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SDR# (rtl2832) signal noise calibrating to peak signal.

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#1
So, I'm trying to calibrate my NooElec RTL-SDR RTL2832U & R820T for SDSharp, using the stock antenna, which actually gets pretty darned good reception.

Problem is, upon calibrating my signal, so it sits center of the peak, I get this high pitch noise. If I readjust the signal so it sits a bit off center, the noise disappears. I'm using my local NOAA (162.550) signal to calibrate.

I'm new to this, so I was just wondering if this noise is normal upon calibrating your signal to dead center of the peak.

Also, is it better to keep the signal directly centered, or better to keep off-center a bit?

I posted a video showing the noise I'm experiencing, if anyone's interested in watching.

Any help or suggestions is appreciated. Thanks!

VIDEO CAPTURE LINK --> RTL-SDR RTL2832U & R820T singal noise. - YouTube
 
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#2
The centre frequency is 'zero' or DC. When you try to put the carrier in the centre what you are hearing is actually the carrier frequency converted down to slightly off-centre - therefore the tone. IMHO the sensitivity doesn't change much as you move away from the centre and as long as you are receiving the signal (you have the red vertical line on the signal carrier frequency and you have left-clicked) the "S" meter should be indicating the carrier level. Is NOAA NFM? I'd go for an AM station that has a carrier that doesn't vary in amplitude (much?)
 

rbm

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#3
Are you using a good antenna, or the little whip that came with the dongle?
Do you get that same noise on other signals?

I noticed in your video that you have the 'Filter bandwidth' set to 8000 (8khz).
That's very narrow for NOAA broadcasts.

I normally use 12500 so I don't get adjacent channel interference.
If I reduce my 'Filter bandwith' to 8000, I start to hear some noise and distortion on some NOAA stations.

But the 'official' recommendation is:
"The NWS recommends the public purchase receivers with a 0.5 to 1.0 microvolt sensitivity for 20 decibels (dB) quieting, a selectivity of 45 to 70 dB down at +/- 25 kilohertz (kHz)"
NOAA All Hazards Weather Radio

I'm using an outside antenna with lots of signal.
The little antenna that comes with it doesn't work well at all. A clip lead is better.

You can see the settings I use for NOAA broadcasts in this post:
http://forums.radioreference.com/software-defined-radio/259610-sdr-nfm.html#post1908232

Also, it's not good practice to precisely center the frequency in the display.
The process of Correcting IQ will create some 'artifacts' in the signal.

Rich
 
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#5
@majoco, thanks for your input. You stated the tone I'm hearing is a result of the carrier frequency converted down to slightly off-centre. Though, I was wondering if others with my same dongle, hear that same tone after calibrating to center on NOAA.

Are you using a good antenna, or the little whip that came with the dongle?
Do you get that same noise on other signals?
Yes, I'm using the antenna that came with my dongle, and it receives suprising well, especially compared to past scanners I've had. I'm on the 2nd floor of a private home, so that probably adds to the good reception.

Do you get that same noise on other signals?
No. I don't receive that noise on any FM stations, which come in very clear, also I don't receive that noise on other frequencies, for instance police and fire. So far, I just hear that particular type noise (tone) when I center on NOAA.


I noticed in your video that you have the 'Filter bandwidth' set to 8000 (8khz).
That's very narrow for NOAA broadcasts.

I normally use 12500 so I don't get adjacent channel interference.
If I reduce my 'Filter bandwith' to 8000, I start to hear some noise and distortion on some NOAA stations.
I've tried all types of adjustments for bandwidth filter, from 1000 through 20000. The noise does not go away on any setting.


But the 'official' recommendation is:
"The NWS recommends the public purchase receivers with a 0.5 to 1.0 microvolt sensitivity for 20 decibels (dB) quieting, a selectivity of 45 to 70 dB down at +/- 25 kilohertz (kHz)"
NOAA All Hazards Weather Radio

I'm using an outside antenna with lots of signal.
The little antenna that comes with it doesn't work well at all. A clip lead is better.
Ya, I was mainly wondering if it's normal to hear that type of tone upon centering your signal for NOAA with these dongles. I was hoping others would say, "ya, we all hear that tone", though it's looking like it's either my setup, dongle, or antenna.


Also, it's not good practice to precisely center the frequency in the display.
The process of Correcting IQ will create some 'artifacts' in the signal.
I'll keep that in mind, and thanks for all your useful input!

Thanks for your suggestion, though I already had the Noise Reduction plugin installed. It does work well, but it does nothing to remove the noise I'm inquiring about.
 
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#6
I've been playing with my dongle (lol), to see if anything removes that tone from the NOAA broadcast. What I found, is plugging the dongle into different USB ports, seems to reduce the tone.

So far, the USB 2.0 ports on the front of my desktop PC, has the least amount of noise in terms of that tone I'm experiencing. I tried the ports on the back of my PC, and the tone was still considerably heard. The powered USB 2.0 port is where I originally had my dongle plugged in, and that, so far seems to emit the strongest tone of all my ports.

For now, I'll just be keeping the dongle in my powered hub, and just keep the signal off-set from center to reduce the tone.
 

AM909

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Dec 10, 2015
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#8
Are you within range of two different NOAA transmitters on that channel (remember they are fairly high-power and high-elevation)? You could be hearing a heterodyne of the two signals. You could also have some local source of interference that happens to be on that frequency. No way to tell unless the primary goes off the air for maintenance.
 
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