Re: LF: OPERA Question

[Markus Vester <markusvester@aol.com>]

Hi Graham, Mike,

 

given the ratio of occupied signal bandwidth (a few Hz) to filter bandwidth in question (a few 100 Hz), I don't believe there will be any significant impact of analog filters on signal shape. Neither on background noise, directly.

 

However in the presence of impulsive noise, eg lightning statics, the effectiveness of a noiseblanker will be severely degraded if the duration of the pulses is prolonged by narrow filters with long ringdown. The sharper the better - especially at VLF where spherics are dominated by the short cracks from return strokes. However strong carriers should be excluded from the passband before noiseblanker action, as otherwise they will be contaminated by sidebands from the cutouts.

 

Best 73,

MArkus (DF6NM) 

-----Ursprüngliche Mitteilung-----
Von: Graham <g8fzk@g8fzk.fsnet.co.uk>
An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
Cc: james.moritz <james.moritz@btopenworld.com>
Verschickt: Mi, 11 Jul 2012 4:15 pm
Betreff: 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..




--------------------------------------------------
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
> ================
>
>
>