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Date: Thu, 5 Mar 2009 19:31:51 +0000
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From: Andy Talbot <andy.g4jnt@googlemail.com>
To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: Deep copy...
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It all depends on your resolution bandwith which you don't specify.
Pnoise =3D kTB

With my SDR-IQ set to 190kHz span and 131072 point FFT, resolution
bandwidth =3D 1.5Hz and Rx set to +10dB gain to get the best  noise
figure:
-145dBm with a 50R load.  With the Antenna connected this rises to
-115dBm (measured at 1920 UTC) with no obvious lines or peaks
contributing.

If you normalise to the standard 2.5kHz noise measurement bandwidth,
that equates to -112 (50R)  and -82dBm.(antenna) respectively

Thermal noise at reference temperature of 290K is -174dBm/Hz, so for
2.5kHz that gives -140dBm suggesting the noise figure of the SDR-IQ
could be 140 - 112 =3D 28dB.    It probably isn't this low as we're in
the quantisation noise with no input signal, but a NF of 15 - 20dB
would probably be a rough guesstimate.

Andy  G4JNT
www.g4jnt.com



2009/3/5 dave.riley3 <dave.riley3@verizon.net>:
> 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...=A0 What is the expected ambient noise=20=
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=A0into a 50 ohm load and set a long
> integrate ( 64+ ) =A0and saw approx -134 dbm of baseband noise with NO
> signals...
>
> I then connected to the outside antenna with no external pre-amp or any no=
n
> 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 th=
e
> SNR with the hope that the most offending noises will not be in the path o=
f
> a desired signal...
>
> TNX and ain't this fun???
>
> Dave @ WD2XSH/17
>
>
>
>
>
>
>
>
>
> ----- Original Message -----
> From: Bill de Carle
> To: rsgb_lf_group@blacksheep.org
> 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 bypa=
ss
> some heavy static hits as well...
>
> I've noticed that when doing HF frequency measurement tests (working to th=
e
> nearest milliHertz) - turning off the AGC under high static conditions see=
ms
> to improve the accuracy of the measurement, at least with the software I
> use.=A0 One plausible explanation is that AGC action necessarily introduce=
s
> amplitude modulation (on all signals in the passband).=A0 When I process t=
he
> 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.=A0 If the power in those sidebands is comparable to
> that of the signal whose frequency I'm looking for, the FFT algorithm (whi=
ch
> assumes the real signal has the largest amplitude) gets confused and comes
> up with an estimated frequency somewhere between the correct value and tha=
t
> of a nearby sideband.=A0 The effect is small however because the AGC pumpi=
ng
> action doesn't occur very fast so the added sidebands are seen to be only
> some milliHertz away from the signal.=A0 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.=A0 It should be
> possible to model the AGC action of a particular receiver and compensate f=
or
> it in the software.=A0 No doubt the phenomenon becomes less significant wi=
th
> smaller FFT's or shorter integration times.=A0 Changing the AGC setting
> between SLOW-FAST-OFF might help under some conditions.=A0 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
>