Message-ID: <DC1FE246FEE64024A0A4797938CD13F6@AGB>
From: "Graham" <g8fzk@g8fzk.fsnet.co.uk>
To: <rsgb_lf_group@blacksheep.org>
References: <20101101233708.02b5f67d@opc1> <AANLkTi=chbDySSJR=D2_453gDhjwgxZ8D19xS=GOF5hE@mail.gmail.com> <007b01cb7a91$ceb8df60$4001a8c0@lark> <187038.8742.qm@web86705.mail.ird.yahoo.com> <op.vlrpsvoyyzqh0k@pc-roelof> <20101106233741.0895ce48@opc1> <op.vlsqwpu6yzqh0k@pc-roelof> <A9F62956427C4E57A04F2D2CEFFAE126@JimPC>
In-Reply-To: <A9F62956427C4E57A04F2D2CEFFAE126@JimPC>
Date: Sun, 7 Nov 2010 13:59:47 -0000
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Subject: Re: LF: WSPR T/A hole discussion
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Jim,

Yes  good  points  ,  note the narrow  front  end  ..looks good on a plot 
..no use for  s/n   LC in front of the  mixer = more  noise  , same argument 
over  tuned/un-tuned  rx  antennas ... look  at these  min-whips  on mf

''with A/D conversion immediately after a "roofing" filter for data 
signals'',

I would think,  these days , the  cost  of a  good roofing filter  compared 
to a  A/D  convertor  , pushes this  argument one  step on to  eliminating 
the  filter as well , taking a  few pointers  from the  'world of radar' the 
sooner you  can digitise the better ,

AGC , off  and on are not  quite the  full  story,  attack and  hang  time 
serve to  produce   IMD by  altering the  transmitted  pulse shape , 
switching  from short to  long on the  ra1778  introduces  app  10 dB  more 
IMD on a psk31  signal  by  tracking the  envelope, the  some what 
'mechanical'  sample and  hold  AGC in the  ra6790  makes little  difference

For my  $ the  sound card  is an area  most overlooked , SURE 444  V  XTAL 
mic  syndrome  and  speech clipping , you  'have'  to have good audio 
before you  (rf)  clip  .. but  how  many  assume because its clipped than 
the  start  quality is not  important ?

G ..

hobby  .?. more like  being at work  !

--------------------------------------------------
From: "James Moritz" <james.moritz@btopenworld.com>
Sent: Sunday, November 07, 2010 11:41 AM
To: <rsgb_lf_group@blacksheep.org>
Subject: Re: LF: WSPR T/A hole discussion

>
> Dear John, Roelof, LF Group,
>
> I think this problem and the discussion just reflects the fact that 
> receivers are not really designed for use with digital modes, particularly 
> ones that are very narrow-band compared to the traditional phone/CW modes. 
> There does not seem to be a fundamental reason why WSPR signals of greatly 
> differing strength should not be successfully simultaneously decoded, 
> provided the transmitted signals are "clean", and the receive stages up to 
> and including the DSP processing have good linearity and low phase noise, 
> etc.
>
> There has been an obsession in the amateur world for receivers with 
> enormous dynamic range, but this is only normally applied to the RX front 
> end, up to the point where the final IF bandwidth is achieved - the 
> designers assume that the IF bandwidth is similar to the wanted signal 
> bandwidth, and filtering represents all that can be done to remove 
> unwanted signals. It is assumed that all unwanted signals that it is 
> possible to remove have been eliminated after this stage. The post-filter 
> stages only need to be linear enough not to significantly impair the 
> wanted signal, and there has to be a trade-off with other inherently 
> non-linear functions too, such as AGC.
>
> But for many modern "digital" modes, this is no longer the case - 
> normally, several signals will be present in the RX IF and audio channel 
> simultaneously. In the case where spread-spectrum or CDMA techniques are 
> used, multiple signals inherently use the same bandwidth. Linearity must 
> be maintained throughout the receiver. So RX design lags behind the 
> requirements for modern transmission modes. I think in the long term this 
> means seperating the audible reception channel from the data reception 
> channel, with A/D conversion immediately after a "roofing" filter for data 
> signals, and AGC, etc. confined to the "analogue" channel. For current 
> receivers this can't easily be done, and the best practical thing to do in 
> my experience seems to be to ensure the AGC has been disabled, and that 
> the RF/IF gain is operated at a much lower level than would normally be 
> used for audible reception. This applies to QRSS, etc as well as WSPR. I 
> have found that with careful gain adjustment, a lot of the problems due to 
> strong local signals can be eliminated.
>
> In the case of WSPR, the other practical thing that can be done is to 
> reduce the TX duty cycle - so if a 20% duty cycle is used, 80% of time 
> slots will be unnaffected by a local ground-wave-blasting station; if 
> there are two such stations, 64% of slots remain unaffected, and even for 
> three, 51% are OK.
>
> Cheers, Jim Moritz
> 73 de M0BMU
>
>
>