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Re: LF: WSPR T/A hole discussion

To: <rsgb_lf_group@blacksheep.org>
Subject: Re: LF: WSPR T/A hole discussion
From: "Graham" <g8fzk@g8fzk.fsnet.co.uk>
Date: Sun, 7 Nov 2010 13:59:47 -0000
Importance: Normal
In-reply-to: <A9F62956427C4E57A04F2D2CEFFAE126@JimPC>
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>
Reply-to: rsgb_lf_group@blacksheep.org
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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






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