|
Thanks to ALL who responded with quick and cogent
findings...
It has seemed for a time that with the receiver AGC
and NBs OFF that I generally can see a deeper signal, depending on
conditions...
The final audio filter assures that the program does
not have to deal with noise outside of the passband of
interest...
TNX
Here is today's gnawing question... What is the
expected ambient noise say @ 600M one should expect??
Rural, City, Country, remote battery operated, etc.
combinations... Especially well away from AC neutral
wires...
Today I ran the input to the
SDR-IQ into a 50 ohm load and set a long integrate ( 64+ ) and saw approx
-134 dbm of baseband noise with NO signals...
I then connected to the outside antenna with no
external pre-amp or any non linear device and saw -117 dbm of noise across 10
khz. with a few weak and floating predictable BB noises..
At night it is sure to be up to near -100 dbm
depending on conditions and noises present...
What do YOU get for a noise difference between a
terminated antenna input and your regular receive
antenna??
I'm about to place several e-probes about this place
in order to mix and match phase and amplitudes in order to see what net gain can
be made to the SNR with the hope that the most offending noises will not be in
the path of a desired signal...
TNX and ain't this fun???
Dave @ WD2XSH/17
----- Original Message -----
Sent: Thursday, March 05, 2009 9:58
AM
Subject: Re: LF: Deep copy...
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
| 