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Re: LF: Deep copy...

To: [email protected]
Subject: Re: LF: Deep copy...
From: Bill de Carle <[email protected]>
Date: Thu, 05 Mar 2009 09:58:04 -0500
In-reply-to: <FA25C6BCA43E4B89B3C17DA7F30F45EE@DR2>
References: <000a01c99a91$a54a1180$1402a8c0@e7010> <FA25C6BCA43E4B89B3C17DA7F30F45EE@DR2>
Reply-to: [email protected]
Sender: [email protected]
At 09:25 PM 3/4/2009, you wrote:
I seem to get best results while trying to pull signals out of the noise when the AGC of the receiver is OFF as well as the Noise Blanker and Noise reduction features being OFF...
 
Do you concur??
 
Also setting a good audio filter to the passband of interest seems to bypass some heavy static hits as well...

I've noticed that when doing HF frequency measurement tests (working to the nearest milliHertz) - turning off the AGC under high static conditions seems to improve the accuracy of the measurement, at least with the software I use.  One plausible explanation is that AGC action necessarily introduces amplitude modulation (on all signals in the passband).  When I process the AGC'd signal with what amounts to a very narrow DSP filter the added amplitude modulation shows up as apparent sidebands close-in on the signal I'm trying to measure.  If the power in those sidebands is comparable to that of the signal whose frequency I'm looking for, the FFT algorithm (which assumes the real signal has the largest amplitude) gets confused and comes up with an estimated frequency somewhere between the correct value and that of a nearby sideband.  The effect is small however because the AGC pumping action doesn't occur very fast so the added sidebands are seen to be only some milliHertz away from the signal.  The sidebands occur on both sides of the "real" signal, so one might expect them to cancel out but in practice they don't because the amount of error depends on where the "real" signal falls with respect to the fixed frequency bins of the FFT.  It should be possible to model the AGC action of a particular receiver and compensate for it in the software.  No doubt the phenomenon becomes less significant with smaller FFT's or shorter integration times.  Changing the AGC setting between SLOW-FAST-OFF might help under some conditions.  With QRSS-60 signaling rates it can take a long time to find out which setting is optimum, especially when band conditions are changing or the QSB period is close to a bit time, hi!

VE2IQ
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