signal information between
them. This why the further toward the antenna the blanking occurs the better
it works. However there are big trade-offs. The further toward the antenna
you blank(or cancel) the less selectivity is allowed in the sampling channel
and this makes the blanker more susceptable to interference. The trade-off
we hear most often is that a strong signal(carrier) with-in the pass band of
the selectivity in front of the blanking gate is modulated by the blanking
pulse, creating sidebands that interfere with the desired signal ...
sounding like the original noise and sometimes worse the original
interference.
The reason I bring this up is that there seems to be a suggestion that if
noise cancellation (subtraction) is done in the digital domain than these
interference issues will not exist.... I challange that. When noise is
subtracted there will be a modulation of interfering signals that will spill
over and interfere with the desired signal.

>
>> ---------------------------------------------------------------------
> W8JI: >By the way, none of the data or measurements I have been able to
find
> >indicate these software-based radios are as good overall as what we have
> >now. 40dB TOI is absolutely meaningless unless we know a whole lot more.
An
> >absolute piece of receiving-junk could have 40dB TOI.
>
>
> ---------------------------------------------------------------------
>
> W8JI: >As for BW. Ringing is a direct function of bandwidth, the shape of
the
> >bandwidth slope, and varying group delay times through the system as
> >frequency is changed. As W4ZV pointed out, we can't possibly use 20Hz or
> >even 100Hz  BW with "brick wall" skirts for normal CW. The filter MUST
pass
> >all sidebands generated by the rise and fall time, or it extends the rise
> >and fall!!!
> >We have to pass the significant sidebands, or the signal "rings". The
> >spacing of the sidebands is dictated by the rise time and fall time. This
> >would also apply to any noise pulses going through the filter. Narrower
> >selectivity lengthens the duration of noise pulses...even if it is a
> >software filter....and makes noise "ring". The more it rings, the more
> noise
> >sounds like CW.

 I   100 percent agree with Tom. If someone likes 20Hz. selectivity it is
because he does not have 20Hz. selectivity.
If reciever is operating in the linear mode, and interference is not a
problem, then it makes no difference if the selectivity is in the antenna
lead of in the headphone lead. As interference increases and the active
stages begin to be driven into non-linearity, then the further forward the
selectivity is moved the better it works. This is why every DSP radio I have
tested (That includes most of them) is a poor performer under heavy
interference unless the DSP is protected by some form of selectivity
(sometimes called a roofing filter).

 John K9DX______________________________________________
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