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Message-ID: <6876ACB6572F47BC801F2268111C7E07@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <4BBE16CB.23244.11610C0@mike.dennison.ntlworld.com>
In-Reply-To: <4BBE16CB.23244.11610C0@mike.dennison.ntlworld.com>
Date: Thu, 8 Apr 2010 22:21:25 +0100
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Subject: LF: Re: QRSS limiter
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Dear Mike, LF Group,

I have also used this technique on 137kHz, by the crude but reasonably 
effective method of turning off the AGC on my RA1792 RX and turning the RF 
gain control to max with AF gain fairly low, resulting in limiting in the IF 
amplifier. However, on many amateur-type rigs, it isn't really possible to 
completely disable the AGC in this way, so you will have to provide some 
external form of limiting as Mike has done.

> The background noise was much less 'clean'  and Loran lines were a
> little fainter, but when testing during the recent Russian evening I
> got much better results on weak stations. Strong (audible) stations
> showed some distortion, but it was easy to turn the receiver gain
> down if necessary.

Limiting seems to "blur" the spectral lines in the waterfall, I presume due 
to the QRN impulses and other noise effectively randomly modulating the 
narrow-band signals and so adding a form of noise sidebands to them. But 
overall, the effect is beneficial under conditions of high QRN, provided no 
strong signals are present in the RX passband. It would be worth trying the 
Spectrum Lab limiter (with the RX gain turned back down of course), since 
this should be more tolerant of the presence of strong signals of varying 
levels.

Cheers, Jim Moritz
73 de M0BMU