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From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <4C7C52CC.7000506@telus.net> <7.0.1.0.1.20100901090820.0188dc40@magma.ca>
In-Reply-To: <7.0.1.0.1.20100901090820.0188dc40@magma.ca>
Date: Wed, 1 Sep 2010 22:52:36 +0100
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Subject: LF: Re: Re: VE7TIL TX Loop tests continue.............
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Dear Bill, LF Group,

----- Original Message ----- 
From: "Bill de Carle" <ve2iq@magma.ca>

>   I tried turning off the AGC for a while but had to manually cut the RF 
> gain so much to avoid clipping it didn't seem like a good idea.  With the 
> AGC on at least there is some possibility of weak-signal energy getting 
> through between crashes if there are QRN-lulls here and there.

I think you may find that you get better results under these conditions if 
you crank up the RF gain until clipping occurs all the time. This results in 
some loss of signal level, but a greater reduction in QRN level on the 
spectrogram, leading to several dB SNR improvement overall. I suspect how 
well this works depends on how quickly the RX gain stages recover from a 
short-term overload; if the amplifiers clip in a well-behaved way, then the 
QRN transients during which the signal is lost will have a duration 
determined by the impulse response of the IF filter. But the overdriving of 
the amplifiers that occurs during the transient might upset the DC biasing, 
leading to a long recovery "tail" due to the time constants of coupling and 
decoupling Rs, Cs, etc., and greater loss of signal energy.  Operating the 
receiver with the IF amplifier in this kind of limiting mode was quite 
successful with the old RA1792 receiver, which has gain-block IC IF 
amplifiers which probably work quite well as limiting amplifiers.

This kind of clipping does not work well if there are strong unwanted 
signals present in the bandwidth being clipped - either the clipping 
threshold has to be increased to above the level of the unwanted signal, 
making it less effective at reducing the QRN, or the strong signal 
"captures" the bandwidth as in an FM receiver limiting IF, reducing the 
level of the wanted signal as well as the QRN. so in Europe the bandwidth 
around 136kHz has to be restricted to 1kHz or so to exclude DCF39 et al.

Cheers, Jim Moritz
73 de M0BMU