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 -----
From: Bill de Carle
To: [email protected]
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
