Signal Stronger at left edge of waterfall nooelec Smartee

Paul-Nuneaton

Newbie
Joined
Oct 3, 2020
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4
Hi,
I am just dipping my toe into the world of SDR and this looks like a great forum to pick up tome tips.

I have bought a nooelec NESDR Smartee V2 as a start.

Unfortunately i live in a ground floor apartment and external antennae are not an option. I wish I had thought about that before i moved in but I have not had a valid Ham licence for 20 years and receiving RF was not in my mind. So, I am using the bundled VHF antenna that came with the nooelec on my desk.

Anyway, while trying out CubicSDR on my Macbook Pro, I was listening around the VHF Airband and noticed a continuous tone on 117.450.
I don't know what it is, and it doesn't matter for for now. I'm sure it is something spurious.

I noticed it at the left edge of the waterfall and it was a reasonably good signal.

But when I scrolled the window sideways to centre that frequency, it went off completely.

So on the far left, it is strongest, but if I move it closer to the centre by a 12th of the width of the window (The window is showing the full width that the nooelec can do, about 2mhz), it has gone completely. I see the signal get weaker as I move it.

What does this indicate? I have a RSP1A on order and it will be interesting to compare the signal on both.

Does it show a software setting problem or is it a limitation of the hardware?
 

Ubbe

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Sep 8, 2006
Messages
5,040
Location
Stockholm, Sweden
It is spurioses in the receiver. All SDR receivers have more or less of them and the cheap ones have a lot.
If the signal strengh changes depending of where in the spectrum it is or if it stationary even when you move the spectrums frequency then it is internal signals in the SDR receiver and can be ignored. More expensive receivers like RSP1A are pretty good with very few internal signals, but then it is also much more expensive.

/Ubbe
 

lwvmobile

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Apr 26, 2020
Messages
109
Location
Lafayette County, FL
I noticed it at the left edge of the waterfall and it was a reasonably good signal.

But when I scrolled the window sideways to center that frequency, it went off completely.
I've observed the same type of behavior. What I've discovered is that if you have a really REALLY strong signal nearby where you are centered, but not quite in the range of your waterfall, then it will show up as a slightly weaker signal on the other side of the waterfall from where it should be when scrolling though. It can be quite misleading, and I admit, it used to trick me all the time.

For example, here is a really strong signal at 851.8

gqrx-sig-1.png

If I scroll it just off the screen, then I can get its phantom to show up on the other side of the waterfall.
gqrx-sig2.png
That 'signal' on the very far right is in fact 851.8

Don't ask me why this happens with cheap RTL dongles, but I'm sure there is a good technical explanation as to why it happens.
 

Paul-Nuneaton

Newbie
Joined
Oct 3, 2020
Messages
4
Thank you both for the replies.

I had noticed multiple signals that appeared and disappeared at the same time. I assumed it was just being transmitted on multiple frequencies, but thinking about it, I did only notice it on very string signals.

After I posted this thread I tried a few more tests and found another signal which must have been real as it peaked in the centre and dropped off slightly at the edges the same as the noise floor seems to drop off.

Hopefully this will stop me chasing phantoms :)

My RSP1A arrived today so I will be able to do some comparisons of the spurious and phantom signals.
 

Ubbe

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Sep 8, 2006
Messages
5,040
Location
Stockholm, Sweden
When the internal signals are strong they will work as an oscillator and mix with the external signal and produce mirror frequencies. These will move in the opposite direction when you move the spectrums frequency. If you have a signal below the middle of the spectrum and you move the spectrum to get that frequency to the center, the signal will move even further down in frequency.

There's lots of false signals in cheap SDR dongles, some will suddenly disappear when you reduce the gain slightly, and you can't really trust any of those signals until you verify them with a conventional standard type of receiver.

/Ubbe
 

slicerwizard

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Joined
Sep 19, 2002
Messages
6,638
Location
Toronto, Ontario
Don't ask me why this happens with cheap RTL dongles, but I'm sure there is a good technical explanation as to why it happens.
It's a simple Nyquist violation. The signal is more than 1.2 MHz from the dongle's oscillator and it's pushing through the cheesy bandpass filter, so the mixing product is greater than 1.2 MHz, which cannot be properly represented by a 2.4 MHz sampling rate. Simple aliasing/folding into the +/- 1.2 MHz spectrum that a 2.4 Msps I/Q stream can convey.
 
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