Message-ID: <6E301500F9494DF7BE3128B1F13E69D0@AGB>
From: "Graham" <g8fzk@g8fzk.fsnet.co.uk>
To: <rsgb_lf_group@blacksheep.org>
References: <C3F731D3B3D442FDBAA8720F953F74A5@AGB>, <4FFD5313.6103.333D978@mike.dennison.ntlworld.com>, <B1EECEA279364E4E9084F3C395B74761@AGB> <4FFDB2F7.23256.4AA6AB0@mike.dennison.ntlworld.com>
In-Reply-To: <4FFDB2F7.23256.4AA6AB0@mike.dennison.ntlworld.com>
Date: Wed, 11 Jul 2012 17:33:26 +0100
MIME-Version: 1.0
Importance: Normal
Subject: Re: LF:  OPERA  Question
Content-Type: text/plain;
	format=flowed;
	charset="iso-8859-1";
	reply-type=original
Content-Transfer-Encoding: 8bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
X-AOL-SCOLL-URL_COUNT: 0  
x-aol-sid: 3039ac1d40574ffdab125ab7
X-AOL-IP: 195.171.43.25
X-AOL-SPF: domain : blacksheep.org SPF : none


Ok Mike

AGC , yes  'off' is the  preferred   mode , to  enable the  full  range of 
the  dsp  to  be  used .  wide/no agc  feels a  bit  odd but  that's the 
recommended  setting's ...better  still  go  SDR  , but there are limits , 
I  brought one of the  Ј10  donglels  , last weekend  , monitoring   CR 
repeater  on 433.150  , using  HDSDR , the  dongle  gave  better  reception 
than the  FT-897  ...   some things  are  better  not  known .. : (

I found  a  odd  thing , with the  RA1778  and the  RA6790 , if the  AGC 
was  set to  'short'  then the  IMD
level  shown  on  PSK31  was  something  like   10 dB  worse , than  with 
the  AGC  set to  medium/long  or  off.
The  agc  was  following the  modulation  envelope  and flat-topping the 
signal ...

This  coming   winter  should  be  action  packed  on MF / LF , its  mid 
summer  and 500  is  full  already !

73 -G.



--------------------------------------------------
From: "Mike Dennison" <mike.dennison@ntlworld.com>
Sent: Wednesday, July 11, 2012 5:08 PM
To: <rsgb_lf_group@blacksheep.org>
Subject: Re: LF:  OPERA  Question

> 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
>> > ================
>> >
>> >
>> >
>>
>
>
>
>