From: Rik Strobbe <Rik.Strobbe@fys.kuleuven.be>
To: "rsgb_lf_group@blacksheep.org" <rsgb_lf_group@blacksheep.org>
Date: Fri, 9 Apr 2010 19:53:47 +0200
Thread-Topic: Re: QRSS limiter
Thread-Index: AcrXYc/H+0vnNa9xSkeHN98lnK5z5gAq8sye
Message-ID: <BF4A524700075746A6467658DFC7102C19A043E53C@ICTS-S-EXC2-CA.luna.kuleuven.be>
References: <4BBE16CB.23244.11610C0@mike.dennison.ntlworld.com>,<6876ACB6572F47BC801F2268111C7E07@JimPC>
In-Reply-To: <6876ACB6572F47BC801F2268111C7E07@JimPC>
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Subject: LF: RE: Re: QRSS limiter
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Jim, Mike,

what about IMD due to the non linear behavior of the clipping diodes ?

73, Rik  ON7YD - OR7T

________________________________________
Van: owner-rsgb_lf_group@blacksheep.org [owner-rsgb_lf_group@blacksheep.or=
g] namens James Moritz [james.moritz@btopenworld.com]
Verzonden: donderdag 8 april 2010 23:21
Aan: rsgb_lf_group@blacksheep.org
Onderwerp: LF: Re: QRSS limiter

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 du=
e
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 th=
e
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