From: "Mike Dennison" <mike.dennison@ntlworld.com>
To: rsgb_lf_group@blacksheep.org
Date: Wed, 11 Jul 2012 17:08:07 +0100
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Subject: Re: LF:  OPERA  Question
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OK, Graham, Many thanks for the helpful reply. Of course it helps if 
you have a bomb-proof radio that doesn't mind having close-in strong 
QRM. The main problem with my old IC-706 is the inability to switch 
off the AGC.

de Mike
=======

> Welcome to the crazy world of  JR  !
> 
> The  basic  explanation is the  narrow filters  increase the  noise
> power in a limited   bandwidth , ringing  due to the  Q , similar to 
> CW , narrow filters tend to  round  the  CW signal
> 
> The DSP  engine  is  better equipped to  differentiate between 
> carrier and noise and  has a  much  greater  dynamic  range , so 
> optimum results  are obtained , when the  signal is  presented  to the
>  interface, as close to to the  original as  possible , the  DSP
> filter profiles are  tailored to the mode/speed in use
> 
> This can be  noticed  with the  new  generation of   SDR  support
> software , where  audio/voice recovery  can be  superior to
> conventional hardware based systems
> 
> With hardware filtering , there  are  transit  (group) delays which 
> can alter the  amplitude / time  , either  from  on/off  keying  or 
> with FSK, this can  be  observed when  sending  wide  band  FSK ,
> although  the  audio level  remains  constant and  'phase continuous' 
> , its possible  some times to  see a  'am modulation'  envelope  on
> the  carrier    ... $$$$   can solve this  problem , but  for  most 
> Ham  kit its  something  that 'happens'
> 
> Physical  constraints , if  there  is  a  very large  carrier in the 
> pass band  and  this  is pushing  the  hardware into  non-linearity /
> A/D  to over  range  , giving   quantising  errors, then  , yes 
> filtering would help , but  its more  likely  its the  analogue  path 
> that's  causing intermod products .....reducing  the  rf/if gain will 
> provide the  solution
> 
> I think that's  about the  picture , if  Jim's  about , im sure  he 
> will fill in the  gaps
> 
> 73 -G..
> 
> 
> 
> 
> --------------------------------------------------
> From: "Mike Dennison" <mike.dennison@ntlworld.com>
> Sent: Wednesday, July 11, 2012 10:18 AM
> To: <rsgb_lf_group@blacksheep.org>; <rsgb_lf_group@yahoogroups.co.uk>
> Subject: Re: LF:  OPERA  Question
> 
> >> Narrow   IF filters  are  not  desirable  and   reduce the
> >> performance of the  demodulator  , better  simply  use  SSB  filter
> >> GL ..73 -G..
> >
> > Graham,
> >
> > Why is that the case? What does the considerable extra bandwidth
> > achieve? Is the SSB bandwidth optimal, or would it be even better
> > with 10kHz bandwidth, or 100kHz?. Why is 3kHz better than perhaps
> > 1kHz or 2kHz?
> >
> > At first glance it appears crazy to let in all sorts of adjacent
> > channel QRM (the bandwidth is more than ten times the size of the
> > entire Opera window) when using a mode that occupies a fraction of
> > 1Hz.
> >
> > Is it simply that the 1.7kHz Tx tone is high enough for the SSB
> > filter to kill its harmonics, and on receive it is difficult to get
> > a 1.7kHz tone out of a CW filter, even with passband shifting.
> >
> > Am I missing something?
> >
> > 73 de Mike, G3XDV
> > g3xdv.blogspot.co.uk
> > ================
> >
> >
> > 
>