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Message-ID: <17A9542495DE4F9296878968B2D73B29@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <4BBE16CB.23244.11610C0@mike.dennison.ntlworld.com>,<6876ACB6572F47BC801F2268111C7E07@JimPC> <BF4A524700075746A6467658DFC7102C19A043E53C@ICTS-S-EXC2-CA.luna.kuleuven.be>
In-Reply-To: <BF4A524700075746A6467658DFC7102C19A043E53C@ICTS-S-EXC2-CA.luna.kuleuven.be>
Date: Fri, 9 Apr 2010 21:24:19 +0100
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Subject: LF: Re: QRSS limiter
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Dear Rik, LF group,

...>what about IMD due to the non linear behavior of the clipping diodes ?

Yes, some IMD is inherent in any clipping (or noise-blanking) scheme. If 
there is, for example, a strong local CW signal in the passband, "quieting" 
as in a FM limiting IF amplifier occurs, and any weaker signal present is 
heavily attenuated, and IM products generated. So severe cross-modulation of 
the weak signal by the strong one would be present. But where most of the 
power in the passband is more-or-less random noise like QRN, IM will still 
occur, but the products won't have any particularly defined frequency and 
will just add to the wideband noise level, or add noise sidebands to the 
signal being received. I suppose this is what causes the "thickening" of 
traces on the spectrogram. But overall, the reduction in noise obtained by 
clipping the large amplitude QRN impulses more than compensates for this. So 
the technique works quite well when you can only hear noise coming out of 
the RX speaker.

The Spectrum Lab limiter seems to be less susceptible to these effects, I 
guess because the clipping threshold automatically tracks any signal 
present.

Cheers, Jim Moritz
73 de M0BMU