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From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <fc7eccec1002071020g1f1fe66dq4bd363e8d4142c39@mail.gmail.com>
In-Reply-To: <fc7eccec1002071020g1f1fe66dq4bd363e8d4142c39@mail.gmail.com>
Date: Sun, 7 Feb 2010 21:02:29 -0000
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Subject: LF: Re: WSPR S/N Tests
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Dear Andy, LF Group,

I have just tried a quick test here, which more or less confirms your 
findings. My approach was to receive off-air signals as normal with 300Hz 
BW, but to also run Spectrum Lab on the decoding PC, configured in " narrow 
CW filter" mode, with centre frequency set to 1475 Hz (the audio frequency 
G4JNT comes out on) with a BW of 50Hz. The resulting filtered audio was fed 
into a second PC running the WSPR decoder. This has the effect of increasing 
the reported SNR by about 6dB to 13dB when the filtered audio is decoded - 
however, this is smaller than the increase you saw.

Doing this also produces a lot of false decodes - the spectrogram display in 
WSPR shows a couple of broad, bright, noise-like bands where the 
passband-stopband transitions of the audio filter occur, with assorted 
spectral lines that are probably low-level noise from one PC or another. I 
have often noticed the number of false decodes goes up when there are 
narrow-band spectral "features" present, rather than just uniform noise. It 
is curious why the WSPR spectrogram display shows fairly uniform noise 
baseline across the whole 200Hz, with brighter bands of noise where the 
filter attenuation is increasing, rather than a bright band of noise in the 
50Hz passband and darker in the stopband.

Increasing the RX bandwidth to 3kHz makes essentially no difference to the 
normal, un-filtered WSPR decoder over the 300Hz BW.

Cheers, Jim Moritz
73 de M0BMU
----- Original Message ----- 
From: "Andy Talbot" <andy.g4jnt@googlemail.com>
To: <rsgb_lf_group@blacksheep.org>
Cc: <jfell@tesco.net>
Sent: Sunday, February 07, 2010 6:20 PM
Subject: LF: WSPR S/N Tests


> To try to get to grips with the falsely high values of  S/N reported by 
> some
> stations recently, I made a set of controlled measurements over a direct
> link using a range of Rx bandwidths and input signal levels.