Re: LF: OPERA Question

[M0FMT <m0fmt@yahoo.co.uk>]

Hi Graham

 

SDR; doing your best to drag them kicking and screaming into the future. I don't get the resistance..... the QEX (ARRL) files are available to all (unlike other national outfits) on DSP, a technology which has been with us for a decade or two! But easier to implement these days.

73 es GL Pete M0FMT IO91UX

From: Graham <g8fzk@g8fzk.fsnet.co.uk>
To: rsgb_lf_group@blacksheep.org
Cc: james.moritz@btopenworld.com
Sent: Wednesday, 11 July 2012, 16:51
Subject: Re: LF: OPERA Question

''At first glance it appears crazy''

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..




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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
> ================
>
>
>