FM broadcast mirrored to different bands - fault?

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Na svyazi
Posts: 1
Joined: Sun Nov 11, 2018 9:39 pm

FM broadcast mirrored to different bands - fault?

Post by Na svyazi » Sun Nov 11, 2018 10:23 pm

Hi everyone!
I'm new to SDR and the RTL-SDR particularly. I've just received my RTV-SDR v3 dongle and started playing with it.
The first thing I noticed is the strange behavior at different bands when strong signals appear to be mirrored to different frequencies. But when I change tuning of the receiver by shifting the whole spectrum in SDR# these mirrored signals move to different frequencies.
I noticed this behavior in CB, HAM, 800 MHz, 900 MHz, 2400MHz band etc.

For example,
I can see FM broadcast stations by tuning to about 27MZh (see the attached Screenshot 1). I'm able to listen to the FM stations by moving the cursor to the corresponding frequencies and selecting WFM.
The really strange thing happens when I change tuning of the receiver by moving the whole spectrum to the left or to the right. The mirrored ghost FM broadcast stations move to different frequencies (see Screenshot 3 and Screenshot 4).

The same thing happens in different bands mentioned above. I get ghost signals (not from FM stations but from digital stuff in the higher bands) that change frequencies corresponding to tuning of the receiver.

Is that faulty hardware or is it that the expected behavior?
Can it be something wrong with the mixer,filters etc?
Screenshot 1
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Screenshot 3
3_c.png (259.17 KiB) Viewed 1066 times
Screenshot 4
4_c.png (256.69 KiB) Viewed 1066 times

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Posts: 1955
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Re: FM broadcast mirrored to different bands - fault?

Post by rtlsdrblog » Mon Nov 12, 2018 10:50 pm

This behavior is expected with the lost cost 8-bit RTL-SDR dongles. Basically very strong signals will overload the dongle, and this results in those strong signals being imaged to other frequencies.

The possible solutions are:

1) Lower the gain
2) Use an attenuator
3) Use an tuned for the frequency band of interest (i.e not a wideband antenna)
4) Use a bandpass/bandstop filter to remove the strong signals
5) Purchase a more expensive 12-bit SDR which is more resilient to overloading, but can still overload with very strong signals.

For for strong broadcast FM signals we sell a broadcast FM bandstop filter in our store which can help.

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