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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