Message-Id: <7.0.1.0.1.20100903091104.01853b28@magma.ca>
Date: Fri, 03 Sep 2010 09:58:04 -0400
To: rsgb_lf_group@blacksheep.org
From: Bill de Carle <ve2iq@magma.ca>
In-Reply-To: <03CB7191C69D4F29AA7D6E5E4AB10D62@JimPC>
References: <4C7C52CC.7000506@telus.net>
 <7.0.1.0.1.20100901090820.0188dc40@magma.ca>
 <03CB7191C69D4F29AA7D6E5E4AB10D62@JimPC>
Mime-Version: 1.0
Subject: Re: LF: Re: Re: VE7TIL TX Loop tests continue.............
Content-Type: text/plain; charset="us-ascii"; format=flowed
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org


At 05:52 PM 9/1/2010, Jim Moritz wrote:
>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.

Thanks for your comments Jim.  I tried turning off the AGC with RF 
gain at max and some extra gain in front of the Rx to make it clip 
sooner.  Looking at the results of an overnight (Wednesday) session, 
I saw no trace of TIL (worse than the performance on the previous 
overnight runs) but I did see other weak signals.  Of course one 
overnight run shouldn't be taken as definitive, especially when 
propagation on the path from TIL seems to have been steadily 
worsening over the last few nights.  Could be it was just a bad night.

One idea I am kicking around is to pass the 50-ohm signal at the 
antenna connector through the centre of a toroid with a lot of turns 
on the secondary - enough that we see millivolts instead of 
microvolts.  Then I would do the clipping on the hi-Z side using a 
couple of fast op-amps with a variable threshold setting, hoping the 
hi-Z clipping gets reflected back thru the toroid to the 50 ohm 
side.  Not good for HF but at 137 Khz it might work well 
enough.  Then I could tweak the clipping threshold while watching the 
screen to find the best setting.  On the other hand, recording first 
then playing back a QRSS-120 run repeatedly while changing a software 
setting to find the best clipping point would be quite time-consuming.

Since we're trying to adjust display parameters to make it easier for 
the human eye/brain to discern a weak signal against a noisy 
background, it might be best to maintain linearity as much as 
possible then do the clipping after the FFT across all the individual 
frequency component amplitudes.  I don't know if this would be any 
more effective than adjusting the contrast and brightness controls as 
we do now.  The eye-brain is already pretty good at ignoring all the 
background clutter and "seeing" a pattern it is expecting.  And the 
ear-brain is amazing in its ability to "hear" really weak CW signals 
in a band full of louder competing signals.  The signals we're 
looking for are generally weaker than the noise.  I wonder if it 
would be helpful to display brightness proportional to the logarithm 
of amplitude?  I tried that years ago to no great benefit, but the 
on-the-fly contrast/brightness sliders we have today to change the 
whole screen in real-time might make it worth another try.

73,
Bill VE2IQ